Apache Beam Programming Guide
The Beam Programming Guide is intended for Beam users who want to use the Beam SDKs to create data processing pipelines. It provides guidance for using the Beam SDK classes to build and test your pipeline. The programming guide is not intended as an exhaustive reference, but as a language-agnostic, high-level guide to programmatically building your Beam pipeline. As the programming guide is filled out, the text will include code samples in multiple languages to help illustrate how to implement Beam concepts in your pipelines.
If you want a brief introduction to Beam’s basic concepts before reading the programming guide, take a look at the Basics of the Beam model page.
- Java SDK
- Python SDK
- Go SDK
- TypeScript SDK
- Yaml API
The Python SDK supports Python 3.8, 3.9, 3.10, and 3.11.
The Go SDK supports Go v1.20+.
The Typescript SDK supports Node v16+ and is still experimental.
YAML is supported as of Beam 2.52, but is under active development and the most recent SDK is advised.
1. Overview
To use Beam, you need to first create a driver program using the classes in one of the Beam SDKs. Your driver program defines your pipeline, including all of the inputs, transforms, and outputs; it also sets execution options for your pipeline (typically passed in using command-line options). These include the Pipeline Runner, which, in turn, determines what back-end your pipeline will run on.
The Beam SDKs provide a number of abstractions that simplify the mechanics of large-scale distributed data processing. The same Beam abstractions work with both batch and streaming data sources. When you create your Beam pipeline, you can think about your data processing task in terms of these abstractions. They include:
Pipeline
: APipeline
encapsulates your entire data processing task, from start to finish. This includes reading input data, transforming that data, and writing output data. All Beam driver programs must create aPipeline
. When you create thePipeline
, you must also specify the execution options that tell thePipeline
where and how to run.PCollection
: APCollection
represents a distributed data set that your Beam pipeline operates on. The data set can be bounded, meaning it comes from a fixed source like a file, or unbounded, meaning it comes from a continuously updating source via a subscription or other mechanism. Your pipeline typically creates an initialPCollection
by reading data from an external data source, but you can also create aPCollection
from in-memory data within your driver program. From there,PCollection
s are the inputs and outputs for each step in your pipeline.PTransform
: APTransform
represents a data processing operation, or a step, in your pipeline. EveryPTransform
takes one or morePCollection
objects as input, performs a processing function that you provide on the elements of thatPCollection
, and produces zero or more outputPCollection
objects.
Scope
: The Go SDK has an explicit scope variable used to build aPipeline
. APipeline
can return it’s root scope with theRoot()
method. The scope variable is passed toPTransform
functions to place them in thePipeline
that owns theScope
.
- I/O transforms: Beam comes with a number of “IOs” - library
PTransform
s that read or write data to various external storage systems.
A typical Beam driver program works as follows:
- Create a
Pipeline
object and set the pipeline execution options, including the Pipeline Runner. - Create an initial
PCollection
for pipeline data, either using the IOs to read data from an external storage system, or using aCreate
transform to build aPCollection
from in-memory data. - Apply
PTransform
s to eachPCollection
. Transforms can change, filter, group, analyze, or otherwise process the elements in aPCollection
. A transform creates a new outputPCollection
without modifying the input collection. A typical pipeline applies subsequent transforms to each new outputPCollection
in turn until processing is complete. However, note that a pipeline does not have to be a single straight line of transforms applied one after another: think ofPCollection
s as variables andPTransform
s as functions applied to these variables: the shape of the pipeline can be an arbitrarily complex processing graph. - Use IOs to write the final, transformed
PCollection
(s) to an external source. - Run the pipeline using the designated Pipeline Runner.
When you run your Beam driver program, the Pipeline Runner that you designate
constructs a workflow graph of your pipeline based on the PCollection
objects you’ve created and transforms that you’ve applied. That graph is then
executed using the appropriate distributed processing back-end, becoming an
asynchronous “job” (or equivalent) on that back-end.
2. Creating a pipeline
The Pipeline
abstraction encapsulates all the data and steps in your data
processing task. Your Beam driver program typically starts by constructing a
Pipeline
Pipeline
Pipeline
object, and then using that object as the basis for creating the pipeline’s data
sets as PCollection
s and its operations as Transform
s.
To use Beam, your driver program must first create an instance of the Beam SDK
class Pipeline
(typically in the main()
function). When you create your
Pipeline
, you’ll also need to set some configuration options. You can set
your pipeline’s configuration options programmatically, but it’s often easier to
set the options ahead of time (or read them from the command line) and pass them
to the Pipeline
object when you create the object.
For a more in-depth tutorial on creating basic pipelines in the Python SDK, please read and work through this colab notebook.
2.1. Configuring pipeline options
Use the pipeline options to configure different aspects of your pipeline, such as the pipeline runner that will execute your pipeline and any runner-specific configuration required by the chosen runner. Your pipeline options will potentially include information such as your project ID or a location for storing files.
When you run the pipeline on a runner of your choice, a copy of the
PipelineOptions will be available to your code. For example, if you add a PipelineOptions parameter
to a DoFn’s @ProcessElement
method, it will be populated by the system.
2.1.1. Setting PipelineOptions from command-line arguments
While you can configure your pipeline by creating a PipelineOptions
object and
setting the fields directly, the Beam SDKs include a command-line parser that
you can use to set fields in PipelineOptions
using command-line arguments.
To read options from the command-line, construct your PipelineOptions
object
as demonstrated in the following example code:
Use Go flags to parse command line arguments to configure your pipeline. Flags must be parsed
before beam.Init()
is called.
Any Javascript object can be used as pipeline options.
One can either construct one manually, but it is also common to pass an object
created from command line options such as yargs.argv
.
Pipeline options are simply an optional YAML mapping property that is a sibling to the pipeline definition itself. It will be merged with whatever options are passed on the command line.
This interprets command-line arguments that follow the format:
--<option>=<value>
Appending the method
.withValidation
will check for required command-line arguments and validate argument values.
Building your PipelineOptions
this way lets you specify any of the options as
a command-line argument.
Defining flag variables this way lets you specify any of the options as a command-line argument.
Note: The WordCount example pipeline demonstrates how to set pipeline options at runtime by using command-line options.
2.1.2. Creating custom options
You can add your own custom options in addition to the standard
PipelineOptions
.
To add your own options, define an interface with getter and setter methods for each option.
The following example shows how to addinput
and output
custom options:You can also specify a description, which appears when a user passes --help
as
a command-line argument, and a default value.
You set the description and default value using annotations, as follows:
public interface MyOptions extends PipelineOptions {
@Description("Input for the pipeline")
@Default.String("gs://my-bucket/input")
String getInput();
void setInput(String input);
@Description("Output for the pipeline")
@Default.String("gs://my-bucket/output")
String getOutput();
void setOutput(String output);
}
from apache_beam.options.pipeline_options import PipelineOptions
class MyOptions(PipelineOptions):
@classmethod
def _add_argparse_args(cls, parser):
parser.add_argument(
'--input',
default='gs://dataflow-samples/shakespeare/kinglear.txt',
help='The file path for the input text to process.')
parser.add_argument(
'--output', required=True, help='The path prefix for output files.')
For Python, you can also simply parse your custom options with argparse; there is no need to create a separate PipelineOptions subclass.
It’s recommended that you register your interface with PipelineOptionsFactory
and then pass the interface when creating the PipelineOptions
object. When you
register your interface with PipelineOptionsFactory
, the --help
can find
your custom options interface and add it to the output of the --help
command.
PipelineOptionsFactory
will also validate that your custom options are
compatible with all other registered options.
The following example code shows how to register your custom options interface
with PipelineOptionsFactory
:
Now your pipeline can accept --input=value
and --output=value
as command-line arguments.
3. PCollections
The PCollection
PCollection
PCollection
abstraction represents a
potentially distributed, multi-element data set. You can think of a
PCollection
as “pipeline” data; Beam transforms use PCollection
objects as
inputs and outputs. As such, if you want to work with data in your pipeline, it
must be in the form of a PCollection
.
After you’ve created your Pipeline
, you’ll need to begin by creating at least
one PCollection
in some form. The PCollection
you create serves as the input
for the first operation in your pipeline.
3.1. Creating a PCollection
You create a PCollection
by either reading data from an external source using
Beam’s Source API, or you can create a PCollection
of data
stored in an in-memory collection class in your driver program. The former is
typically how a production pipeline would ingest data; Beam’s Source APIs
contain adapters to help you read from external sources like large cloud-based
files, databases, or subscription services. The latter is primarily useful for
testing and debugging purposes.
3.1.1. Reading from an external source
To read from an external source, you use one of the Beam-provided I/O
adapters. The adapters vary in their exact usage, but all of them
read from some external data source and return a PCollection
whose elements
represent the data records in that source.
Each data source adapter has a Read
transform; to read,
you must apply that transform to the Pipeline
object itself.
place this transform in the source
or transforms
portion of the pipeline.
TextIO.Read
io.TextFileSource
textio.Read
textio.ReadFromText
,
ReadFromText
,
for example, reads from an
external text file and returns a PCollection
whose elements
are of type String
where each String
represents one line from the text file. Here’s how you
would apply TextIO.Read
io.TextFileSource
textio.Read
textio.ReadFromText
ReadFromText
to your Pipeline
root to create
a PCollection
:
public static void main(String[] args) {
// Create the pipeline.
PipelineOptions options =
PipelineOptionsFactory.fromArgs(args).create();
Pipeline p = Pipeline.create(options);
// Create the PCollection 'lines' by applying a 'Read' transform.
PCollection<String> lines = p.apply(
"ReadMyFile", TextIO.read().from("gs://some/inputData.txt"));
}
See the section on I/O to learn more about how to read from the various data sources supported by the Beam SDK.
3.1.2. Creating a PCollection from in-memory data
To create a PCollection
from an in-memory Java Collection
, you use the
Beam-provided Create
transform. Much like a data adapter’s Read
, you apply
Create
directly to your Pipeline
object itself.
As parameters, Create
accepts the Java Collection
and a Coder
object. The
Coder
specifies how the elements in the Collection
should be
encoded.
To create a PCollection
from an in-memory list
, you use the Beam-provided
Create
transform. Apply this transform directly to your Pipeline
object
itself.
To create a PCollection
from an in-memory slice
, you use the Beam-provided
beam.CreateList
transform. Pass the pipeline scope
, and the slice
to this transform.
To create a PCollection
from an in-memory array
, you use the Beam-provided
Create
transform. Apply this transform directly to your Root
object.
To create a PCollection
from an in-memory array
, you use the Beam-provided
Create
transform. Specify the elements in the pipeline itself.
The following example code shows how to create a PCollection
from an in-memory
List
list
slice
array
:
public static void main(String[] args) {
// Create a Java Collection, in this case a List of Strings.
final List<String> LINES = Arrays.asList(
"To be, or not to be: that is the question: ",
"Whether 'tis nobler in the mind to suffer ",
"The slings and arrows of outrageous fortune, ",
"Or to take arms against a sea of troubles, ");
// Create the pipeline.
PipelineOptions options =
PipelineOptionsFactory.fromArgs(args).create();
Pipeline p = Pipeline.create(options);
// Apply Create, passing the list and the coder, to create the PCollection.
p.apply(Create.of(LINES)).setCoder(StringUtf8Coder.of());
}
lines := []string{
"To be, or not to be: that is the question: ",
"Whether 'tis nobler in the mind to suffer ",
"The slings and arrows of outrageous fortune, ",
"Or to take arms against a sea of troubles, ",
}
// Create the Pipeline object and root scope.
// It's conventional to use p as the Pipeline variable and
// s as the scope variable.
p, s := beam.NewPipelineWithRoot()
// Pass the slice to beam.CreateList, to create the pcollection.
// The scope variable s is used to add the CreateList transform
// to the pipeline.
linesPCol := beam.CreateList(s, lines)
3.2. PCollection characteristics
A PCollection
is owned by the specific Pipeline
object for which it is
created; multiple pipelines cannot share a PCollection
.
In some respects, a PCollection
functions like
a Collection
class. However, a PCollection
can differ in a few key ways:
3.2.1. Element type
The elements of a PCollection
may be of any type, but must all be of the same
type. However, to support distributed processing, Beam needs to be able to
encode each individual element as a byte string (so elements can be passed
around to distributed workers). The Beam SDKs provide a data encoding mechanism
that includes built-in encoding for commonly-used types as well as support for
specifying custom encodings as needed.
3.2.2. Element schema
In many cases, the element type in a PCollection
has a structure that can be introspected.
Examples are JSON, Protocol Buffer, Avro, and database records. Schemas provide a way to
express types as a set of named fields, allowing for more-expressive aggregations.
3.2.3. Immutability
A PCollection
is immutable. Once created, you cannot add, remove, or change
individual elements. A Beam Transform might process each element of a
PCollection
and generate new pipeline data (as a new PCollection
), but it
does not consume or modify the original input collection.
Note: Beam SDKs avoid unnecessary copying of elements, so
PCollection
contents are logically immutable, not physically immutable. Changes to input elements may be visible to other DoFns executing within the same bundle, and may cause correctness issues. As a rule, it’s not safe to modify values provided to a DoFn.
3.2.4. Random access
A PCollection
does not support random access to individual elements. Instead,
Beam Transforms consider every element in a PCollection
individually.
3.2.5. Size and boundedness
A PCollection
is a large, immutable “bag” of elements. There is no upper limit
on how many elements a PCollection
can contain; any given PCollection
might
fit in memory on a single machine, or it might represent a very large
distributed data set backed by a persistent data store.
A PCollection
can be either bounded or unbounded in size. A
bounded PCollection
represents a data set of a known, fixed size, while an
unbounded PCollection
represents a data set of unlimited size. Whether a
PCollection
is bounded or unbounded depends on the source of the data set that
it represents. Reading from a batch data source, such as a file or a database,
creates a bounded PCollection
. Reading from a streaming or
continuously-updating data source, such as Pub/Sub or Kafka, creates an unbounded
PCollection
(unless you explicitly tell it not to).
The bounded (or unbounded) nature of your PCollection
affects how Beam
processes your data. A bounded PCollection
can be processed using a batch job,
which might read the entire data set once, and perform processing in a job of
finite length. An unbounded PCollection
must be processed using a streaming
job that runs continuously, as the entire collection can never be available for
processing at any one time.
Beam uses windowing to divide a continuously updating unbounded
PCollection
into logical windows of finite size. These logical windows are
determined by some characteristic associated with a data element, such as a
timestamp. Aggregation transforms (such as GroupByKey
and Combine
) work
on a per-window basis — as the data set is generated, they process each
PCollection
as a succession of these finite windows.
3.2.6. Element timestamps
Each element in a PCollection
has an associated intrinsic timestamp. The
timestamp for each element is initially assigned by the Source
that creates the PCollection
. Sources that create an unbounded PCollection
often assign each new element a timestamp that corresponds to when the element
was read or added.
Note: Sources that create a bounded
PCollection
for a fixed data set also automatically assign timestamps, but the most common behavior is to assign every element the same timestamp (Long.MIN_VALUE
).
Timestamps are useful for a PCollection
that contains elements with an
inherent notion of time. If your pipeline is reading a stream of events, like
Tweets or other social media messages, each element might use the time the event
was posted as the element timestamp.
You can manually assign timestamps to the elements of a PCollection
if the
source doesn’t do it for you. You’ll want to do this if the elements have an
inherent timestamp, but the timestamp is somewhere in the structure of the
element itself (such as a “time” field in a server log entry). Beam has
Transforms that take a PCollection
as input and output an
identical PCollection
with timestamps attached; see Adding
Timestamps for more information
about how to do so.
4. Transforms
Transforms are the operations in your pipeline, and provide a generic
processing framework. You provide processing logic in the form of a function
object (colloquially referred to as “user code”), and your user code is applied
to each element of an input PCollection
(or more than one PCollection
).
Depending on the pipeline runner and back-end that you choose, many different
workers across a cluster may execute instances of your user code in parallel.
The user code running on each worker generates the output elements that are
ultimately added to the final output PCollection
that the transform produces.
Aggregation is an important concept to understand when learning about Beam’s transforms. For an introduction to aggregation, see the Basics of the Beam model Aggregation section.
The Beam SDKs contain a number of different transforms that you can apply to
your pipeline’s PCollection
s. These include general-purpose core transforms,
such as ParDo or Combine. There are also pre-written
composite transforms included in the SDKs, which
combine one or more of the core transforms in a useful processing pattern, such
as counting or combining elements in a collection. You can also define your own
more complex composite transforms to fit your pipeline’s exact use case.
For a more in-depth tutorial of applying various transforms in the Python SDK, please read and work through this colab notebook.
4.1. Applying transforms
To invoke a transform, you must apply it to the input PCollection
. Each
transform in the Beam SDKs has a generic apply
method
(or pipe operator |
).
Invoking multiple Beam transforms is similar to method chaining, but with one
slight difference: You apply the transform to the input PCollection
, passing
the transform itself as an argument, and the operation returns the output
PCollection
.
array
In YAML, transforms are applied by listing their inputs.
This takes the general form:
If a transform has more than one (non-error) output, the various outputs can be identified by explicitly giving the output name.
For linear pipelines, this can be further simplified by implicitly determining
the inputs based on by the ordering of the transforms by designating and setting
the type to chain
. For example
Because Beam uses a generic apply
method for PCollection
, you can both chain
transforms sequentially and also apply transforms that contain other transforms
nested within (called composite transforms in the Beam
SDKs).
It’s recommended to create a new variable for each new PCollection
to
sequentially transform input data. Scope
s can be used to create functions
that contain other transforms
(called composite transforms in the Beam SDKs).
How you apply your pipeline’s transforms determines the structure of your
pipeline. The best way to think of your pipeline is as a directed acyclic graph,
where PTransform
nodes are subroutines that accept PCollection
nodes as
inputs and emit PCollection
nodes as outputs.
For example, you can chain together transforms to create a pipeline that successively modifies input data:
For example, you can successively call transforms on PCollections to modify the input data:
The graph of this pipeline looks like the following:
Figure 1: A linear pipeline with three sequential transforms.
However, note that a transform does not consume or otherwise alter the input
collection — remember that a PCollection
is immutable by definition. This means
that you can apply multiple transforms to the same input PCollection
to create
a branching pipeline, like so:
The graph of this branching pipeline looks like the following:
Figure 2: A branching pipeline. Two transforms are applied to a single PCollection of database table rows.
You can also build your own composite transforms that nest multiple transforms inside a single, larger transform. Composite transforms are particularly useful for building a reusable sequence of simple steps that get used in a lot of different places.
The pipe syntax allows one to apply PTransforms to tuple
s and dict
s of
PCollections as well for those transforms accepting multiple inputs (such as
Flatten
and CoGroupByKey
).
PTransforms can also be applied to any PValue
, which include the Root object,
PCollections, arrays of PValue
s, and objects with PValue
values.
One can apply transforms to these composite types by wrapping them with
beam.P
, e.g.
beam.P({left: pcollA, right: pcollB}).apply(transformExpectingTwoPCollections)
.
PTransforms come in two flavors, synchronous and asynchronous, depending on
whether their application* involves asynchronous invocations.
An AsyncTransform
must be applied with applyAsync
and returns a Promise
which must be awaited before further pipeline construction.
4.2. Core Beam transforms
Beam provides the following core transforms, each of which represents a different processing paradigm:
ParDo
GroupByKey
CoGroupByKey
Combine
Flatten
Partition
The Typescript SDK provides some of the most basic of these transforms
as methods on PCollection
itself.
4.2.1. ParDo
ParDo
is a Beam transform for generic parallel processing. The ParDo
processing paradigm is similar to the “Map” phase of a
Map/Shuffle/Reduce-style
algorithm: a ParDo
transform considers each element in the input
PCollection
, performs some processing function (your user code) on that
element, and emits zero, one, or multiple elements to an output PCollection
.
ParDo
is useful for a variety of common data processing operations, including:
- Filtering a data set. You can use
ParDo
to consider each element in aPCollection
and either output that element to a new collection or discard it. - Formatting or type-converting each element in a data set. If your input
PCollection
contains elements that are of a different type or format than you want, you can useParDo
to perform a conversion on each element and output the result to a newPCollection
. - Extracting parts of each element in a data set. If you have a
PCollection
of records with multiple fields, for example, you can use aParDo
to parse out just the fields you want to consider into a newPCollection
. - Performing computations on each element in a data set. You can use
ParDo
to perform simple or complex computations on every element, or certain elements, of aPCollection
and output the results as a newPCollection
.
In such roles, ParDo
is a common intermediate step in a pipeline. You might
use it to extract certain fields from a set of raw input records, or convert raw
input into a different format; you might also use ParDo
to convert processed
data into a format suitable for output, like database table rows or printable
strings.
When you apply a ParDo
transform, you’ll need to provide user code in the form
of a DoFn
object. DoFn
is a Beam SDK class that defines a distributed
processing function.
In Beam YAML, ParDo
operations are expressed by the MapToFields
, Filter
,
and Explode
transform types. These types can take a UDF in the language of your
choice, rather than introducing the notion of a DoFn
.
See the page on mapping fns for more details.
When you create a subclass of
DoFn
, note that your subclass should adhere to the Requirements for writing user code for Beam transforms.
All DoFns should be registered using a generic register.DoFnXxY[...]
function. This allows the Go SDK to infer an encoding from any inputs/outputs,
registers the DoFn for execution on remote runners, and optimizes the runtime
execution of the DoFns via reflection.
// ComputeWordLengthFn is a DoFn that computes the word length of string elements.
type ComputeWordLengthFn struct{}
// ProcessElement computes the length of word and emits the result.
// When creating structs as a DoFn, the ProcessElement method performs the
// work of this step in the pipeline.
func (fn *ComputeWordLengthFn) ProcessElement(ctx context.Context, word string) int {
...
}
func init() {
// 2 inputs and 1 output => DoFn2x1
// Input/output types are included in order in the brackets
register.DoFn2x1[context.Context, string, int](&ComputeWordLengthFn{})
}
4.2.1.1. Applying ParDo
Like all Beam transforms, you apply ParDo
by calling the apply
method on the
input PCollection
and passing ParDo
as an argument, as shown in the
following example code:
Like all Beam transforms, you apply ParDo
by calling the beam.ParDo
on the
input PCollection
and passing the DoFn
as an argument, as shown in the
following example code:
beam.ParDo
applies the passed in DoFn
argument to the input PCollection
,
as shown in the following example code:
// The input PCollection of Strings.
PCollection<String> words = ...;
// The DoFn to perform on each element in the input PCollection.
static class ComputeWordLengthFn extends DoFn<String, Integer> { ... }
// Apply a ParDo to the PCollection "words" to compute lengths for each word.
PCollection<Integer> wordLengths = words.apply(
ParDo
.of(new ComputeWordLengthFn())); // The DoFn to perform on each element, which
// we define above.
# The input PCollection of Strings.
words = ...
# The DoFn to perform on each element in the input PCollection.
class ComputeWordLengthFn(beam.DoFn):
def process(self, element):
return [len(element)]
# Apply a ParDo to the PCollection "words" to compute lengths for each word.
word_lengths = words | beam.ParDo(ComputeWordLengthFn())
// ComputeWordLengthFn is the DoFn to perform on each element in the input PCollection.
type ComputeWordLengthFn struct{}
// ProcessElement is the method to execute for each element.
func (fn *ComputeWordLengthFn) ProcessElement(word string, emit func(int)) {
emit(len(word))
}
// DoFns must be registered with beam.
func init() {
beam.RegisterType(reflect.TypeOf((*ComputeWordLengthFn)(nil)))
// 2 inputs and 0 outputs => DoFn2x0
// 1 input => Emitter1
// Input/output types are included in order in the brackets
register.DoFn2x0[string, func(int)](&ComputeWordLengthFn{})
register.Emitter1[int]()
}
// words is an input PCollection of strings
var words beam.PCollection = ...
wordLengths := beam.ParDo(s, &ComputeWordLengthFn{}, words)
# The input PCollection of Strings.
const words : PCollection<string> = ...
# The DoFn to perform on each element in the input PCollection.
function computeWordLengthFn(): beam.DoFn<string, number> {
return {
process: function* (element) {
yield element.length;
},
};
}
const result = words.apply(beam.parDo(computeWordLengthFn()));
In the example, our input PCollection
contains String
string
values. We apply a
ParDo
transform that specifies a function (ComputeWordLengthFn
) to compute
the length of each string, and outputs the result to a new PCollection
of
Integer
int
values that stores the length of each word.
4.2.1.2. Creating a DoFn
The DoFn
object that you pass to ParDo
contains the processing logic that
gets applied to the elements in the input collection. When you use Beam, often
the most important pieces of code you’ll write are these DoFn
s - they’re what
define your pipeline’s exact data processing tasks.
Note: When you create your
DoFn
, be mindful of the Requirements for writing user code for Beam transforms and ensure that your code follows them. You should avoid time-consuming operations such as reading large files inDoFn.Setup
.
A DoFn
processes one element at a time from the input PCollection
. When you
create a subclass of DoFn
, you’ll need to provide type parameters that match
the types of the input and output elements. If your DoFn
processes incoming
String
elements and produces Integer
elements for the output collection
(like our previous example, ComputeWordLengthFn
), your class declaration would
look like this:
A DoFn
processes one element at a time from the input PCollection
. When you
create a DoFn
struct, you’ll need to provide type parameters that match
the types of the input and output elements in a ProcessElement method.
If your DoFn
processes incoming string
elements and produces int
elements
for the output collection (like our previous example, ComputeWordLengthFn
), your dofn could
look like this:
// ComputeWordLengthFn is a DoFn that computes the word length of string elements.
type ComputeWordLengthFn struct{}
// ProcessElement computes the length of word and emits the result.
// When creating structs as a DoFn, the ProcessElement method performs the
// work of this step in the pipeline.
func (fn *ComputeWordLengthFn) ProcessElement(word string, emit func(int)) {
...
}
func init() {
// 2 inputs and 0 outputs => DoFn2x0
// 1 input => Emitter1
// Input/output types are included in order in the brackets
register.Function2x0(&ComputeWordLengthFn{})
register.Emitter1[int]()
}
Inside your DoFn
subclass, you’ll write a method annotated with
@ProcessElement
where you provide the actual processing logic. You don’t need
to manually extract the elements from the input collection; the Beam SDKs handle
that for you. Your @ProcessElement
method should accept a parameter tagged with
@Element
, which will be populated with the input element. In order to output
elements, the method can also take a parameter of type OutputReceiver
which
provides a method for emitting elements. The parameter types must match the input
and output types of your DoFn
or the framework will raise an error. Note: @Element
and
OutputReceiver
were introduced in Beam 2.5.0; if using an earlier release of Beam, a
ProcessContext
parameter should be used instead.
Inside your DoFn
subclass, you’ll write a method process
where you provide
the actual processing logic. You don’t need to manually extract the elements
from the input collection; the Beam SDKs handle that for you. Your process
method
should accept an argument element
, which is the input element, and return an
iterable with its output values. You can accomplish this by emitting individual
elements with yield
statements, and use yield from
to emit all elements from
an iterable, such as a list or a generator. Using return
statement
with an iterable is also acceptable as long as you don’t mix yield
and
return
statements in the same process
method, since that leads to incorrect behavior.
For your DoFn
type, you’ll write a method ProcessElement
where you provide
the actual processing logic. You don’t need to manually extract the elements
from the input collection; the Beam SDKs handle that for you. Your ProcessElement
method
should accept a parameter element
, which is the input element. In order to output elements,
the method can also take a function parameter, which can be called to emit elements.
The parameter types must match the input and output types of your DoFn
or the framework will raise an error.
// ComputeWordLengthFn is the DoFn to perform on each element in the input PCollection.
type ComputeWordLengthFn struct{}
// ProcessElement is the method to execute for each element.
func (fn *ComputeWordLengthFn) ProcessElement(word string, emit func(int)) {
emit(len(word))
}
// DoFns must be registered with beam.
func init() {
beam.RegisterType(reflect.TypeOf((*ComputeWordLengthFn)(nil)))
// 2 inputs and 0 outputs => DoFn2x0
// 1 input => Emitter1
// Input/output types are included in order in the brackets
register.DoFn2x0[string, func(int)](&ComputeWordLengthFn{})
register.Emitter1[int]()
}
Simple DoFns can also be written as functions.
Note: Whether using a structural
DoFn
type or a functionalDoFn
, they should be registered with beam in aninit
block. Otherwise they may not execute on distributed runners.
Note: If the elements in your input
PCollection
are key/value pairs, you can access the key or value by usingelement.getKey()
orelement.getValue()
, respectively.
Note: If the elements in your input
PCollection
are key/value pairs, your process element method must have two parameters, for each of the key and value, respectively. Similarly, key/value pairs are also output as separate parameters to a singleemitter function
.
A given DoFn
instance generally gets invoked one or more times to process some
arbitrary bundle of elements. However, Beam doesn’t guarantee an exact number of
invocations; it may be invoked multiple times on a given worker node to account
for failures and retries. As such, you can cache information across multiple
calls to your processing method, but if you do so, make sure the implementation
does not depend on the number of invocations.
In your processing method, you’ll also need to meet some immutability requirements to ensure that Beam and the processing back-end can safely serialize and cache the values in your pipeline. Your method should meet the following requirements:
- You should not in any way modify an element returned by
the
@Element
annotation orProcessContext.sideInput()
(the incoming elements from the input collection). - Once you output a value using
OutputReceiver.output()
you should not modify that value in any way.
- You should not in any way modify the
element
argument provided to theprocess
method, or any side inputs. - Once you output a value using
yield
orreturn
, you should not modify that value in any way.
- You should not in any way modify the parameters provided to the
ProcessElement
method, or any side inputs. - Once you output a value using an
emitter function
, you should not modify that value in any way.
4.2.1.3. Lightweight DoFns and other abstractions
If your function is relatively straightforward, you can simplify your use of
ParDo
by providing a lightweight DoFn
in-line, as
an anonymous inner class instance
a lambda function
an anonymous function
a function passed to PCollection.map
or PCollection.flatMap
.
Here’s the previous example, ParDo
with ComputeLengthWordsFn
, with the
DoFn
specified as
an anonymous inner class instance
a lambda function
an anonymous function
a function:
// The input PCollection.
PCollection<String> words = ...;
// Apply a ParDo with an anonymous DoFn to the PCollection words.
// Save the result as the PCollection wordLengths.
PCollection<Integer> wordLengths = words.apply(
"ComputeWordLengths", // the transform name
ParDo.of(new DoFn<String, Integer>() { // a DoFn as an anonymous inner class instance
@ProcessElement
public void processElement(@Element String word, OutputReceiver<Integer> out) {
out.output(word.length());
}
}));
If your ParDo
performs a one-to-one mapping of input elements to output
elements–that is, for each input element, it applies a function that produces
exactly one output element, you can return that
element directly.you can use the higher-level
MapElements
Map
transform.MapElements
can accept an anonymous
Java 8 lambda function for additional brevity.
Here’s the previous example using MapElements
Map
a direct return:
// The input PCollection.
PCollection<String> words = ...;
// Apply a MapElements with an anonymous lambda function to the PCollection words.
// Save the result as the PCollection wordLengths.
PCollection<Integer> wordLengths = words.apply(
MapElements.into(TypeDescriptors.integers())
.via((String word) -> word.length()));
The Go SDK cannot support anonymous functions outside of the deprecated Go Direct runner.
func wordLengths(word string) int { return len(word) }
func init() { register.Function1x1(wordLengths) }
func applyWordLenAnon(s beam.Scope, words beam.PCollection) beam.PCollection {
return beam.ParDo(s, wordLengths, words)
}
Note: You can use Java 8 lambda functions with several other Beam transforms, including
Filter
,FlatMapElements
, andPartition
.
Note: Anonymous function DoFns do not work on distributed runners. It’s recommended to use named functions and register them with
register.FunctionXxY
in aninit()
block.
4.2.1.4. DoFn lifecycle
Here is a sequence diagram that shows the lifecycle of the DoFn during the execution of the ParDo transform. The comments give useful information to pipeline developers such as the constraints that apply to the objects or particular cases such as failover or instance reuse. They also give instantiation use cases. Three key points to note are that:
- Teardown is done on a best effort basis and thus isn’t guaranteed.
- The number of DoFn instances created at runtime is runner-dependent.
- For the Python SDK, the pipeline contents such as DoFn user code,
is serialized into a bytecode. Therefore,
DoFn
s should not reference objects that are not serializable, such as locks. To manage a single instance of an object across multipleDoFn
instances in the same process, use utilities in the shared.py module.
4.2.2. GroupByKey
GroupByKey
is a Beam transform for processing collections of key/value pairs.
It’s a parallel reduction operation, analogous to the Shuffle phase of a
Map/Shuffle/Reduce-style algorithm. The input to GroupByKey
is a collection of
key/value pairs that represents a multimap, where the collection contains
multiple pairs that have the same key, but different values. Given such a
collection, you use GroupByKey
to collect all of the values associated with
each unique key.
GroupByKey
is a good way to aggregate data that has something in common. For
example, if you have a collection that stores records of customer orders, you
might want to group together all the orders from the same postal code (wherein
the “key” of the key/value pair is the postal code field, and the “value” is the
remainder of the record).
Let’s examine the mechanics of GroupByKey
with a simple example case, where
our data set consists of words from a text file and the line number on which
they appear. We want to group together all the line numbers (values) that share
the same word (key), letting us see all the places in the text where a
particular word appears.
Our input is a PCollection
of key/value pairs where each word is a key, and
the value is a line number in the file where the word appears. Here’s a list of
the key/value pairs in the input collection:
cat, 1
dog, 5
and, 1
jump, 3
tree, 2
cat, 5
dog, 2
and, 2
cat, 9
and, 6
...
GroupByKey
gathers up all the values with the same key and outputs a new pair
consisting of the unique key and a collection of all of the values that were
associated with that key in the input collection. If we apply GroupByKey
to
our input collection above, the output collection would look like this:
cat, [1,5,9]
dog, [5,2]
and, [1,2,6]
jump, [3]
tree, [2]
...
Thus, GroupByKey
represents a transform from a multimap (multiple keys to
individual values) to a uni-map (unique keys to collections of values).
Using GroupByKey
is straightforward:
While all SDKs have a GroupByKey
transform, using GroupBy
is
generally more natural.
The GroupBy
transform can be parameterized by the name(s) of properties
on which to group the elements of the PCollection, or a function taking
the each element as input that maps to a key on which to do grouping.
// A PCollection of elements like
// {word: "cat", score: 1}, {word: "dog", score: 5}, {word: "cat", score: 5}, ...
const scores : PCollection<{word: string, score: number}> = ...
// This will produce a PCollection with elements like
// {key: "cat", value: [{ word: "cat", score: 1 },
// { word: "cat", score: 5 }, ...]}
// {key: "dog", value: [{ word: "dog", score: 5 }, ...]}
const grouped_by_word = scores.apply(beam.groupBy("word"));
// This will produce a PCollection with elements like
// {key: 3, value: [{ word: "cat", score: 1 },
// { word: "dog", score: 5 },
// { word: "cat", score: 5 }, ...]}
const by_word_length = scores.apply(beam.groupBy((x) => x.word.length));
4.2.2.1 GroupByKey and unbounded PCollections
If you are using unbounded PCollection
s, you must use either non-global
windowing or an
aggregation trigger in order to perform a GroupByKey
or
CoGroupByKey. This is because a bounded GroupByKey
or
CoGroupByKey
must wait for all the data with a certain key to be collected,
but with unbounded collections, the data is unlimited. Windowing and/or triggers
allow grouping to operate on logical, finite bundles of data within the
unbounded data streams.
If you do apply GroupByKey
or CoGroupByKey
to a group of unbounded
PCollection
s without setting either a non-global windowing strategy, a trigger
strategy, or both for each collection, Beam generates an IllegalStateException
error at pipeline construction time.
When using GroupByKey
or CoGroupByKey
to group PCollection
s that have a
windowing strategy applied, all of the PCollection
s you want to
group must use the same windowing strategy and window sizing. For example, all
of the collections you are merging must use (hypothetically) identical 5-minute
fixed windows, or 4-minute sliding windows starting every 30 seconds.
If your pipeline attempts to use GroupByKey
or CoGroupByKey
to merge
PCollection
s with incompatible windows, Beam generates an
IllegalStateException error at pipeline construction time.
4.2.3. CoGroupByKey
CoGroupByKey
performs a relational join of two or more key/value
PCollection
s that have the same key type.
Design Your Pipeline
shows an example pipeline that uses a join.
Consider using CoGroupByKey
if you have multiple data sets that provide
information about related things. For example, let’s say you have two different
files with user data: one file has names and email addresses; the other file
has names and phone numbers. You can join those two data sets, using the user
name as a common key and the other data as the associated values. After the
join, you have one data set that contains all of the information (email
addresses and phone numbers) associated with each name.
One can also consider using SqlTransform to perform a join.
If you are using unbounded PCollection
s, you must use either non-global
windowing or an
aggregation trigger in order to perform a CoGroupByKey
. See
GroupByKey and unbounded PCollections
for more details.
In the Beam SDK for Java, CoGroupByKey
accepts a tuple of keyed
PCollection
s (PCollection<KV<K, V>>
) as input. For type safety, the SDK
requires you to pass each PCollection
as part of a KeyedPCollectionTuple
.
You must declare a TupleTag
for each input PCollection
in the
KeyedPCollectionTuple
that you want to pass to CoGroupByKey
. As output,
CoGroupByKey
returns a PCollection<KV<K, CoGbkResult>>
, which groups values
from all the input PCollection
s by their common keys. Each key (all of type
K
) will have a different CoGbkResult
, which is a map from TupleTag<T>
to
Iterable<T>
. You can access a specific collection in an CoGbkResult
object
by using the TupleTag
that you supplied with the initial collection.
In the Beam SDK for Python, CoGroupByKey
accepts a dictionary of keyed
PCollection
s as input. As output, CoGroupByKey
creates a single output
PCollection
that contains one key/value tuple for each key in the input
PCollection
s. Each key’s value is a dictionary that maps each tag to an
iterable of the values under they key in the corresponding PCollection
.
In the Beam Go SDK, CoGroupByKey
accepts an arbitrary number of
PCollection
s as input. As output, CoGroupByKey
creates a single output
PCollection
that groups each key with value iterator functions for each
input PCollection
. The iterator functions map to input PCollections
in
the same order they were provided to the CoGroupByKey
.
The following conceptual examples use two input collections to show the mechanics of
CoGroupByKey
.
The first set of data has a TupleTag<String>
called emailsTag
and contains names
and email addresses. The second set of data has a TupleTag<String>
called
phonesTag
and contains names and phone numbers.
The first set of data contains names and email addresses. The second set of data contains names and phone numbers.
final List<KV<String, String>> emailsList =
Arrays.asList(
KV.of("amy", "amy@example.com"),
KV.of("carl", "carl@example.com"),
KV.of("julia", "julia@example.com"),
KV.of("carl", "carl@email.com"));
final List<KV<String, String>> phonesList =
Arrays.asList(
KV.of("amy", "111-222-3333"),
KV.of("james", "222-333-4444"),
KV.of("amy", "333-444-5555"),
KV.of("carl", "444-555-6666"));
PCollection<KV<String, String>> emails = p.apply("CreateEmails", Create.of(emailsList));
PCollection<KV<String, String>> phones = p.apply("CreatePhones", Create.of(phonesList));
emails_list = [
('amy', 'amy@example.com'),
('carl', 'carl@example.com'),
('julia', 'julia@example.com'),
('carl', 'carl@email.com'),
]
phones_list = [
('amy', '111-222-3333'),
('james', '222-333-4444'),
('amy', '333-444-5555'),
('carl', '444-555-6666'),
]
emails = p | 'CreateEmails' >> beam.Create(emails_list)
phones = p | 'CreatePhones' >> beam.Create(phones_list)
type stringPair struct {
K, V string
}
func splitStringPair(e stringPair) (string, string) {
return e.K, e.V
}
func init() {
// Register DoFn.
register.Function1x2(splitStringPair)
}
// CreateAndSplit is a helper function that creates
func CreateAndSplit(s beam.Scope, input []stringPair) beam.PCollection {
initial := beam.CreateList(s, input)
return beam.ParDo(s, splitStringPair, initial)
}
var emailSlice = []stringPair{
{"amy", "amy@example.com"},
{"carl", "carl@example.com"},
{"julia", "julia@example.com"},
{"carl", "carl@email.com"},
}
var phoneSlice = []stringPair{
{"amy", "111-222-3333"},
{"james", "222-333-4444"},
{"amy", "333-444-5555"},
{"carl", "444-555-6666"},
}
emails := CreateAndSplit(s.Scope("CreateEmails"), emailSlice)
phones := CreateAndSplit(s.Scope("CreatePhones"), phoneSlice)
const emails_list = [
{ name: "amy", email: "amy@example.com" },
{ name: "carl", email: "carl@example.com" },
{ name: "julia", email: "julia@example.com" },
{ name: "carl", email: "carl@email.com" },
];
const phones_list = [
{ name: "amy", phone: "111-222-3333" },
{ name: "james", phone: "222-333-4444" },
{ name: "amy", phone: "333-444-5555" },
{ name: "carl", phone: "444-555-6666" },
];
const emails = root.apply(
beam.withName("createEmails", beam.create(emails_list))
);
const phones = root.apply(
beam.withName("createPhones", beam.create(phones_list))
);
- type: Create
name: CreateEmails
config:
elements:
- { name: "amy", email: "amy@example.com" }
- { name: "carl", email: "carl@example.com" }
- { name: "julia", email: "julia@example.com" }
- { name: "carl", email: "carl@email.com" }
- type: Create
name: CreatePhones
config:
elements:
- { name: "amy", phone: "111-222-3333" }
- { name: "james", phone: "222-333-4444" }
- { name: "amy", phone: "333-444-5555" }
- { name: "carl", phone: "444-555-6666" }
After CoGroupByKey
, the resulting data contains all data associated with each
unique key from any of the input collections.
final TupleTag<String> emailsTag = new TupleTag<>();
final TupleTag<String> phonesTag = new TupleTag<>();
final List<KV<String, CoGbkResult>> expectedResults =
Arrays.asList(
KV.of(
"amy",
CoGbkResult.of(emailsTag, Arrays.asList("amy@example.com"))
.and(phonesTag, Arrays.asList("111-222-3333", "333-444-5555"))),
KV.of(
"carl",
CoGbkResult.of(emailsTag, Arrays.asList("carl@email.com", "carl@example.com"))
.and(phonesTag, Arrays.asList("444-555-6666"))),
KV.of(
"james",
CoGbkResult.of(emailsTag, Arrays.asList())
.and(phonesTag, Arrays.asList("222-333-4444"))),
KV.of(
"julia",
CoGbkResult.of(emailsTag, Arrays.asList("julia@example.com"))
.and(phonesTag, Arrays.asList())));
results = [
(
'amy',
{
'emails': ['amy@example.com'],
'phones': ['111-222-3333', '333-444-5555']
}),
(
'carl',
{
'emails': ['carl@email.com', 'carl@example.com'],
'phones': ['444-555-6666']
}),
('james', {
'emails': [], 'phones': ['222-333-4444']
}),
('julia', {
'emails': ['julia@example.com'], 'phones': []
}),
]
results := beam.CoGroupByKey(s, emails, phones)
contactLines := beam.ParDo(s, formatCoGBKResults, results)
// Synthetic example results of a cogbk.
results := []struct {
Key string
Emails, Phones []string
}{
{
Key: "amy",
Emails: []string{"amy@example.com"},
Phones: []string{"111-222-3333", "333-444-5555"},
}, {
Key: "carl",
Emails: []string{"carl@email.com", "carl@example.com"},
Phones: []string{"444-555-6666"},
}, {
Key: "james",
Emails: []string{},
Phones: []string{"222-333-4444"},
}, {
Key: "julia",
Emails: []string{"julia@example.com"},
Phones: []string{},
},
}
const results = [
{
name: "amy",
values: {
emails: [{ name: "amy", email: "amy@example.com" }],
phones: [
{ name: "amy", phone: "111-222-3333" },
{ name: "amy", phone: "333-444-5555" },
],
},
},
{
name: "carl",
values: {
emails: [
{ name: "carl", email: "carl@example.com" },
{ name: "carl", email: "carl@email.com" },
],
phones: [{ name: "carl", phone: "444-555-6666" }],
},
},
{
name: "james",
values: {
emails: [],
phones: [{ name: "james", phone: "222-333-4444" }],
},
},
{
name: "julia",
values: {
emails: [{ name: "julia", email: "julia@example.com" }],
phones: [],
},
},
];
The following code example joins the two PCollection
s with CoGroupByKey
,
followed by a ParDo
to consume the result. Then, the code uses tags to look up
and format data from each collection.
The following code example joins the two PCollection
s with CoGroupByKey
,
followed by a ParDo
to consume the result. The ordering of the DoFn
iterator
parameters maps to the ordering of the CoGroupByKey
inputs.
PCollection<KV<String, CoGbkResult>> results =
KeyedPCollectionTuple.of(emailsTag, emails)
.and(phonesTag, phones)
.apply(CoGroupByKey.create());
PCollection<String> contactLines =
results.apply(
ParDo.of(
new DoFn<KV<String, CoGbkResult>, String>() {
@ProcessElement
public void processElement(ProcessContext c) {
KV<String, CoGbkResult> e = c.element();
String name = e.getKey();
Iterable<String> emailsIter = e.getValue().getAll(emailsTag);
Iterable<String> phonesIter = e.getValue().getAll(phonesTag);
String formattedResult =
Snippets.formatCoGbkResults(name, emailsIter, phonesIter);
c.output(formattedResult);
}
}));
# The result PCollection contains one key-value element for each key in the
# input PCollections. The key of the pair will be the key from the input and
# the value will be a dictionary with two entries: 'emails' - an iterable of
# all values for the current key in the emails PCollection and 'phones': an
# iterable of all values for the current key in the phones PCollection.
results = ({'emails': emails, 'phones': phones} | beam.CoGroupByKey())
def join_info(name_info):
(name, info) = name_info
return '%s; %s; %s' %\
(name, sorted(info['emails']), sorted(info['phones']))
contact_lines = results | beam.Map(join_info)
func formatCoGBKResults(key string, emailIter, phoneIter func(*string) bool) string {
var s string
var emails, phones []string
for emailIter(&s) {
emails = append(emails, s)
}
for phoneIter(&s) {
phones = append(phones, s)
}
// Values have no guaranteed order, sort for deterministic output.
sort.Strings(emails)
sort.Strings(phones)
return fmt.Sprintf("%s; %s; %s", key, formatStringIter(emails), formatStringIter(phones))
}
func init() {
register.Function3x1(formatCoGBKResults)
// 1 input of type string => Iter1[string]
register.Iter1[string]()
}
// Synthetic example results of a cogbk.
results := []struct {
Key string
Emails, Phones []string
}{
{
Key: "amy",
Emails: []string{"amy@example.com"},
Phones: []string{"111-222-3333", "333-444-5555"},
}, {
Key: "carl",
Emails: []string{"carl@email.com", "carl@example.com"},
Phones: []string{"444-555-6666"},
}, {
Key: "james",
Emails: []string{},
Phones: []string{"222-333-4444"},
}, {
Key: "julia",
Emails: []string{"julia@example.com"},
Phones: []string{},
},
}
- type: MapToFields
name: PrepareEmails
input: CreateEmails
config:
language: python
fields:
name: name
email: "[email]"
phone: "[]"
- type: MapToFields
name: PreparePhones
input: CreatePhones
config:
language: python
fields:
name: name
email: "[]"
phone: "[phone]"
- type: Combine
name: CoGropuBy
input: [PrepareEmails, PreparePhones]
config:
group_by: [name]
combine:
email: concat
phone: concat
- type: MapToFields
name: FormatResults
input: CoGropuBy
config:
language: python
fields:
formatted:
"'%s; %s; %s' % (name, sorted(email), sorted(phone))"
The formatted data looks like this:
4.2.4. Combine
Combine
Combine
Combine
Combine
is a Beam transform for combining collections of elements or values in your
data. Combine
has variants that work on entire PCollection
s, and some that
combine the values for each key in PCollection
s of key/value pairs.
When you apply a Combine
transform, you must provide the function that
contains the logic for combining the elements or values. The combining function
should be commutative and associative, as the function is not necessarily
invoked exactly once on all values with a given key. Because the input data
(including the value collection) may be distributed across multiple workers, the
combining function might be called multiple times to perform partial combining
on subsets of the value collection. The Beam SDK also provides some pre-built
combine functions for common numeric combination operations such as sum, min,
and max.
Simple combine operations, such as sums, can usually be implemented as a simple
function. More complex combination operations might require you to create a
subclass of CombineFn
that has an accumulation type distinct from the input/output type.
The associativity and commutativity of a CombineFn
allows runners to
automatically apply some optimizations:
- Combiner lifting: This is the most significant optimization. Input elements are combined per key and window before they are shuffled, so the volume of data shuffled might be reduced by many orders of magnitude. Another term for this optimization is “mapper-side combine.”
- Incremental combining: When you have a
CombineFn
that reduces the data size by a lot, it is useful to combine elements as they emerge from a streaming shuffle. This spreads out the cost of doing combines over the time that your streaming computation might be idle. Incremental combining also reduces the storage of intermediate accumulators.
4.2.4.1. Simple combinations using simple functions
Beam YAML has the following buit-in CombineFns: count, sum, min, max, mean, any, all, group, and concat. CombineFns from other languages can also be referenced as described in the (full docs on aggregation)[https://beam.apache.org/documentation/sdks/yaml-combine/]. The following example code shows a simple combine function. Combining is done by modifying a grouping transform with the `combining` method. This method takes three parameters: the value to combine (either as a named property of the input elements, or a function of the entire input), the combining operation (either a binary function or a `CombineFn`), and finally a name for the combined value in the output object.// Sum a collection of Integer values. The function SumInts implements the interface SerializableFunction.
public static class SumInts implements SerializableFunction<Iterable<Integer>, Integer> {
@Override
public Integer apply(Iterable<Integer> input) {
int sum = 0;
for (int item : input) {
sum += item;
}
return sum;
}
}
func sumInts(a, v int) int {
return a + v
}
func init() {
register.Function2x1(sumInts)
}
func globallySumInts(s beam.Scope, ints beam.PCollection) beam.PCollection {
return beam.Combine(s, sumInts, ints)
}
type boundedSum struct {
Bound int
}
func (fn *boundedSum) MergeAccumulators(a, v int) int {
sum := a + v
if fn.Bound > 0 && sum > fn.Bound {
return fn.Bound
}
return sum
}
func init() {
register.Combiner1[int](&boundedSum{})
}
func globallyBoundedSumInts(s beam.Scope, bound int, ints beam.PCollection) beam.PCollection {
return beam.Combine(s, &boundedSum{Bound: bound}, ints)
}
All Combiners should be registered using a generic register.CombinerX[...]
function. This allows the Go SDK to infer an encoding from any inputs/outputs,
registers the Combiner for execution on remote runners, and optimizes the runtime
execution of the Combiner via reflection.
Combiner1 should be used when your accumulator, input, and output are all of the
same type. It can be called with register.Combiner1[T](&CustomCombiner{})
where T
is the type of the input/accumulator/output.
Combiner2 should be used when your accumulator, input, and output are 2 distinct
types. It can be called with register.Combiner2[T1, T2](&CustomCombiner{})
where
T1
is the type of the accumulator and T2
is the other type.
Combiner3 should be used when your accumulator, input, and output are 3 distinct
types. It can be called with register.Combiner3[T1, T2, T3](&CustomCombiner{})
where T1
is the type of the accumulator, T2
is the type of the input, and T3
is
the type of the output.
4.2.4.2. Advanced combinations using CombineFn
For more complex combine functions, you can define a
subclass ofCombineFn
.
You should use a CombineFn
if the combine function requires a more sophisticated
accumulator, must perform additional pre- or post-processing, might change the
output type, or takes the key into account.
A general combining operation consists of five operations. When you create a
subclass of
CombineFn
, you must provide five operations by overriding the
corresponding methods. Only MergeAccumulators
is a required method. The
others will have a default interpretation based on the accumulator type. The
lifecycle methods are:
Create Accumulator creates a new “local” accumulator. In the example case, taking a mean average, a local accumulator tracks the running sum of values (the numerator value for our final average division) and the number of values summed so far (the denominator value). It may be called any number of times in a distributed fashion.
Add Input adds an input element to an accumulator, returning the accumulator value. In our example, it would update the sum and increment the count. It may also be invoked in parallel.
Merge Accumulators merges several accumulators into a single accumulator; this is how data in multiple accumulators is combined before the final calculation. In the case of the mean average computation, the accumulators representing each portion of the division are merged together. It may be called again on its outputs any number of times.
Extract Output performs the final computation. In the case of computing a mean average, this means dividing the combined sum of all the values by the number of values summed. It is called once on the final, merged accumulator.
Compact returns a more compact represenation of the accumulator. This is called before an accumulator is sent across the wire, and can be useful in cases where values are buffered or otherwise lazily kept unprocessed when added to the accumulator. Compact should return an equivalent, though possibly modified, accumulator. In most cases, Compact is not necessary. For a real world example of using Compact, see the Python SDK implementation of TopCombineFn
The following example code shows how to define a CombineFn
that computes a
mean average:
public class AverageFn extends CombineFn<Integer, AverageFn.Accum, Double> {
public static class Accum {
int sum = 0;
int count = 0;
}
@Override
public Accum createAccumulator() { return new Accum(); }
@Override
public Accum addInput(Accum accum, Integer input) {
accum.sum += input;
accum.count++;
return accum;
}
@Override
public Accum mergeAccumulators(Iterable<Accum> accums) {
Accum merged = createAccumulator();
for (Accum accum : accums) {
merged.sum += accum.sum;
merged.count += accum.count;
}
return merged;
}
@Override
public Double extractOutput(Accum accum) {
return ((double) accum.sum) / accum.count;
}
// No-op
@Override
public Accum compact(Accum accum) { return accum; }
}
pc = ...
class AverageFn(beam.CombineFn):
def create_accumulator(self):
return (0.0, 0)
def add_input(self, sum_count, input):
(sum, count) = sum_count
return sum + input, count + 1
def merge_accumulators(self, accumulators):
sums, counts = zip(*accumulators)
return sum(sums), sum(counts)
def extract_output(self, sum_count):
(sum, count) = sum_count
return sum / count if count else float('NaN')
def compact(self, accumulator):
# No-op
return accumulator
type averageFn struct{}
type averageAccum struct {
Count, Sum int
}
func (fn *averageFn) CreateAccumulator() averageAccum {
return averageAccum{0, 0}
}
func (fn *averageFn) AddInput(a averageAccum, v int) averageAccum {
return averageAccum{Count: a.Count + 1, Sum: a.Sum + v}
}
func (fn *averageFn) MergeAccumulators(a, v averageAccum) averageAccum {
return averageAccum{Count: a.Count + v.Count, Sum: a.Sum + v.Sum}
}
func (fn *averageFn) ExtractOutput(a averageAccum) float64 {
if a.Count == 0 {
return math.NaN()
}
return float64(a.Sum) / float64(a.Count)
}
func (fn *averageFn) Compact(a averageAccum) averageAccum {
// No-op
return a
}
func init() {
register.Combiner3[averageAccum, int, float64](&averageFn{})
}
const meanCombineFn: beam.CombineFn<number, [number, number], number> =
{
createAccumulator: () => [0, 0],
addInput: ([sum, count]: [number, number], i: number) => [
sum + i,
count + 1,
],
mergeAccumulators: (accumulators: [number, number][]) =>
accumulators.reduce(([sum0, count0], [sum1, count1]) => [
sum0 + sum1,
count0 + count1,
]),
extractOutput: ([sum, count]: [number, number]) => sum / count,
};
4.2.4.3. Combining a PCollection into a single value
Use the global combine to transform all of the elements in a given PCollection
into a single value, represented in your pipeline as a new PCollection
containing one element. The following example code shows how to apply the Beam
provided sum combine function to produce a single sum value for a PCollection
of integers.
4.2.4.4. Combine and global windowing
If your input PCollection
uses the default global windowing, the default
behavior is to return a PCollection
containing one item. That item’s value
comes from the accumulator in the combine function that you specified when
applying Combine
. For example, the Beam provided sum combine function returns
a zero value (the sum of an empty input), while the min combine function returns
a maximal or infinite value.
To have Combine
instead return an empty PCollection
if the input is empty,
specify .withoutDefaults
when you apply your Combine
transform, as in the
following code example:
func returnSideOrDefault(d float64, iter func(*float64) bool) float64 {
var c float64
if iter(&c) {
// Side input has a value, so return it.
return c
}
// Otherwise, return the default
return d
}
func init() { register.Function2x1(returnSideOrDefault) }
func globallyAverageWithDefault(s beam.Scope, ints beam.PCollection) beam.PCollection {
// Setting combine defaults has requires no helper function in the Go SDK.
average := beam.Combine(s, &averageFn{}, ints)
// To add a default value:
defaultValue := beam.Create(s, float64(0))
return beam.ParDo(s, returnSideOrDefault, defaultValue, beam.SideInput{Input: average})
}
const pcoll = root.apply(
beam.create([
{ player: "alice", accuracy: 1.0 },
{ player: "bob", accuracy: 0.99 },
{ player: "eve", accuracy: 0.5 },
{ player: "eve", accuracy: 0.25 },
])
);
const result = pcoll.apply(
beam
.groupGlobally()
.combining("accuracy", combiners.mean, "mean")
.combining("accuracy", combiners.max, "max")
);
const expected = [{ max: 1.0, mean: 0.685 }];
4.2.4.5. Combine and non-global windowing
If your PCollection
uses any non-global windowing function, Beam does not
provide the default behavior. You must specify one of the following options when
applying Combine
:
- Specify
.withoutDefaults
, where windows that are empty in the inputPCollection
will likewise be empty in the output collection. - Specify
.asSingletonView
, in which the output is immediately converted to aPCollectionView
, which will provide a default value for each empty window when used as a side input. You’ll generally only need to use this option if the result of your pipeline’sCombine
is to be used as a side input later in the pipeline.
If your PCollection
uses any non-global windowing function, the Beam Go SDK
behaves the same way as with global windowing. Windows that are empty in the input
PCollection
will likewise be empty in the output collection.
4.2.4.6. Combining values in a keyed PCollection
After creating a keyed PCollection (for example, by using a GroupByKey
transform), a common pattern is to combine the collection of values associated
with each key into a single, merged value. Drawing on the previous example from
GroupByKey
, a key-grouped PCollection
called groupedWords
looks like this:
cat, [1,5,9]
dog, [5,2]
and, [1,2,6]
jump, [3]
tree, [2]
...
In the above PCollection
, each element has a string key (for example, “cat”)
and an iterable of integers for its value (in the first element, containing [1,
5, 9]). If our pipeline’s next processing step combines the values (rather than
considering them individually), you can combine the iterable of integers to
create a single, merged value to be paired with each key. This pattern of a
GroupByKey
followed by merging the collection of values is equivalent to
Beam’s Combine PerKey transform. The combine function you supply to Combine
PerKey must be an associative reduction function or a
subclass of CombineFn
.
// PCollection is grouped by key and the Double values associated with each key are combined into a Double.
PCollection<KV<String, Double>> salesRecords = ...;
PCollection<KV<String, Double>> totalSalesPerPerson =
salesRecords.apply(Combine.<String, Double, Double>perKey(
new Sum.SumDoubleFn()));
// The combined value is of a different type than the original collection of values per key. PCollection has
// keys of type String and values of type Integer, and the combined value is a Double.
PCollection<KV<String, Integer>> playerAccuracy = ...;
PCollection<KV<String, Double>> avgAccuracyPerPlayer =
playerAccuracy.apply(Combine.<String, Integer, Double>perKey(
new MeanInts())));
const pcoll = root.apply(
beam.create([
{ player: "alice", accuracy: 1.0 },
{ player: "bob", accuracy: 0.99 },
{ player: "eve", accuracy: 0.5 },
{ player: "eve", accuracy: 0.25 },
])
);
const result = pcoll.apply(
beam
.groupBy("player")
.combining("accuracy", combiners.mean, "mean")
.combining("accuracy", combiners.max, "max")
);
const expected = [
{ player: "alice", mean: 1.0, max: 1.0 },
{ player: "bob", mean: 0.99, max: 0.99 },
{ player: "eve", mean: 0.375, max: 0.5 },
];
4.2.5. Flatten
Flatten
Flatten
Flatten
Flatten
is a Beam transform for PCollection
objects that store the same data type.
Flatten
merges multiple PCollection
objects into a single logical
PCollection
.
The following example shows how to apply a Flatten
transform to merge multiple
PCollection
objects.
// Flatten takes a PCollectionList of PCollection objects of a given type.
// Returns a single PCollection that contains all of the elements in the PCollection objects in that list.
PCollection<String> pc1 = ...;
PCollection<String> pc2 = ...;
PCollection<String> pc3 = ...;
PCollectionList<String> collections = PCollectionList.of(pc1).and(pc2).and(pc3);
PCollection<String> merged = collections.apply(Flatten.<String>pCollections());
// Flatten taken an array of PCollection objects, wrapped in beam.P(...)
// Returns a single PCollection that contains a union of all of the elements in all input PCollections.
const fib = root.apply(
beam.withName("createFib", beam.create([1, 1, 2, 3, 5, 8]))
);
const pow = root.apply(
beam.withName("createPow", beam.create([1, 2, 4, 8, 16, 32]))
);
const result = beam.P([fib, pow]).apply(beam.flatten());
In Beam YAML explicit flattens are not usually needed as one can list multiple inputs for any transform which will be implicitly flattened.
4.2.5.1. Data encoding in merged collections
By default, the coder for the output PCollection
is the same as the coder for
the first PCollection
in the input PCollectionList
. However, the input
PCollection
objects can each use different coders, as long as they all contain
the same data type in your chosen language.
4.2.5.2. Merging windowed collections
When using Flatten
to merge PCollection
objects that have a windowing
strategy applied, all of the PCollection
objects you want to merge must use a
compatible windowing strategy and window sizing. For example, all the
collections you’re merging must all use (hypothetically) identical 5-minute
fixed windows or 4-minute sliding windows starting every 30 seconds.
If your pipeline attempts to use Flatten
to merge PCollection
objects with
incompatible windows, Beam generates an IllegalStateException
error when your
pipeline is constructed.
4.2.6. Partition
Partition
Partition
Partition
Partition
is a Beam transform for PCollection
objects that store the same data
type. Partition
splits a single PCollection
into a fixed number of smaller
collections.
Often in the Typescript SDK the Split
transform is more natural to use.
Partition
divides the elements of a PCollection
according to a partitioning
function that you provide. The partitioning function contains the logic that
determines how to split up the elements of the input PCollection
into each
resulting partition PCollection
. The number of partitions must be determined
at graph construction time. You can, for example, pass the number of partitions
as a command-line option at runtime (which will then be used to build your
pipeline graph), but you cannot determine the number of partitions in
mid-pipeline (based on data calculated after your pipeline graph is constructed,
for instance).
The following example divides a PCollection
into percentile groups.
// Provide an int value with the desired number of result partitions, and a PartitionFn that represents the
// partitioning function. In this example, we define the PartitionFn in-line. Returns a PCollectionList
// containing each of the resulting partitions as individual PCollection objects.
PCollection<Student> students = ...;
// Split students up into 10 partitions, by percentile:
PCollectionList<Student> studentsByPercentile =
students.apply(Partition.of(10, new PartitionFn<Student>() {
public int partitionFor(Student student, int numPartitions) {
return student.getPercentile() // 0..99
* numPartitions / 100;
}}));
// You can extract each partition from the PCollectionList using the get method, as follows:
PCollection<Student> fortiethPercentile = studentsByPercentile.get(4);
# Provide an int value with the desired number of result partitions, and a partitioning function (partition_fn in this example).
# Returns a tuple of PCollection objects containing each of the resulting partitions as individual PCollection objects.
students = ...
def partition_fn(student, num_partitions):
return int(get_percentile(student) * num_partitions / 100)
by_decile = students | beam.Partition(partition_fn, 10)
# You can extract each partition from the tuple of PCollection objects as follows:
fortieth_percentile = by_decile[4]
func decileFn(student Student) int {
return int(float64(student.Percentile) / float64(10))
}
func init() {
register.Function1x1(decileFn)
}
// Partition returns a slice of PCollections
studentsByPercentile := beam.Partition(s, 10, decileFn, students)
// Each partition can be extracted by indexing into the slice.
fortiethPercentile := studentsByPercentile[4]
Note that in Beam YAML, PCollections
are partitioned via string rather than integer values.
4.3. Requirements for writing user code for Beam transforms
When you build user code for a Beam transform, you should keep in mind the distributed nature of execution. For example, there might be many copies of your function running on a lot of different machines in parallel, and those copies function independently, without communicating or sharing state with any of the other copies. Depending on the Pipeline Runner and processing back-end you choose for your pipeline, each copy of your user code function may be retried or run multiple times. As such, you should be cautious about including things like state dependency in your user code.
In general, your user code must fulfill at least these requirements:
- Your function object must be serializable.
- Your function object must be thread-compatible, and be aware that the Beam SDKs are not thread-safe.
In addition, it’s recommended that you make your function object idempotent. Non-idempotent functions are supported by Beam, but require additional thought to ensure correctness when there are external side effects.
Note: These requirements apply to subclasses of
DoFn
(a function object used with the ParDo transform),CombineFn
(a function object used with the Combine transform), andWindowFn
(a function object used with the Window transform).
Note: These requirements apply to
DoFn
s (a function object used with the ParDo transform),CombineFn
s (a function object used with the Combine transform), andWindowFn
s (a function object used with the Window transform).
4.3.1. Serializability
Any function object you provide to a transform must be fully serializable.
This is because a copy of the function needs to be serialized and transmitted to
a remote worker in your processing cluster.
The base classes for user code, such
as DoFn
, CombineFn
, and WindowFn
, already implement Serializable
;
however, your subclass must not add any non-serializable members.
Funcs are serializable as long as
they are registered with register.FunctionXxY
(for simple functions) or
register.DoFnXxY
(for structural DoFns), and are not closures. Structural
DoFn
s will have all exported fields serialized. Unexported fields are unable to
be serialized, and will be silently ignored.
The Typescript SDK use ts-serialize-closures
to serialize functions (and other objects).
This works out of the box for functions that are not closures, and also works
for closures as long as the function in question (and any closures it references)
are compiled with the
ts-closure-transform
hooks
(e.g. by using ttsc
in place of tsc
).
One can alternatively call
requireForSerialization("importableModuleDefiningFunc", {func})
to register a function directly by name which can be less error-prone.
Note that if, as is often the case in Javascript, func
returns objects that
contain closures, it is not sufficient to register func
alone–its return
value must be registered if used.
Some other serializability factors you should keep in mind are:
- TransientUnexported fields in your function object are not transmitted to worker instances, because they are not automatically serialized.
- Avoid loading a field with a large amount of data before serialization.
- Individual instances of your function object cannot share data.
- Mutating a function object after it gets applied will have no effect.
Note: Take care when declaring your function object inline by using an anonymous inner class instance. In a non-static context, your inner class instance will implicitly contain a pointer to the enclosing class and that class’ state. That enclosing class will also be serialized, and thus the same considerations that apply to the function object itself also apply to this outer class.
Note: There’s no way to detect if a function is a closure. Closures will cause runtime errors and pipeline failures. Avoid using anonymous functions when possible.
4.3.2. Thread-compatibility
Your function object should be thread-compatible. Each instance of your function object is accessed by a single thread at a time on a worker instance, unless you explicitly create your own threads. Note, however, that the Beam SDKs are not thread-safe. If you create your own threads in your user code, you must provide your own synchronization. Note that static members in your function object are not passed to worker instances and that multiple instances of your function may be accessed from different threads.
4.3.3. Idempotence
It’s recommended that you make your function object idempotent–that is, that it can be repeated or retried as often as necessary without causing unintended side effects. Non-idempotent functions are supported, however the Beam model provides no guarantees as to the number of times your user code might be invoked or retried; as such, keeping your function object idempotent keeps your pipeline’s output deterministic, and your transforms’ behavior more predictable and easier to debug.
4.4. Side inputs
In addition to the main input PCollection
, you can provide additional inputs
to a ParDo
transform in the form of side inputs. A side input is an additional
input that your DoFn
can access each time it processes an element in the input
PCollection
. When you specify a side input, you create a view of some other
data that can be read from within the ParDo
transform’s DoFn
while processing
each element.
Side inputs are useful if your ParDo
needs to inject additional data when
processing each element in the input PCollection
, but the additional data
needs to be determined at runtime (and not hard-coded). Such values might be
determined by the input data, or depend on a different branch of your pipeline.
All side input iterables should be registered using a generic register.IterX[...]
function. This optimizes runtime execution of the iterable.
4.4.1. Passing side inputs to ParDo
// Pass side inputs to your ParDo transform by invoking .withSideInputs.
// Inside your DoFn, access the side input by using the method DoFn.ProcessContext.sideInput.
// The input PCollection to ParDo.
PCollection<String> words = ...;
// A PCollection of word lengths that we'll combine into a single value.
PCollection<Integer> wordLengths = ...; // Singleton PCollection
// Create a singleton PCollectionView from wordLengths using Combine.globally and View.asSingleton.
final PCollectionView<Integer> maxWordLengthCutOffView =
wordLengths.apply(Combine.globally(new Max.MaxIntFn()).asSingletonView());
// Apply a ParDo that takes maxWordLengthCutOffView as a side input.
PCollection<String> wordsBelowCutOff =
words.apply(ParDo
.of(new DoFn<String, String>() {
@ProcessElement
public void processElement(@Element String word, OutputReceiver<String> out, ProcessContext c) {
// In our DoFn, access the side input.
int lengthCutOff = c.sideInput(maxWordLengthCutOffView);
if (word.length() <= lengthCutOff) {
out.output(word);
}
}
}).withSideInputs(maxWordLengthCutOffView)
);
# Side inputs are available as extra arguments in the DoFn's process method or Map / FlatMap's callable.
# Optional, positional, and keyword arguments are all supported. Deferred arguments are unwrapped into their
# actual values. For example, using pvalue.AsIteor(pcoll) at pipeline construction time results in an iterable
# of the actual elements of pcoll being passed into each process invocation. In this example, side inputs are
# passed to a FlatMap transform as extra arguments and consumed by filter_using_length.
words = ...
# Callable takes additional arguments.
def filter_using_length(word, lower_bound, upper_bound=float('inf')):
if lower_bound <= len(word) <= upper_bound:
yield word
# Construct a deferred side input.
avg_word_len = (
words
| beam.Map(len)
| beam.CombineGlobally(beam.combiners.MeanCombineFn()))
# Call with explicit side inputs.
small_words = words | 'small' >> beam.FlatMap(filter_using_length, 0, 3)
# A single deferred side input.
larger_than_average = (
words | 'large' >> beam.FlatMap(
filter_using_length, lower_bound=pvalue.AsSingleton(avg_word_len))
)
# Mix and match.
small_but_nontrivial = words | beam.FlatMap(
filter_using_length,
lower_bound=2,
upper_bound=pvalue.AsSingleton(avg_word_len))
# We can also pass side inputs to a ParDo transform, which will get passed to its process method.
# The first two arguments for the process method would be self and element.
class FilterUsingLength(beam.DoFn):
def process(self, element, lower_bound, upper_bound=float('inf')):
if lower_bound <= len(element) <= upper_bound:
yield element
small_words = words | beam.ParDo(FilterUsingLength(), 0, 3)
...
// Side inputs are provided using `beam.SideInput` in the DoFn's ProcessElement method.
// Side inputs can be arbitrary PCollections, which can then be iterated over per element
// in a DoFn.
// Side input parameters appear after main input elements, and before any output emitters.
words = ...
// avgWordLength is a PCollection containing a single element, a singleton.
avgWordLength := stats.Mean(s, wordLengths)
// Side inputs are added as with the beam.SideInput option to beam.ParDo.
wordsAboveCutOff := beam.ParDo(s, filterWordsAbove, words, beam.SideInput{Input: avgWordLength})
wordsBelowCutOff := beam.ParDo(s, filterWordsBelow, words, beam.SideInput{Input: avgWordLength})
// filterWordsAbove is a DoFn that takes in a word,
// and a singleton side input iterator as of a length cut off
// and only emits words that are beneath that cut off.
//
// If the iterator has no elements, an error is returned, aborting processing.
func filterWordsAbove(word string, lengthCutOffIter func(*float64) bool, emitAboveCutoff func(string)) error {
var cutOff float64
ok := lengthCutOffIter(&cutOff)
if !ok {
return fmt.Errorf("no length cutoff provided")
}
if float64(len(word)) > cutOff {
emitAboveCutoff(word)
}
return nil
}
// filterWordsBelow is a DoFn that takes in a word,
// and a singleton side input of a length cut off
// and only emits words that are beneath that cut off.
//
// If the side input isn't a singleton, a runtime panic will occur.
func filterWordsBelow(word string, lengthCutOff float64, emitBelowCutoff func(string)) {
if float64(len(word)) <= lengthCutOff {
emitBelowCutoff(word)
}
}
func init() {
register.Function3x1(filterWordsAbove)
register.Function3x0(filterWordsBelow)
// 1 input of type string => Emitter1[string]
register.Emitter1[string]()
// 1 input of type float64 => Iter1[float64]
register.Iter1[float64]()
}
// The Go SDK doesn't support custom ViewFns.
// See https://github.com/apache/beam/issues/18602 for details
// on how to contribute them!
// Side inputs are provided by passing an extra context object to
// `map`, `flatMap`, or `parDo` transforms. This object will get passed as an
// extra argument to the provided function (or `process` method of the `DoFn`).
// `SideInputParam` properties (generally created with `pardo.xxxSideInput(...)`)
// have a `lookup` method that can be invoked from within the process method.
// Let words be a PCollection of strings.
const words : PCollection<string> = ...
// meanLengthPColl will contain a single number whose value is the
// average length of the words
const meanLengthPColl: PCollection<number> = words
.apply(
beam
.groupGlobally<string>()
.combining((word) => word.length, combiners.mean, "mean")
)
.map(({ mean }) => mean);
// Now we use this as a side input to yield only words that are
// smaller than average.
const smallWords = words.flatMap(
// This is the function, taking context as a second argument.
function* keepSmall(word, context) {
if (word.length < context.meanLength.lookup()) {
yield word;
}
},
// This is the context that will be passed as a second argument.
{ meanLength: pardo.singletonSideInput(meanLengthPColl) }
);
4.4.2. Side inputs and windowing
A windowed PCollection
may be infinite and thus cannot be compressed into a
single value (or single collection class). When you create a PCollectionView
of a windowed PCollection
, the PCollectionView
represents a single entity
per window (one singleton per window, one list per window, etc.).
Beam uses the window(s) for the main input element to look up the appropriate window for the side input element. Beam projects the main input element’s window into the side input’s window set, and then uses the side input from the resulting window. If the main input and side inputs have identical windows, the projection provides the exact corresponding window. However, if the inputs have different windows, Beam uses the projection to choose the most appropriate side input window.
For example, if the main input is windowed using fixed-time windows of one minute, and the side input is windowed using fixed-time windows of one hour, Beam projects the main input window against the side input window set and selects the side input value from the appropriate hour-long side input window.
If the main input element exists in more than one window, then processElement
gets called multiple times, once for each window. Each call to processElement
projects the “current” window for the main input element, and thus might provide
a different view of the side input each time.
If the side input has multiple trigger firings, Beam uses the value from the latest trigger firing. This is particularly useful if you use a side input with a single global window and specify a trigger.
4.5. Additional outputs
While ParDo
always produces a main output PCollection
(as the return value
from apply
), you can also have your ParDo
produce any number of additional
output PCollection
s. If you choose to have multiple outputs, your ParDo
returns all of the output PCollection
s (including the main output) bundled
together.
While beam.ParDo
always produces an output PCollection
, your DoFn
can produce any
number of additional output PCollections
s, or even none at all.
If you choose to have multiple outputs, your DoFn
needs to be called with the ParDo
function that matches the number of outputs. beam.ParDo2
for two output PCollection
s,
beam.ParDo3
for three and so on until beam.ParDo7
. If you need more, you can
use beam.ParDoN
which will return a []beam.PCollection
.
While ParDo
always produces a main output PCollection
(as the return value
from apply
). If you want to have multiple outputs, emit an object with distinct
properties in your ParDo
operation and follow this operation with a Split
to break it into multiple PCollection
s.
In Beam YAML, one obtains multiple outputs by emitting all outputs to a single
PCollection
, possibly with an extra field, and then using Partition
to
split this single PCollection
into multiple distinct PCollection
outputs.
4.5.1. Tags for multiple outputs
The Split
PTransform will take a PCollection of elements of the form
{tagA?: A, tagB?: B, ...}
and return a object
{tagA: PCollection<A>, tagB: PCollection<B>, ...}
.
The set of expected tags is passed to the operation; how multiple or
unknown tags are handled can be specified by passing a non-default
SplitOptions
instance.
The Go SDK doesn’t use output tags, and instead uses positional ordering for multiple output PCollections.
// To emit elements to multiple output PCollections, create a TupleTag object to identify each collection
// that your ParDo produces. For example, if your ParDo produces three output PCollections (the main output
// and two additional outputs), you must create three TupleTags. The following example code shows how to
// create TupleTags for a ParDo with three output PCollections.
// Input PCollection to our ParDo.
PCollection<String> words = ...;
// The ParDo will filter words whose length is below a cutoff and add them to
// the main output PCollection<String>.
// If a word is above the cutoff, the ParDo will add the word length to an
// output PCollection<Integer>.
// If a word starts with the string "MARKER", the ParDo will add that word to an
// output PCollection<String>.
final int wordLengthCutOff = 10;
// Create three TupleTags, one for each output PCollection.
// Output that contains words below the length cutoff.
final TupleTag<String> wordsBelowCutOffTag =
new TupleTag<String>(){};
// Output that contains word lengths.
final TupleTag<Integer> wordLengthsAboveCutOffTag =
new TupleTag<Integer>(){};
// Output that contains "MARKER" words.
final TupleTag<String> markedWordsTag =
new TupleTag<String>(){};
// Passing Output Tags to ParDo:
// After you specify the TupleTags for each of your ParDo outputs, pass the tags to your ParDo by invoking
// .withOutputTags. You pass the tag for the main output first, and then the tags for any additional outputs
// in a TupleTagList. Building on our previous example, we pass the three TupleTags for our three output
// PCollections to our ParDo. Note that all of the outputs (including the main output PCollection) are
// bundled into the returned PCollectionTuple.
PCollectionTuple results =
words.apply(ParDo
.of(new DoFn<String, String>() {
// DoFn continues here.
...
})
// Specify the tag for the main output.
.withOutputTags(wordsBelowCutOffTag,
// Specify the tags for the two additional outputs as a TupleTagList.
TupleTagList.of(wordLengthsAboveCutOffTag)
.and(markedWordsTag)));
# To emit elements to multiple output PCollections, invoke with_outputs() on the ParDo, and specify the
# expected tags for the outputs. with_outputs() returns a DoOutputsTuple object. Tags specified in
# with_outputs are attributes on the returned DoOutputsTuple object. The tags give access to the
# corresponding output PCollections.
results = (
words
| beam.ParDo(ProcessWords(), cutoff_length=2, marker='x').with_outputs(
'above_cutoff_lengths',
'marked strings',
main='below_cutoff_strings'))
below = results.below_cutoff_strings
above = results.above_cutoff_lengths
marked = results['marked strings'] # indexing works as well
# The result is also iterable, ordered in the same order that the tags were passed to with_outputs(),
# the main tag (if specified) first.
below, above, marked = (words
| beam.ParDo(
ProcessWords(), cutoff_length=2, marker='x')
.with_outputs('above_cutoff_lengths',
'marked strings',
main='below_cutoff_strings'))
// beam.ParDo3 returns PCollections in the same order as
// the emit function parameters in processWords.
below, above, marked := beam.ParDo3(s, processWords, words)
// processWordsMixed uses both a standard return and an emitter function.
// The standard return produces the first PCollection from beam.ParDo2,
// and the emitter produces the second PCollection.
length, mixedMarked := beam.ParDo2(s, processWordsMixed, words)
4.5.2. Emitting to multiple outputs in your DoFn
Call emitter functions as needed to produce 0 or more elements for its matching
PCollection
. The same value can be emitted with multiple emitters.
As normal, do not mutate values after emitting them from any emitter.
All emitters should be registered using a generic register.EmitterX[...]
function. This optimizes runtime execution of the emitter.
DoFns can also return a single element via the standard return. The standard return is always the first PCollection returned from beam.ParDo. Other emitters output to their own PCollections in their defined parameter order.
MapToFields
is always one-to-one. To perform a one-to-many mapping one can
first map a field to an iterable type and then follow this transform with an
Explode
transform that will emit multiple values, one per value of the
exploded field.
// Inside your ParDo's DoFn, you can emit an element to a specific output PCollection by providing a
// MultiOutputReceiver to your process method, and passing in the appropriate TupleTag to obtain an OutputReceiver.
// After your ParDo, extract the resulting output PCollections from the returned PCollectionTuple.
// Based on the previous example, this shows the DoFn emitting to the main output and two additional outputs.
.of(new DoFn<String, String>() {
public void processElement(@Element String word, MultiOutputReceiver out) {
if (word.length() <= wordLengthCutOff) {
// Emit short word to the main output.
// In this example, it is the output with tag wordsBelowCutOffTag.
out.get(wordsBelowCutOffTag).output(word);
} else {
// Emit long word length to the output with tag wordLengthsAboveCutOffTag.
out.get(wordLengthsAboveCutOffTag).output(word.length());
}
if (word.startsWith("MARKER")) {
// Emit word to the output with tag markedWordsTag.
out.get(markedWordsTag).output(word);
}
}}));
# Inside your ParDo's DoFn, you can emit an element to a specific output by wrapping the value and the output tag (str).
# using the pvalue.OutputValue wrapper class.
# Based on the previous example, this shows the DoFn emitting to the main output and two additional outputs.
class ProcessWords(beam.DoFn):
def process(self, element, cutoff_length, marker):
if len(element) <= cutoff_length:
# Emit this short word to the main output.
yield element
else:
# Emit this word's long length to the 'above_cutoff_lengths' output.
yield pvalue.TaggedOutput('above_cutoff_lengths', len(element))
if element.startswith(marker):
# Emit this word to a different output with the 'marked strings' tag.
yield pvalue.TaggedOutput('marked strings', element)
# Producing multiple outputs is also available in Map and FlatMap.
# Here is an example that uses FlatMap and shows that the tags do not need to be specified ahead of time.
def even_odd(x):
yield pvalue.TaggedOutput('odd' if x % 2 else 'even', x)
if x % 10 == 0:
yield x
results = numbers | beam.FlatMap(even_odd).with_outputs()
evens = results.even
odds = results.odd
tens = results[None] # the undeclared main output
// processWords is a DoFn that has 3 output PCollections. The emitter functions
// are matched in positional order to the PCollections returned by beam.ParDo3.
func processWords(word string, emitBelowCutoff, emitAboveCutoff, emitMarked func(string)) {
const cutOff = 5
if len(word) < cutOff {
emitBelowCutoff(word)
} else {
emitAboveCutoff(word)
}
if isMarkedWord(word) {
emitMarked(word)
}
}
// processWordsMixed demonstrates mixing an emitter, with a standard return.
// If a standard return is used, it will always be the first returned PCollection,
// followed in positional order by the emitter functions.
func processWordsMixed(word string, emitMarked func(string)) int {
if isMarkedWord(word) {
emitMarked(word)
}
return len(word)
}
func init() {
register.Function4x0(processWords)
register.Function2x1(processWordsMixed)
// 1 input of type string => Emitter1[string]
register.Emitter1[string]()
}
4.5.3. Accessing additional parameters in your DoFn
In addition to the element and the OutputReceiver
, Beam will populate other parameters to your DoFn’s @ProcessElement
method.
Any combination of these parameters can be added to your process method in any order.
In addition to the element, Beam will populate other parameters to your DoFn’s process
method.
Any combination of these parameters can be added to your process method in any order.
In addition to the element, Beam will populate other parameters to your DoFn’s process
method.
These are available by placing accessors in the context argument, just as for side inputs.
In addition to the element, Beam will populate other parameters to your DoFn’s ProcessElement
method.
Any combination of these parameters can be added to your process method in a standard order.
context.Context:
To support consolidated logging and user defined metrics, a context.Context
parameter can be requested.
Per Go conventions, if present it’s required to be the first parameter of the DoFn
method.
Timestamp:
To access the timestamp of an input element, add a parameter annotated with @Timestamp
of type Instant
. For example:
Timestamp:
To access the timestamp of an input element, add a keyword parameter default to DoFn.TimestampParam
. For example:
Timestamp:
To access the timestamp of an input element, add a beam.EventTime
parameter before the element. For example:
Timestamp:
To access the window an input element falls into, add a pardo.windowParam()
to the context argument.
Window:
To access the window an input element falls into, add a parameter of the type of the window used for the input PCollection
.
If the parameter is a window type (a subclass of BoundedWindow
) that does not match the input PCollection
, then an error
will be raised. If an element falls in multiple windows (for example, this will happen when using SlidingWindows
), then the
@ProcessElement
method will be invoked multiple time for the element, once for each window. For example, when fixed windows
are being used, the window is of type IntervalWindow
.
Window:
To access the window an input element falls into, add a keyword parameter default to DoFn.WindowParam
.
If an element falls in multiple windows (for example, this will happen when using SlidingWindows
), then the
process
method will be invoked multiple time for the element, once for each window.
Window:
To access the window an input element falls into, add a beam.Window
parameter before the element.
If an element falls in multiple windows (for example, this will happen when using SlidingWindows),
then the ProcessElement
method will be invoked multiple time for the element, once for each window.
Since beam.Window
is an interface it’s possible to type assert to the concrete implementation of the window.
For example, when fixed windows are being used, the window is of type window.IntervalWindow
.
Window:
To access the window an input element falls into, add a pardo.windowParam()
to the context argument.
If an element falls in multiple windows (for example, this will happen when using SlidingWindows
), then the
function will be invoked multiple time for the element, once for each window.
PaneInfo:
When triggers are used, Beam provides a PaneInfo
object that contains information about the current firing. Using PaneInfo
you can determine whether this is an early or a late firing, and how many times this window has already fired for this key.
PaneInfo:
When triggers are used, Beam provides a DoFn.PaneInfoParam
object that contains information about the current firing. Using DoFn.PaneInfoParam
you can determine whether this is an early or a late firing, and how many times this window has already fired for this key.
This feature implementation in Python SDK is not fully completed; see more at Issue 17821.
PaneInfo:
When triggers are used, Beam provides beam.PaneInfo
object that contains information about the current firing. Using beam.PaneInfo
you can determine whether this is an early or a late firing, and how many times this window has already fired for this key.
Window:
To access the window an input element falls into, add a pardo.paneInfoParam()
to the context argument.
Using beam.PaneInfo
you can determine whether this is an early or a late firing,
and how many times this window has already fired for this key.
func extractWordsFn(pn beam.PaneInfo, line string, emitWords func(string)) {
if pn.Timing == typex.PaneEarly || pn.Timing == typex.PaneOnTime {
// ... perform operation ...
}
if pn.Timing == typex.PaneLate {
// ... perform operation ...
}
if pn.IsFirst {
// ... perform operation ...
}
if pn.IsLast {
// ... perform operation ...
}
words := strings.Split(line, " ")
for _, w := range words {
emitWords(w)
}
}
PipelineOptions:
The PipelineOptions
for the current pipeline can always be accessed in a process method by adding it
as a parameter:
@OnTimer
methods can also access many of these parameters. Timestamp, Window, key, PipelineOptions
, OutputReceiver
, and
MultiOutputReceiver
parameters can all be accessed in an @OnTimer
method. In addition, an @OnTimer
method can take
a parameter of type TimeDomain
which tells whether the timer is based on event time or processing time.
Timers are explained in more detail in the
Timely (and Stateful) Processing with Apache Beam blog post.
Timer and State: In addition to aforementioned parameters, user defined Timer and State parameters can be used in a stateful DoFn. Timers and States are explained in more detail in the Timely (and Stateful) Processing with Apache Beam blog post.
Timer and State: User defined State and Timer parameters can be used in a stateful DoFn. Timers and States are explained in more detail in the Timely (and Stateful) Processing with Apache Beam blog post.
Timer and State: This feature isn’t yet implemented in the Typescript SDK, but we welcome contributions. In the meantime, Typescript pipelines wishing to use state and timers can do so using cross-language transforms.
class StatefulDoFn(beam.DoFn):
"""An example stateful DoFn with state and timer"""
BUFFER_STATE_1 = BagStateSpec('buffer1', beam.BytesCoder())
BUFFER_STATE_2 = BagStateSpec('buffer2', beam.VarIntCoder())
WATERMARK_TIMER = TimerSpec('watermark_timer', TimeDomain.WATERMARK)
def process(self,
element,
timestamp=beam.DoFn.TimestampParam,
window=beam.DoFn.WindowParam,
buffer_1=beam.DoFn.StateParam(BUFFER_STATE_1),
buffer_2=beam.DoFn.StateParam(BUFFER_STATE_2),
watermark_timer=beam.DoFn.TimerParam(WATERMARK_TIMER)):
# Do your processing here
key, value = element
# Read all the data from buffer1
all_values_in_buffer_1 = [x for x in buffer_1.read()]
if StatefulDoFn._is_clear_buffer_1_required(all_values_in_buffer_1):
# clear the buffer data if required conditions are met.
buffer_1.clear()
# add the value to buffer 2
buffer_2.add(value)
if StatefulDoFn._all_condition_met():
# Clear the timer if certain condition met and you don't want to trigger
# the callback method.
watermark_timer.clear()
yield element
@on_timer(WATERMARK_TIMER)
def on_expiry_1(self,
timestamp=beam.DoFn.TimestampParam,
window=beam.DoFn.WindowParam,
key=beam.DoFn.KeyParam,
buffer_1=beam.DoFn.StateParam(BUFFER_STATE_1),
buffer_2=beam.DoFn.StateParam(BUFFER_STATE_2)):
# Window and key parameters are really useful especially for debugging issues.
yield 'expired1'
@staticmethod
def _all_condition_met():
# some logic
return True
@staticmethod
def _is_clear_buffer_1_required(buffer_1_data):
# Some business logic
return True
// stateAndTimersFn is an example stateful DoFn with state and a timer.
type stateAndTimersFn struct {
Buffer1 state.Bag[string]
Buffer2 state.Bag[int64]
Watermark timers.EventTime
}
func (s *stateAndTimersFn) ProcessElement(sp state.Provider, tp timers.Provider, w beam.Window, key string, value int64, emit func(string, int64)) error {
// ... handle processing elements here, set a callback timer...
// Read all the data from Buffer1 in this window.
vals, ok, err := s.Buffer1.Read(sp)
if err != nil {
return err
}
if ok && s.shouldClearBuffer(vals) {
// clear the buffer data if required conditions are met.
s.Buffer1.Clear(sp)
}
// Add the value to Buffer2.
s.Buffer2.Add(sp, value)
if s.allConditionsMet() {
// Clear the timer if certain condition met and you don't want to trigger
// the callback method.
s.Watermark.Clear(tp)
}
emit(key, value)
return nil
}
func (s *stateAndTimersFn) OnTimer(sp state.Provider, tp timers.Provider, w beam.Window, key string, timer timers.Context, emit func(string, int64)) error {
// Window and key parameters are really useful especially for debugging issues.
switch timer.Family {
case s.Watermark.Family:
// timer expired, emit a different signal
emit(key, -1)
}
return nil
}
func (s *stateAndTimersFn) shouldClearBuffer([]string) bool {
// some business logic
return false
}
func (s *stateAndTimersFn) allConditionsMet() bool {
// other business logic
return true
}
4.6. Composite transforms
Transforms can have a nested structure, where a complex transform performs
multiple simpler transforms (such as more than one ParDo
, Combine
,
GroupByKey
, or even other composite transforms). These transforms are called
composite transforms. Nesting multiple transforms inside a single composite
transform can make your code more modular and easier to understand.
The Beam SDK comes packed with many useful composite transforms. See the API reference pages for a list of transforms:
- Pre-written Beam transforms for Java
- Pre-written Beam transforms for Python
- Pre-written Beam transforms for Go
4.6.1. An example composite transform
The CountWords
transform in the WordCount example program
is an example of a composite transform. CountWords
is a PTransform
subclass that consists
of multiple nested transforms.
In its expand
method, the
The CountWords
transform applies the following
transform operations:
- It applies a
ParDo
on the inputPCollection
of text lines, producing an outputPCollection
of individual words. - It applies the Beam SDK library transform
Count
on thePCollection
of words, producing aPCollection
of key/value pairs. Each key represents a word in the text, and each value represents the number of times that word appeared in the original data.
public static class CountWords extends PTransform<PCollection<String>,
PCollection<KV<String, Long>>> {
@Override
public PCollection<KV<String, Long>> expand(PCollection<String> lines) {
// Convert lines of text into individual words.
PCollection<String> words = lines.apply(
ParDo.of(new ExtractWordsFn()));
// Count the number of times each word occurs.
PCollection<KV<String, Long>> wordCounts =
words.apply(Count.<String>perElement());
return wordCounts;
}
}
# The CountWords Composite Transform inside the WordCount pipeline.
@beam.ptransform_fn
def CountWords(pcoll):
return (
pcoll
# Convert lines of text into individual words.
| 'ExtractWords' >> beam.ParDo(ExtractWordsFn())
# Count the number of times each word occurs.
| beam.combiners.Count.PerElement()
# Format each word and count into a printable string.
| 'FormatCounts' >> beam.ParDo(FormatCountsFn()))
// CountWords is a function that builds a composite PTransform
// to count the number of times each word appears.
func CountWords(s beam.Scope, lines beam.PCollection) beam.PCollection {
// A subscope is required for a function to become a composite transform.
// We assign it to the original scope variable s to shadow the original
// for the rest of the CountWords function.
s = s.Scope("CountWords")
// Since the same subscope is used for the following transforms,
// they are in the same composite PTransform.
// Convert lines of text into individual words.
words := beam.ParDo(s, extractWordsFn, lines)
// Count the number of times each word occurs.
wordCounts := stats.Count(s, words)
// Return any PCollections that should be available after
// the composite transform.
return wordCounts
}
Note: Because
Count
is itself a composite transform,CountWords
is also a nested composite transform.
4.6.2. Creating a composite transform
A PTransform in the Typescript SDK is simply a function that accepts and
returns PValue
s such as PCollection
s.
To create your own composite transform, create a subclass of the PTransform
class and override the expand
method to specify the actual processing logic.
You can then use this transform just as you would a built-in transform from the
Beam SDK.
For the PTransform
class type parameters, you pass the PCollection
types
that your transform takes as input, and produces as output. To take multiple
PCollection
s as input, or produce multiple PCollection
s as output, use one
of the multi-collection types for the relevant type parameter.
To create your own composite PTransform
call the Scope
method on the current
pipeline scope variable. Transforms passed this new sub-Scope
will be a part of
the same composite PTransform
.
To be able to re-use your Composite, build it inside a normal Go function or method.
This function is passed a scope and input PCollections, and returns any
output PCollections it produces. Note: Such functions cannot be passed directly to
ParDo
functions.
The following code sample shows how to declare a PTransform
that accepts a
PCollection
of String
s for input, and outputs a PCollection
of Integer
s:
// CountWords is a function that builds a composite PTransform
// to count the number of times each word appears.
func CountWords(s beam.Scope, lines beam.PCollection) beam.PCollection {
// A subscope is required for a function to become a composite transform.
// We assign it to the original scope variable s to shadow the original
// for the rest of the CountWords function.
s = s.Scope("CountWords")
// Since the same subscope is used for the following transforms,
// they are in the same composite PTransform.
// Convert lines of text into individual words.
words := beam.ParDo(s, extractWordsFn, lines)
// Count the number of times each word occurs.
wordCounts := stats.Count(s, words)
// Return any PCollections that should be available after
// the composite transform.
return wordCounts
}
Within your PTransform
subclass, you’ll need to override the expand
method.
The expand
method is where you add the processing logic for the PTransform
.
Your override of expand
must accept the appropriate type of input
PCollection
as a parameter, and specify the output PCollection
as the return
value.
The following code sample shows how to override expand
for the
ComputeWordLengths
class declared in the previous example:
The following code sample shows how to call the CountWords
composite PTransform,
adding it to your pipeline:
As long as you override the expand
method in your PTransform
subclass to
accept the appropriate input PCollection
(s) and return the corresponding
output PCollection
(s), you can include as many transforms as you want. These
transforms can include core transforms, composite transforms, or the transforms
included in the Beam SDK libraries.
Your composite PTransform
s can include as many transforms as you want. These
transforms can include core transforms, other composite transforms, or the transforms
included in the Beam SDK libraries. They can also consume and return as many
PCollection
s as are necessary.
Your composite transform’s parameters and return value must match the initial input type and final return type for the entire transform, even if the transform’s intermediate data changes type multiple times.
Note: The expand
method of a PTransform
is not meant to be invoked
directly by the user of a transform. Instead, you should call the apply
method
on the PCollection
itself, with the transform as an argument. This allows
transforms to be nested within the structure of your pipeline.
4.6.3. PTransform Style Guide
The PTransform Style Guide contains additional information not included here, such as style guidelines, logging and testing guidance, and language-specific considerations. The guide is a useful starting point when you want to write new composite PTransforms.
5. Pipeline I/O
When you create a pipeline, you often need to read data from some external source, such as a file or a database. Likewise, you may want your pipeline to output its result data to an external storage system. Beam provides read and write transforms for a number of common data storage types. If you want your pipeline to read from or write to a data storage format that isn’t supported by the built-in transforms, you can implement your own read and write transforms.
5.1. Reading input data
Read transforms read data from an external source and return a PCollection
representation of the data for use by your pipeline. You can use a read
transform at any point while constructing your pipeline to create a new
PCollection
, though it will be most common at the start of your pipeline.
5.2. Writing output data
Write transforms write the data in a PCollection
to an external data source.
You will most often use write transforms at the end of your pipeline to output
your pipeline’s final results. However, you can use a write transform to output
a PCollection
’s data at any point in your pipeline.
5.3. File-based input and output data
5.3.1. Reading from multiple locations
Many read transforms support reading from multiple input files matching a glob
operator you provide. Note that glob operators are filesystem-specific and obey
filesystem-specific consistency models. The following TextIO example uses a glob
operator (*
) to read all matching input files that have prefix “input-” and the
suffix “.csv” in the given location:
To read data from disparate sources into a single PCollection
, read each one
independently and then use the Flatten transform to create a single
PCollection
.
5.3.2. Writing to multiple output files
For file-based output data, write transforms write to multiple output files by default. When you pass an output file name to a write transform, the file name is used as the prefix for all output files that the write transform produces. You can append a suffix to each output file by specifying a suffix.
The following write transform example writes multiple output files to a location. Each file has the prefix “numbers”, a numeric tag, and the suffix “.csv”.
5.4. Beam-provided I/O transforms
See the Beam-provided I/O Transforms page for a list of the currently available I/O transforms.
6. Schemas
Often, the types of the records being processed have an obvious structure. Common Beam sources produce JSON, Avro, Protocol Buffer, or database row objects; all of these types have well defined structures, structures that can often be determined by examining the type. Even within a SDK pipeline, Simple Java POJOs (or equivalent structures in other languages) are often used as intermediate types, and these also have a clear structure that can be inferred by inspecting the class. By understanding the structure of a pipeline’s records, we can provide much more concise APIs for data processing.
6.1. What is a schema?
Most structured records share some common characteristics:
- They can be subdivided into separate named fields. Fields usually have string names, but sometimes - as in the case of indexed tuples - have numerical indices instead.
- There is a confined list of primitive types that a field can have. These often match primitive types in most programming languages: int, long, string, etc.
- Often a field type can be marked as optional (sometimes referred to as nullable) or required.
Often records have a nested structure. A nested structure occurs when a field itself has subfields so the type of the field itself has a schema. Fields that are array or map types is also a common feature of these structured records.
For example, consider the following schema, representing actions in a fictitious e-commerce company:
Purchase
Field Name | Field Type |
---|---|
userId | STRING |
itemId | INT64 |
shippingAddress | ROW(ShippingAddress) |
cost | INT64 |
transactions | ARRAY[ROW(Transaction)] |
ShippingAddress
Field Name | Field Type |
---|---|
streetAddress | STRING |
city | STRING |
state | nullable STRING |
country | STRING |
postCode | STRING |
Transaction
Field Name | Field Type |
---|---|
bank | STRING |
purchaseAmount | DOUBLE |
Purchase event records are represented by the above purchase schema. Each purchase event contains a shipping address, which is a nested row containing its own schema. Each purchase also contains an array of credit-card transactions (a list, because a purchase might be split across multiple credit cards); each item in the transaction list is a row with its own schema.
This provides an abstract description of the types involved, one that is abstracted away from any specific programming language.
Schemas provide us a type-system for Beam records that is independent of any specific programming-language type. There might be multiple Java classes that all have the same schema (for example a Protocol-Buffer class or a POJO class), and Beam will allow us to seamlessly convert between these types. Schemas also provide a simple way to reason about types across different programming-language APIs.
A PCollection
with a schema does not need to have a Coder
specified, as Beam knows how to encode and decode
Schema rows; Beam uses a special coder to encode schema types.
6.2. Schemas for programming language types
While schemas themselves are language independent, they are designed to embed naturally into the programming languages of the Beam SDK being used. This allows Beam users to continue using native types while reaping the advantage of having Beam understand their element schemas.
In Java you could use the following set of classes to represent the purchase schema. Beam will automatically infer the correct schema based on the members of the class.
In Python you can use the following set of classes to represent the purchase schema. Beam will automatically infer the correct schema based on the members of the class.
In Go, schema encoding is used by default for struct types, with Exported fields becoming part of the schema. Beam will automatically infer the schema based on the fields and field tags of the struct, and their order.
In Typescript, JSON objects are used to represent schema’d data. Unfortunately type information in Typescript is not propagated to the runtime layer, so it needs to be manually specified in some places (e.g. when using cross-language pipelines).
In Beam YAML, all transforms produce and accept schema’d data which is used to validate the pipeline.
In some cases, Beam is unable to figure out the output type of a mapping function. In this case, you can specify it manually using JSON schema syntax.
@DefaultSchema(JavaBeanSchema.class)
public class Purchase {
public String getUserId(); // Returns the id of the user who made the purchase.
public long getItemId(); // Returns the identifier of the item that was purchased.
public ShippingAddress getShippingAddress(); // Returns the shipping address, a nested type.
public long getCostCents(); // Returns the cost of the item.
public List<Transaction> getTransactions(); // Returns the transactions that paid for this purchase (returns a list, since the purchase might be spread out over multiple credit cards).
@SchemaCreate
public Purchase(String userId, long itemId, ShippingAddress shippingAddress, long costCents,
List<Transaction> transactions) {
...
}
}
@DefaultSchema(JavaBeanSchema.class)
public class ShippingAddress {
public String getStreetAddress();
public String getCity();
@Nullable public String getState();
public String getCountry();
public String getPostCode();
@SchemaCreate
public ShippingAddress(String streetAddress, String city, @Nullable String state, String country,
String postCode) {
...
}
}
@DefaultSchema(JavaBeanSchema.class)
public class Transaction {
public String getBank();
public double getPurchaseAmount();
@SchemaCreate
public Transaction(String bank, double purchaseAmount) {
...
}
}
import typing
class Purchase(typing.NamedTuple):
user_id: str # The id of the user who made the purchase.
item_id: int # The identifier of the item that was purchased.
shipping_address: ShippingAddress # The shipping address, a nested type.
cost_cents: int # The cost of the item
transactions: typing.Sequence[Transaction] # The transactions that paid for this purchase (a list, since the purchase might be spread out over multiple credit cards).
class ShippingAddress(typing.NamedTuple):
street_address: str
city: str
state: typing.Optional[str]
country: str
postal_code: str
class Transaction(typing.NamedTuple):
bank: str
purchase_amount: float
type Purchase struct {
// ID of the user who made the purchase.
UserID string `beam:"userId"`
// Identifier of the item that was purchased.
ItemID int64 `beam:"itemId"`
// The shipping address, a nested type.
ShippingAddress ShippingAddress `beam:"shippingAddress"`
// The cost of the item in cents.
Cost int64 `beam:"cost"`
// The transactions that paid for this purchase.
// A slice since the purchase might be spread out over multiple
// credit cards.
Transactions []Transaction `beam:"transactions"`
}
type ShippingAddress struct {
StreetAddress string `beam:"streetAddress"`
City string `beam:"city"`
State *string `beam:"state"`
Country string `beam:"country"`
PostCode string `beam:"postCode"`
}
type Transaction struct {
Bank string `beam:"bank"`
PurchaseAmount float64 `beam:"purchaseAmount"`
}
Using JavaBean classes as above is one way to map a schema to Java classes. However multiple Java classes might have
the same schema, in which case the different Java types can often be used interchangeably. Beam will add implicit
conversions between types that have matching schemas. For example, the above
Transaction
class has the same schema as the following class:
So if we had two PCollection
s as follows
Then these two PCollection
s would have the same schema, even though their Java types would be different. This means
for example the following two code snippets are valid:
and
Even though the in both cases the @Element
parameter differs from the PCollection
’s Java type, since the
schemas are the same Beam will automatically make the conversion. The built-in Convert
transform can also be used
to translate between Java types of equivalent schemas, as detailed below.
6.3. Schema definition
The schema for a PCollection
defines elements of that PCollection
as an ordered list of named fields. Each field
has a name, a type, and possibly a set of user options. The type of a field can be primitive or composite. The following
are the primitive types currently supported by Beam:
Type | Description |
---|---|
BYTE | An 8-bit signed value |
INT16 | A 16-bit signed value |
INT32 | A 32-bit signed value |
INT64 | A 64-bit signed value |
DECIMAL | An arbitrary-precision decimal type |
FLOAT | A 32-bit IEEE 754 floating point number |
DOUBLE | A 64-bit IEEE 754 floating point number |
STRING | A string |
DATETIME | A timestamp represented as milliseconds since the epoch |
BOOLEAN | A boolean value |
BYTES | A raw byte array |
A field can also reference a nested schema. In this case, the field will have type ROW, and the nested schema will be an attribute of this field type.
Three collection types are supported as field types: ARRAY, ITERABLE and MAP:
- ARRAY This represents a repeated value type, where the repeated elements can have any supported type. Arrays of nested rows are supported, as are arrays of arrays.
- ITERABLE This is very similar to the array type, it represents a repeated value, but one in which the full list of
items is not known until iterated over. This is intended for the case where an iterable might be larger than the
available memory, and backed by external storage (for example, this can happen with the iterable returned by a
GroupByKey
). The repeated elements can have any supported type. - MAP This represents an associative map from keys to values. All schema types are supported for both keys and values. Values that contain map types cannot be used as keys in any grouping operation.
6.4. Logical types
Users can extend the schema type system to add custom logical types that can be used as a field. A logical type is identified by a unique identifier and an argument. A logical type also specifies an underlying schema type to be used for storage, along with conversions to and from that type. As an example, a logical union can always be represented as a row with nullable fields, where the user ensures that only one of those fields is ever set at a time. However this can be tedious and complex to manage. The OneOf logical type provides a value class that makes it easier to manage the type as a union, while still using a row with nullable fields as its underlying storage. Each logical type also has a unique identifier, so they can be interpreted by other languages as well. More examples of logical types are listed below.
6.4.1. Defining a logical type
To define a logical type you must specify a Schema type to be used to represent the underlying type as well as a unique identifier for that type. A logical type imposes additional semantics on top a schema type. For example, a logical type to represent nanosecond timestamps is represented as a schema containing an INT64 and an INT32 field. This schema alone does not say anything about how to interpret this type, however the logical type tells you that this represents a nanosecond timestamp, with the INT64 field representing seconds and the INT32 field representing nanoseconds.
Logical types are also specified by an argument, which allows creating a class of related types. For example, a limited-precision decimal type would have an integer argument indicating how many digits of precision are represented. The argument is represented by a schema type, so can itself be a complex type.
In Java, a logical type is specified as a subclass of the LogicalType
class. A custom Java class can be specified to represent the logical type and conversion functions must be supplied to convert back and forth between this Java class and the underlying Schema type representation. For example, the logical type representing nanosecond timestamp might be implemented as follows
In Go, a logical type is specified with a custom implementation of the beam.SchemaProvider
interface.
For example, the logical type provider representing nanosecond timestamps
might be implemented as follows
In Typescript, a logical type defined by the LogicalTypeInfo interface which associates a logical type’s URN with its representation and its conversion to and from this representation.
// A Logical type using java.time.Instant to represent the logical type.
public class TimestampNanos implements LogicalType<Instant, Row> {
// The underlying schema used to represent rows.
private final Schema SCHEMA = Schema.builder().addInt64Field("seconds").addInt32Field("nanos").build();
@Override public String getIdentifier() { return "timestampNanos"; }
@Override public FieldType getBaseType() { return schema; }
// Convert the representation type to the underlying Row type. Called by Beam when necessary.
@Override public Row toBaseType(Instant instant) {
return Row.withSchema(schema).addValues(instant.getEpochSecond(), instant.getNano()).build();
}
// Convert the underlying Row type to an Instant. Called by Beam when necessary.
@Override public Instant toInputType(Row base) {
return Instant.of(row.getInt64("seconds"), row.getInt32("nanos"));
}
...
}
// Define a logical provider like so:
// TimestampNanos is a logical type using time.Time, but
// encodes as a schema type.
type TimestampNanos time.Time
func (tn TimestampNanos) Seconds() int64 {
return time.Time(tn).Unix()
}
func (tn TimestampNanos) Nanos() int32 {
return int32(time.Time(tn).UnixNano() % 1000000000)
}
// tnStorage is the storage schema for TimestampNanos.
type tnStorage struct {
Seconds int64 `beam:"seconds"`
Nanos int32 `beam:"nanos"`
}
var (
// reflect.Type of the Value type of TimestampNanos
tnType = reflect.TypeOf((*TimestampNanos)(nil)).Elem()
tnStorageType = reflect.TypeOf((*tnStorage)(nil)).Elem()
)
// TimestampNanosProvider implements the beam.SchemaProvider interface.
type TimestampNanosProvider struct{}
// FromLogicalType converts checks if the given type is TimestampNanos, and if so
// returns the storage type.
func (p *TimestampNanosProvider) FromLogicalType(rt reflect.Type) (reflect.Type, error) {
if rt != tnType {
return nil, fmt.Errorf("unable to provide schema.LogicalType for type %v, want %v", rt, tnType)
}
return tnStorageType, nil
}
// BuildEncoder builds a Beam schema encoder for the TimestampNanos type.
func (p *TimestampNanosProvider) BuildEncoder(rt reflect.Type) (func(any, io.Writer) error, error) {
if _, err := p.FromLogicalType(rt); err != nil {
return nil, err
}
enc, err := coder.RowEncoderForStruct(tnStorageType)
if err != nil {
return nil, err
}
return func(iface any, w io.Writer) error {
v := iface.(TimestampNanos)
return enc(tnStorage{
Seconds: v.Seconds(),
Nanos: v.Nanos(),
}, w)
}, nil
}
// BuildDecoder builds a Beam schema decoder for the TimestampNanos type.
func (p *TimestampNanosProvider) BuildDecoder(rt reflect.Type) (func(io.Reader) (any, error), error) {
if _, err := p.FromLogicalType(rt); err != nil {
return nil, err
}
dec, err := coder.RowDecoderForStruct(tnStorageType)
if err != nil {
return nil, err
}
return func(r io.Reader) (any, error) {
s, err := dec(r)
if err != nil {
return nil, err
}
tn := s.(tnStorage)
return TimestampNanos(time.Unix(tn.Seconds, int64(tn.Nanos))), nil
}, nil
}
// Register it like so:
beam.RegisterSchemaProvider(tnType, &TimestampNanosProvider{})
// Register a logical type:
class Foo {
constructor(public value: string) {}
}
requireForSerialization("apache-beam", { Foo });
row_coder.registerLogicalType({
urn: "beam:logical_type:typescript_foo:v1",
reprType: row_coder.RowCoder.inferTypeFromJSON("string", false),
toRepr: (foo) => foo.value,
fromRepr: (value) => new Foo(value),
});
// And use it as follows:
const pcoll = root
.apply(beam.create([new Foo("a"), new Foo("b")]))
// Use beamLogicalType in the exemplar to indicate its use.
.apply(
beam.withRowCoder({
beamLogicalType: "beam:logical_type:typescript_foo:v1",
} as any)
);
6.4.2. Useful logical types
Currently the Python SDK provides minimal convenience logical types,
other than to handle MicrosInstant
.
Currently the Go SDK provides minimal convenience logical types,
other than to handle additional integer primitives, and time.Time
.
EnumerationType
This convenience builder doesn’t yet exist for the Python SDK.
This convenience builder doesn’t yet exist for the Go SDK.
This logical type allows creating an enumeration type consisting of a set of named constants.
The value of this field is stored in the row as an INT32 type, however the logical type defines a value type that lets you access the enumeration either as a string or a value. For example:
Given a row object with an enumeration field, you can also extract the field as the enumeration value.
Automatic schema inference from Java POJOs and JavaBeans automatically converts Java enums to EnumerationType logical types.
OneOfType
This convenience builder doesn’t yet exist for the Python SDK.
This convenience builder doesn’t yet exist for the Go SDK.
OneOfType allows creating a disjoint union type over a set of schema fields. For example:
The value of this field is stored in the row as another Row type, where all the fields are marked as nullable. The logical type however defines a Value object that contains an enumeration value indicating which field was set and allows getting just that field:
// Returns an enumeration indicating all possible case values for the enum.
// For the above example, this will be
// EnumerationType.create("intField", "stringField", "bytesField");
EnumerationType oneOfEnum = onOfType.getCaseEnumType();
// Creates an instance of the union with the string field set.
OneOfType.Value oneOfValue = oneOfType.createValue("stringField", "foobar");
// Handle the oneof
switch (oneOfValue.getCaseEnumType().toString()) {
case "intField":
return processInt(oneOfValue.getValue(Integer.class));
case "stringField":
return processString(oneOfValue.getValue(String.class));
case "bytesField":
return processBytes(oneOfValue.getValue(bytes[].class));
}
In the above example we used the field names in the switch statement for clarity, however the enum integer values could also be used.
6.5. Creating Schemas
In order to take advantage of schemas, your PCollection
s must have a schema attached to it.
Often, the source itself will attach a schema to the PCollection.
For example, when using AvroIO
to read Avro files, the source can automatically infer a Beam schema from the Avro schema and attach that to the Beam PCollection
.
However not all sources produce schemas.
In addition, often Beam pipelines have intermediate stages and types, and those also can benefit from the expressiveness of schemas.
6.5.1. Inferring schemas
- Java SDK
- Python SDK
- Go SDK
- TypeScript SDK
Unfortunately, Beam is unable to access Typescript’s type information at runtime.
Schemas must be manually declared with beam.withRowCoder
.
On the other hand, schema-aware operations such as GroupBy
can be used
without an explicit schema declared.
Beam is able to infer schemas from a variety of common Java types.
The @DefaultSchema
annotation can be used to tell Beam to infer schemas from a specific type.
The annotation takes a SchemaProvider
as an argument, and SchemaProvider
classes are already built in for common Java types.
The SchemaRegistry
can also be invoked programmatically for cases where it is not practical to annotate the Java type itself.
Java POJOs
A POJO (Plain Old Java Object) is a Java object that is not bound by any restriction other than the Java Language Specification. A POJO can contain member variables that are primitives, that are other POJOs, or are collections maps or arrays thereof. POJOs do not have to extend prespecified classes or extend any specific interfaces.
If a POJO class is annotated with @DefaultSchema(JavaFieldSchema.class)
, Beam will automatically infer a schema for
this class. Nested classes are supported as are classes with List
, array, and Map
fields.
For example, annotating the following class tells Beam to infer a schema from this POJO class and apply it to any
PCollection<TransactionPojo>
.
@DefaultSchema(JavaFieldSchema.class)
public class TransactionPojo {
public final String bank;
public final double purchaseAmount;
@SchemaCreate
public TransactionPojo(String bank, double purchaseAmount) {
this.bank = bank;
this.purchaseAmount = purchaseAmount;
}
}
// Beam will automatically infer the correct schema for this PCollection. No coder is needed as a result.
PCollection<TransactionPojo> pojos = readPojos();
The @SchemaCreate
annotation tells Beam that this constructor can be used to create instances of TransactionPojo,
assuming that constructor parameters have the same names as the field names. @SchemaCreate
can also be used to annotate
static factory methods on the class, allowing the constructor to remain private. If there is no @SchemaCreate
annotation then all the fields must be non-final and the class must have a zero-argument constructor.
There are a couple of other useful annotations that affect how Beam infers schemas. By default the schema field names
inferred will match that of the class field names. However @SchemaFieldName
can be used to specify a different name to
be used for the schema field. @SchemaIgnore
can be used to mark specific class fields as excluded from the inferred
schema. For example, it’s common to have ephemeral fields in a class that should not be included in a schema
(e.g. caching the hash value to prevent expensive recomputation of the hash), and @SchemaIgnore
can be used to
exclude these fields. Note that ignored fields will not be included in the encoding of these records.
In some cases it is not convenient to annotate the POJO class, for example if the POJO is in a different package that is not owned by the Beam pipeline author. In these cases the schema inference can be triggered programmatically in pipeline’s main function as follows:
Java Beans
Java Beans are a de-facto standard for creating reusable property classes in Java. While the full
standard has many characteristics, the key ones are that all properties are accessed via getter and setter classes, and
the name format for these getters and setters is standardized. A Java Bean class can be annotated with
@DefaultSchema(JavaBeanSchema.class)
and Beam will automatically infer a schema for this class. For example:
@DefaultSchema(JavaBeanSchema.class)
public class TransactionBean {
public TransactionBean() { … }
public String getBank() { … }
public void setBank(String bank) { … }
public double getPurchaseAmount() { … }
public void setPurchaseAmount(double purchaseAmount) { … }
}
// Beam will automatically infer the correct schema for this PCollection. No coder is needed as a result.
PCollection<TransactionBean> beans = readBeans();
The @SchemaCreate
annotation can be used to specify a constructor or a static factory method, in which case the
setters and zero-argument constructor can be omitted.
@SchemaFieldName
and @SchemaIgnore
can be used to alter the schema inferred, just like with POJO classes.
AutoValue
Java value classes are notoriously difficult to generate correctly. There is a lot of boilerplate you must create in order to properly implement a value class. AutoValue is a popular library for easily generating such classes by implementing a simple abstract base class.
Beam can infer a schema from an AutoValue class. For example:
This is all that’s needed to generate a simple AutoValue class, and the above @DefaultSchema
annotation tells Beam to
infer a schema from it. This also allows AutoValue elements to be used inside of PCollection
s.
@SchemaFieldName
and @SchemaIgnore
can be used to alter the schema inferred.
Beam has a few different mechanisms for inferring schemas from Python code.
NamedTuple classes
A NamedTuple
class is a Python class that wraps a tuple
, assigning a name to each element
and restricting it to a particular type. Beam will automatically infer the
schema for PCollections with NamedTuple
output types. For example:
beam.Row and Select
There are also methods for creating ad-hoc schema declarations. First, you can
use a lambda that returns instances of beam.Row
:
Sometimes it can be more concise to express the same logic with the
Select
transform:
Note that these declaration don’t include any specific information about the
types of the bank
and purchase_amount
fields, so Beam will attempt to infer
type information. If it’s unable to it will fall back to the generic type
Any
. Sometimes this is not ideal, you can use casts to make sure Beam
correctly infers types with beam.Row
or with Select
:
Beam currently only infers schemas for exported fields in Go structs.
Structs
Beam will automatically infer schemas for all Go structs used as PCollection elements, and default to encoding them using schema encoding.
Unexported fields are ignored, and cannot be automatically inferred as part of the schema. Fields of type func, channel, unsafe.Pointer, or uintptr will be ignored by inference. Fields of interface types are ignored, unless a schema provider is registered for them.
By default, schema field names will match the exported struct field names. In the above example, “Bank” and “PurchaseAmount” are the schema field names. A schema field name can be overridden with a struct tag for the field.
Overriding schema field names is useful for compatibility cross language transforms, as schema fields may have different requirements or restrictions from Go exported fields.
6.6. Using Schema Transforms
A schema on a PCollection
enables a rich variety of relational transforms. The fact that each record is composed of
named fields allows for simple and readable aggregations that reference fields by name, similar to the aggregations in
a SQL expression.
Beam does not yet support Schema transforms natively in Go. However, it will be implemented with the following behavior.
6.6.1. Field selection syntax
The advantage of schemas is that they allow referencing of element fields by name. Beam provides a selection syntax for referencing fields, including nested and repeated fields. This syntax is used by all of the schema transforms when referencing the fields they operate on. The syntax can also be used inside of a DoFn to specify which schema fields to process.
Addressing fields by name still retains type safety as Beam will check that schemas match at the time the pipeline graph is constructed. If a field is specified that does not exist in the schema, the pipeline will fail to launch. In addition, if a field is specified with a type that does not match the type of that field in the schema, the pipeline will fail to launch.
The following characters are not allowed in field names: . * [ ] { }
Top-level fields
In order to select a field at the top level of a schema, the name of the field is specified. For example, to select just
the user ids from a PCollection
of purchases one would write (using the Select
transform)
Nested fields
Support for Nested fields hasn’t been developed for the Python SDK yet.
Support for Nested fields hasn’t been developed for the Go SDK yet.
Individual nested fields can be specified using the dot operator. For example, to select just the postal code from the shipping address one would write
Wildcards
Support for wildcards hasn’t been developed for the Python SDK yet.
Support for wildcards hasn’t been developed for the Go SDK yet.
The * operator can be specified at any nesting level to represent all fields at that level. For example, to select all shipping-address fields one would write
Arrays
An array field, where the array element type is a row, can also have subfields of the element type addressed. When selected, the result is an array of the selected subfield type. For example
Support for Array fields hasn’t been developed for the Python SDK yet.
Support for Array fields hasn’t been developed for the Go SDK yet.
Will result in a row containing an array field with element-type string, containing the list of banks for each transaction.
While the use of [] brackets in the selector is recommended, to make it clear that array elements are being selected, they can be omitted for brevity. In the future, array slicing will be supported, allowing selection of portions of the array.
Maps
A map field, where the value type is a row, can also have subfields of the value type addressed. When selected, the result is a map where the keys are the same as in the original map but the value is the specified type. Similar to arrays, the use of {} curly brackets in the selector is recommended, to make it clear that map value elements are being selected, they can be omitted for brevity. In the future, map key selectors will be supported, allowing selection of specific keys from the map. For example, given the following schema:
PurchasesByType
Field Name | Field Type |
---|---|
purchases | MAP{STRING, ROW{PURCHASE} |
The following
Support for Map fields hasn’t been developed for the Python SDK yet.
Support for Map fields hasn’t been developed for the Go SDK yet.
Will result in a row containing a map field with key-type string and value-type string. The selected map will contain all of the keys from the original map, and the values will be the userId contained in the purchase record.
While the use of {} brackets in the selector is recommended, to make it clear that map value elements are being selected, they can be omitted for brevity. In the future, map slicing will be supported, allowing selection of specific keys from the map.
6.6.2. Schema transforms
Beam provides a collection of transforms that operate natively on schemas. These transforms are very expressive, allowing selections and aggregations in terms of named schema fields. Following are some examples of useful schema transforms.
Selecting input
Often a computation is only interested in a subset of the fields in an input PCollection
. The Select
transform allows
one to easily project out only the fields of interest. The resulting PCollection
has a schema containing each selected
field as a top-level field. Both top-level and nested fields can be selected. For example, in the Purchase schema, one
could select only the userId and streetAddress fields as follows
Support for Nested fields hasn’t been developed for the Python SDK yet.
Support for Nested fields hasn’t been developed for the Go SDK yet.
The resulting PCollection
will have the following schema
Field Name | Field Type |
---|---|
userId | STRING |
streetAddress | STRING |
The same is true for wildcard selections. The following
Support for Wildcards hasn’t been developed for the Python SDK yet.
Support for Wildcards hasn’t been developed for the Go SDK yet.
Will result in the following schema
Field Name | Field Type |
---|---|
userId | STRING |
streetAddress | STRING |
city | STRING |
state | nullable STRING |
country | STRING |
postCode | STRING |
When selecting fields nested inside of an array, the same rule applies that each selected field appears separately as a top-level field in the resulting row. This means that if multiple fields are selected from the same nested row, each selected field will appear as its own array field. For example
Support for nested fields hasn’t been developed for the Python SDK yet.
Support for nested fields hasn’t been developed for the Go SDK yet.
Will result in the following schema
Field Name | Field Type |
---|---|
bank | ARRAY[STRING] |
purchaseAmount | ARRAY[DOUBLE] |
Wildcard selections are equivalent to separately selecting each field.
Selecting fields nested inside of maps have the same semantics as arrays. If you select multiple fields from a map , then each selected field will be expanded to its own map at the top level. This means that the set of map keys will be copied, once for each selected field.
Sometimes different nested rows will have fields with the same name. Selecting multiple of these fields would result in
a name conflict, as all selected fields are put in the same row schema. When this situation arises, the
Select.withFieldNameAs
builder method can be used to provide an alternate name for the selected field.
Another use of the Select transform is to flatten a nested schema into a single flat schema. For example
Support for nested fields hasn’t been developed for the Python SDK yet.
Support for nested fields hasn’t been developed for the Go SDK yet.
Will result in the following schema
Field Name | Field Type |
---|---|
userId | STRING |
itemId | STRING |
shippingAddress_streetAddress | STRING |
shippingAddress_city | nullable STRING |
shippingAddress_state | STRING |
shippingAddress_country | STRING |
shippingAddress_postCode | STRING |
costCents | INT64 |
transactions_bank | ARRAY[STRING] |
transactions_purchaseAmount | ARRAY[DOUBLE] |
Grouping aggregations
The Group
transform allows simply grouping data by any number of fields in the input schema, applying aggregations to
those groupings, and storing the result of those aggregations in a new schema field. The output of the Group
transform
has a schema with one field corresponding to each aggregation performed.
The GroupBy
transform allows simply grouping data by any number of fields in the input schema, applying aggregations to
those groupings, and storing the result of those aggregations in a new schema field. The output of the GroupBy
transform
has a schema with one field corresponding to each aggregation performed.
The simplest usage of Group
specifies no aggregations, in which case all inputs matching the provided set of fields
are grouped together into an ITERABLE
field. For example
The simplest usage of GroupBy
specifies no aggregations, in which case all inputs matching the provided set of fields
are grouped together into an ITERABLE
field. For example
Support for schema-aware grouping hasn’t been developed for the Go SDK yet.
The output schema of this is:
Field Name | Field Type |
---|---|
key | ROW{userId:STRING, bank:STRING} |
values | ITERABLE[ROW[Purchase]] |
The key field contains the grouping key and the values field contains a list of all the values that matched that key.
The names of the key and values fields in the output schema can be controlled using this withKeyField and withValueField builders, as follows:
It is quite common to apply one or more aggregations to the grouped result. Each aggregation can specify one or more fields to aggregate, an aggregation function, and the name of the resulting field in the output schema. For example, the following application computes three aggregations grouped by userId, with all aggregations represented in a single output schema:
Support for schema-aware grouping hasn’t been developed for the Go SDK yet.
The result of this aggregation will have the following schema:
Field Name | Field Type |
---|---|
key | ROW{userId:STRING} |
value | ROW{numPurchases: INT64, totalSpendCents: INT64, topPurchases: ARRAY[INT64]} |
Often Selected.flattenedSchema
will be use to flatten the result into a non-nested, flat schema.
Joins
Beam supports equijoins on schema PCollections
- namely joins where the join condition depends on the equality of a
subset of fields. For example, the following examples uses the Purchases schema to join transactions with the reviews
that are likely associated with that transaction (both the user and product match that in the transaction). This is a
“natural join” - one in which the same field names are used on both the left-hand and right-hand sides of the join -
and is specified with the using
keyword:
Support for joins hasn’t been developed for the Python SDK yet.
Support for joins hasn’t been developed for the Go SDK yet.
The resulting schema is the following:
Field Name | Field Type |
---|---|
lhs | ROW{Transaction} |
rhs | ROW{Review} |
Each resulting row contains one Transaction and one Review that matched the join condition.
If the fields to match in the two schemas have different names, then the on function can be used. For example, if the Review schema named those fields differently than the Transaction schema, then we could write the following:
Support for joins hasn’t been developed for the Python SDK yet.
Support for joins hasn’t been developed for the Go SDK yet.
In addition to inner joins, the Join transform supports full outer joins, left outer joins, and right outer joins.
Complex joins
While most joins tend to be binary joins - joining two inputs together - sometimes you have more than two input
streams that all need to be joined on a common key. The CoGroup
transform allows joining multiple PCollections
together based on equality of schema fields. Each PCollection
can be marked as required or optional in the final
join record, providing a generalization of outer joins to joins with greater than two input PCollection
s. The output
can optionally be expanded - providing individual joined records, as in the Join
transform. The output can also be
processed in unexpanded format - providing the join key along with Iterables of all records from each input that matched
that key.
Support for joins hasn’t been developed for the Python SDK yet.
Support for joins hasn’t been developed for the Go SDK yet.
Filtering events
The Filter
transform can be configured with a set of predicates, each one based one specified fields. Only records for
which all predicates return true will pass the filter. For example the following
Will produce all purchases made from Germany with a purchase price of greater than twenty cents.
Adding fields to a schema
The AddFields transform can be used to extend a schema with new fields. Input rows will be extended to the new schema by inserting null values for the new fields, though alternate default values can be specified; if the default null value is used then the new field type will be marked as nullable. Nested subfields can be added using the field selection syntax, including nested fields inside arrays or map values.
For example, the following application
Results in a PCollection
with an expanded schema. All of the rows and fields of the input, but also with the specified
fields added to the schema. All resulting rows will have null values filled in for the timeOfDaySeconds and the
shippingAddress.deliveryNotes fields, and a false value filled in for the transactions.isFlagged field.
Removing fields from a schema
DropFields
allows specific fields to be dropped from a schema. Input rows will have their schemas truncated, and any
values for dropped fields will be removed from the output. Nested fields can also be dropped using the field selection
syntax.
For example, the following snippet
Results in a copy of the input with those two fields and their corresponding values removed.
Renaming schema fields
RenameFields
allows specific fields in a schema to be renamed. The field values in input rows are left unchanged, only
the schema is modified. This transform is often used to prepare records for output to a schema-aware sink, such as an
RDBMS, to make sure that the PCollection
schema field names match that of the output. It can also be used to rename
fields generated by other transforms to make them more usable (similar to SELECT AS in SQL). Nested fields can also be
renamed using the field-selection syntax.
For example, the following snippet
Results in the same set of unmodified input elements, however the schema on the PCollection has been changed to rename userId to userIdentifier and shippingAddress.streetAddress to shippingAddress.street.
Converting between types
As mentioned, Beam can automatically convert between different Java types, as long as those types have equivalent
schemas. One way to do this is by using the Convert
transform, as follows.
Beam will validate that the inferred schema for PurchasePojo
matches that of the input PCollection
, and will
then cast to a PCollection<PurchasePojo>
.
Since the Row
class can support any schema, any PCollection
with schema can be cast to a PCollection
of rows, as
follows.
If the source type is a single-field schema, Convert will also convert to the type of the field if asked, effectively
unboxing the row. For example, give a schema with a single INT64 field, the following will convert it to a
PCollection<Long>
In all cases, type checking is done at pipeline graph construction, and if the types do not match the schema then the pipeline will fail to launch.
6.6.3. Schemas in ParDo
A PCollection
with a schema can apply a ParDo
, just like any other PCollection
. However the Beam runner is aware
of schemas when applying a ParDo
, which enables additional functionality.
Input conversion
Beam does not yet support input conversion in Go.
Since Beam knows the schema of the source PCollection
, it can automatically convert the elements to any Java type for
which a matching schema is known. For example, using the above-mentioned Transaction schema, say we have the following
PCollection
:
If there were no schema, then the applied DoFn
would have to accept an element of type TransactionPojo
. However
since there is a schema, you could apply the following DoFn:
Even though the @Element
parameter does not match the Java type of the PCollection
, since it has a matching schema
Beam will automatically convert elements. If the schema does not match, Beam will detect this at graph-construction time
and will fail the job with a type error.
Since every schema can be represented by a Row type, Row can also be used here:
Input selection
Since the input has a schema, you can also automatically select specific fields to process in the DoFn.
Given the above purchases PCollection
, say you want to process just the userId and the itemId fields. You can do these
using the above-described selection expressions, as follows:
You can also select nested fields, as follows.
For more information, see the section on field-selection expressions. When selecting subschemas, Beam will automatically convert to any matching schema type, just like when reading the entire row.
7. Data encoding and type safety
- Java SDK
- Python SDK
- Go SDK
- TypeScript SDK
When Beam runners execute your pipeline, they often need to materialize the
intermediate data in your PCollection
s, which requires converting elements to
and from byte strings. The Beam SDKs use objects called Coder
s to describe how
the elements of a given PCollection
may be encoded and decoded.
Note that coders are unrelated to parsing or formatting data when interacting with external data sources or sinks. Such parsing or formatting should typically be done explicitly, using transforms such as
ParDo
orMapElements
.
In the Beam SDK for Java, the type Coder
provides the methods required for
encoding and decoding data. The SDK for Java provides a number of Coder
subclasses that work with a variety of standard Java types, such as Integer,
Long, Double, StringUtf8 and more. You can find all of the available Coder
subclasses in the Coder package.
In the Beam SDK for Python, the type Coder
provides the methods required for
encoding and decoding data. The SDK for Python provides a number of Coder
subclasses that work with a variety of standard Python types, such as primitive
types, Tuple, Iterable, StringUtf8 and more. You can find all of the available
Coder subclasses in the
apache_beam.coders
package.
Standard Go types like int
, int64
float64
, []byte
, and string
and more are coded using builtin coders.
Structs and pointers to structs default using Beam Schema Row encoding.
However, users can build and register custom coders with beam.RegisterCoder
.
You can find available Coder functions in the
coder
package.
Standard Typescript types like number
, UInt8Array
and string
and more are coded using builtin coders.
Json objects and arrays are encoded via a BSON encoding.
For these types, coders need not be specified unless interacting with cross-language transforms.
Users can build custom coders by extending beam.coders.Coder
for use with withCoderInternal
, but generally logical types are preferred for this case.
Note that coders do not necessarily have a 1:1 relationship with types. For example, the Integer type can have multiple valid coders, and input and output data can use different Integer coders. A transform might have Integer-typed input data that uses BigEndianIntegerCoder, and Integer-typed output data that uses VarIntCoder.
7.1. Specifying coders
The Beam SDKs require a coder for every PCollection
in your pipeline. In most
cases, the Beam SDK is able to automatically infer a Coder
for a PCollection
based on its element type or the transform that produces it, however, in some
cases the pipeline author will need to specify a Coder
explicitly, or develop
a Coder
for their custom type.
You can explicitly set the coder for an existing PCollection
by using the
method PCollection.setCoder
. Note that you cannot call setCoder
on a
PCollection
that has been finalized (e.g. by calling .apply
on it).
You can get the coder for an existing PCollection
by using the method
getCoder
. This method will fail with an IllegalStateException
if a coder has
not been set and cannot be inferred for the given PCollection
.
Beam SDKs use a variety of mechanisms when attempting to automatically infer the
Coder
for a PCollection
.
Each pipeline object has a CoderRegistry
. The CoderRegistry
represents a
mapping of Java types to the default coders that the pipeline should use for
PCollection
s of each type.
The Beam SDK for Python has a CoderRegistry
that represents a mapping of
Python types to the default coder that should be used for PCollection
s of each
type.
The Beam SDK for Go allows users to register default coder
implementations with beam.RegisterCoder
.
By default, the Beam SDK for Java automatically infers the Coder
for the
elements of a PCollection
produced by a PTransform
using the type parameter
from the transform’s function object, such as DoFn
. In the case of ParDo
,
for example, a DoFn<Integer, String>
function object accepts an input element
of type Integer
and produces an output element of type String
. In such a
case, the SDK for Java will automatically infer the default Coder
for the
output PCollection<String>
(in the default pipeline CoderRegistry
, this is
StringUtf8Coder
).
By default, the Beam SDK for Python automatically infers the Coder
for the
elements of an output PCollection
using the typehints from the transform’s
function object, such as DoFn
. In the case of ParDo
, for example a DoFn
with the typehints @beam.typehints.with_input_types(int)
and
@beam.typehints.with_output_types(str)
accepts an input element of type int
and produces an output element of type str. In such a case, the Beam SDK for
Python will automatically infer the default Coder
for the output PCollection
(in the default pipeline CoderRegistry
, this is BytesCoder
).
By default, the Beam SDK for Go automatically infers the Coder
for the elements of an output PCollection
by the output of the transform’s function object, such as a DoFn
.
In the case of ParDo
, for example a DoFn
with the parameters of v int, emit func(string)
accepts an input element of type int
and produces an output element of type string
.
In such a case, the Beam SDK for Go will automatically infer the default Coder
for the output PCollection
to be the string_utf8
coder.
Note: If you create your
PCollection
from in-memory data by using theCreate
transform, you cannot rely on coder inference and default coders.Create
does not have access to any typing information for its arguments, and may not be able to infer a coder if the argument list contains a value whose exact run-time class doesn’t have a default coder registered.
When using Create
, the simplest way to ensure that you have the correct coder
is by invoking withCoder
when you apply the Create
transform.
7.2. Default coders and the CoderRegistry
Each Pipeline object has a CoderRegistry
object, which maps language types to
the default coder the pipeline should use for those types. You can use the
CoderRegistry
yourself to look up the default coder for a given type, or to
register a new default coder for a given type.
CoderRegistry
contains a default mapping of coders to standard
JavaPython
types for any pipeline you create using the Beam SDK for
JavaPython.
The following table shows the standard mapping:
Java Type | Default Coder |
---|---|
Double | DoubleCoder |
Instant | InstantCoder |
Integer | VarIntCoder |
Iterable | IterableCoder |
KV | KvCoder |
List | ListCoder |
Map | MapCoder |
Long | VarLongCoder |
String | StringUtf8Coder |
TableRow | TableRowJsonCoder |
Void | VoidCoder |
byte[ ] | ByteArrayCoder |
TimestampedValue | TimestampedValueCoder |
Python Type | Default Coder |
---|---|
int | VarIntCoder |
float | FloatCoder |
str | BytesCoder |
bytes | StrUtf8Coder |
Tuple | TupleCoder |
7.2.1. Looking up a default coder
You can use the method CoderRegistry.getCoder
to determine the default
Coder for a Java type. You can access the CoderRegistry
for a given pipeline
by using the method Pipeline.getCoderRegistry
. This allows you to determine
(or set) the default Coder for a Java type on a per-pipeline basis: i.e. “for
this pipeline, verify that Integer values are encoded using
BigEndianIntegerCoder
.”
You can use the method CoderRegistry.get_coder
to determine the default Coder
for a Python type. You can use coders.registry
to access the CoderRegistry
.
This allows you to determine (or set) the default Coder for a Python type.
You can use the beam.NewCoder
function to determine the default Coder for a Go type.
7.2.2. Setting the default coder for a type
To set the default Coder for a
JavaPython
type for a particular pipeline, you obtain and modify the pipeline’s
CoderRegistry
. You use the method
Pipeline.getCoderRegistry
coders.registry
to get the CoderRegistry
object, and then use the method
CoderRegistry.registerCoder
CoderRegistry.register_coder
to register a new Coder
for the target type.
To set the default Coder for a Go type you use the function beam.RegisterCoder
to register a encoder and decoder functions for the target type.
However, built in types like int
, string
, float64
, etc cannot have their coders override.
The following example code demonstrates how to set a default Coder, in this case
BigEndianIntegerCoder
, for
Integerint
values for a pipeline.
The following example code demonstrates how to set a custom Coder for MyCustomType
elements.
7.2.3. Annotating a custom data type with a default coder
If your pipeline program defines a custom data type, you can use the
@DefaultCoder
annotation to specify the coder to use with that type.
By default, Beam will use SerializableCoder
which uses Java serialization,
but it has drawbacks:
It is inefficient in encoding size and speed. See this comparison of Java serialization methods.
It is non-deterministic: it may produce different binary encodings for two equivalent objects.
For key/value pairs, the correctness of key-based operations (GroupByKey, Combine) and per-key State depends on having a deterministic coder for the key
You can use the @DefaultCoder
annotation to set a new default as follows:
If you’ve created a custom coder to match your data type, and you want to use
the @DefaultCoder
annotation, your coder class must implement a static
Coder.of(Class<T>)
factory method.
The Beam SDK for PythonGo does not support annotating data types with a default coder. If you would like to set a default coder, use the method described in the previous section, Setting the default coder for a type.
8. Windowing
Windowing subdivides a PCollection
according to the timestamps of its
individual elements. Transforms that aggregate multiple elements, such as
GroupByKey
and Combine
, work implicitly on a per-window basis — they process
each PCollection
as a succession of multiple, finite windows, though the
entire collection itself may be of unbounded size.
A related concept, called triggers, determines when to emit the results of
aggregation as unbounded data arrives. You can use triggers to refine the
windowing strategy for your PCollection
. Triggers allow you to deal with
late-arriving data or to provide early results. See the triggers
section for more information.
8.1. Windowing basics
Some Beam transforms, such as GroupByKey
and Combine
, group multiple
elements by a common key. Ordinarily, that grouping operation groups all of the
elements that have the same key within the entire data set. With an unbounded
data set, it is impossible to collect all of the elements, since new elements
are constantly being added and may be infinitely many (e.g. streaming data). If
you are working with unbounded PCollection
s, windowing is especially useful.
In the Beam model, any PCollection
(including unbounded PCollection
s) can be
subdivided into logical windows. Each element in a PCollection
is assigned to
one or more windows according to the PCollection
’s windowing function, and
each individual window contains a finite number of elements. Grouping transforms
then consider each PCollection
’s elements on a per-window basis. GroupByKey
,
for example, implicitly groups the elements of a PCollection
by key and
window.
Caution: Beam’s default windowing behavior is to assign all elements of a
PCollection
to a single, global window and discard late data, even for
unbounded PCollection
s. Before you use a grouping transform such as
GroupByKey
on an unbounded PCollection
, you must do at least one of the
following:
- Set a non-global windowing function. See Setting your PCollection’s windowing function.
- Set a non-default trigger. This allows the global window to emit results under other conditions, since the default windowing behavior (waiting for all data to arrive) will never occur.
If you don’t set a non-global windowing function or a non-default trigger for
your unbounded PCollection
and subsequently use a grouping transform such as
GroupByKey
or Combine
, your pipeline will generate an error upon
construction and your job will fail.
8.1.1. Windowing constraints
After you set the windowing function for a PCollection
, the elements’ windows
are used the next time you apply a grouping transform to that PCollection
.
Window grouping occurs on an as-needed basis. If you set a windowing function
using the Window
transform, each element is assigned to a window, but the
windows are not considered until GroupByKey
or Combine
aggregates across a
window and key. This can have different effects on your pipeline. Consider the
example pipeline in the figure below:
Figure 3: Pipeline applying windowing
In the above pipeline, we create an unbounded PCollection
by reading a set of
key/value pairs using KafkaIO
, and then apply a windowing function to that
collection using the Window
transform. We then apply a ParDo
to the
collection, and then later group the result of that ParDo
using GroupByKey
.
The windowing function has no effect on the ParDo
transform, because the
windows are not actually used until they’re needed for the GroupByKey
.
Subsequent transforms, however, are applied to the result of the GroupByKey
–
data is grouped by both key and window.
8.1.2. Windowing with bounded PCollections
You can use windowing with fixed-size data sets in bounded PCollection
s.
However, note that windowing considers only the implicit timestamps attached to
each element of a PCollection
, and data sources that create fixed data sets
(such as TextIO
) assign the same timestamp to every element. This means that
all the elements are by default part of a single, global window.
To use windowing with fixed data sets, you can assign your own timestamps to
each element. To assign timestamps to elements, use a ParDo
transform with a
DoFn
that outputs each element with a new timestamp (for example, the
WithTimestamps
transform in the Beam SDK for Java).
To illustrate how windowing with a bounded PCollection
can affect how your
pipeline processes data, consider the following pipeline:
Figure 4: GroupByKey
and ParDo
without windowing, on a bounded collection.
In the above pipeline, we create a bounded PCollection
by reading lines from a
file using TextIO
. We then group the collection using GroupByKey
,
and apply a ParDo
transform to the grouped PCollection
. In this example, the
GroupByKey
creates a collection of unique keys, and then ParDo
gets applied
exactly once per key.
Note that even if you don’t set a windowing function, there is still a window –
all elements in your PCollection
are assigned to a single global window.
Now, consider the same pipeline, but using a windowing function:
Figure 5: GroupByKey
and ParDo
with windowing, on a bounded collection.
As before, the pipeline creates a bounded PCollection
by reading lines from a
file. We then set a windowing function
for that PCollection
. The GroupByKey
transform groups the elements of the
PCollection
by both key and window, based on the windowing function. The
subsequent ParDo
transform gets applied multiple times per key, once for each
window.
8.2. Provided windowing functions
You can define different kinds of windows to divide the elements of your
PCollection
. Beam provides several windowing functions, including:
- Fixed Time Windows
- Sliding Time Windows
- Per-Session Windows
- Single Global Window
- Calendar-based Windows (not supported by the Beam SDK for Python or Go)
You can also define your own WindowFn
if you have a more complex need.
Note that each element can logically belong to more than one window, depending
on the windowing function you use. Sliding time windowing, for example, can
create overlapping windows wherein a single element can be assigned to multiple
windows. However, each element in a PCollection
can only be in one window, so
if an element is assigned to multiple windows, the element is conceptually
duplicated into each of the windows and each element is identical except for its
window.
8.2.1. Fixed time windows
The simplest form of windowing is using fixed time windows: given a
timestamped PCollection
which might be continuously updating, each window
might capture (for example) all elements with timestamps that fall into a 30
second interval.
A fixed time window represents a consistent duration, non overlapping time
interval in the data stream. Consider windows with a 30 second duration: all
of the elements in your unbounded PCollection
with timestamp values from
0:00:00 up to (but not including) 0:00:30 belong to the first window, elements
with timestamp values from 0:00:30 up to (but not including) 0:01:00 belong to
the second window, and so on.
Figure 6: Fixed time windows, 30s in duration.
8.2.2. Sliding time windows
A sliding time window also represents time intervals in the data stream; however, sliding time windows can overlap. For example, each window might capture 60 seconds worth of data, but a new window starts every 30 seconds. The frequency with which sliding windows begin is called the period. Therefore, our example would have a window duration of 60 seconds and a period of 30 seconds.
Because multiple windows overlap, most elements in a data set will belong to more than one window. This kind of windowing is useful for taking running averages of data; using sliding time windows, you can compute a running average of the past 60 seconds’ worth of data, updated every 30 seconds, in our example.
Figure 7: Sliding time windows, with 1 minute window duration and 30s window period.
8.2.3. Session windows
A session window function defines windows that contain elements that are within a certain gap duration of another element. Session windowing applies on a per-key basis and is useful for data that is irregularly distributed with respect to time. For example, a data stream representing user mouse activity may have long periods of idle time interspersed with high concentrations of clicks. If data arrives after the minimum specified gap duration time, this initiates the start of a new window.
Figure 8: Session windows, with a minimum gap duration. Note how each data key has different windows, according to its data distribution.
8.2.4. The single global window
By default, all data in a PCollection
is assigned to the single global window,
and late data is discarded. If your data set is of a fixed size, you can use the
global window default for your PCollection
.
You can use the single global window if you are working with an unbounded data set
(e.g. from a streaming data source) but use caution when applying aggregating
transforms such as GroupByKey
and Combine
. The single global window with a
default trigger generally requires the entire data set to be available before
processing, which is not possible with continuously updating data. To perform
aggregations on an unbounded PCollection
that uses global windowing, you
should specify a non-default trigger for that PCollection
.
8.3. Setting your PCollection’s windowing function
You can set the windowing function for a PCollection
by applying the Window
transform. When you apply the Window
transform, you must provide a WindowFn
.
The WindowFn
determines the windowing function your PCollection
will use for
subsequent grouping transforms, such as a fixed or sliding time window.
When you set a windowing function, you may also want to set a trigger for your
PCollection
. The trigger determines when each individual window is aggregated
and emitted, and helps refine how the windowing function performs with respect
to late data and computing early results. See the triggers section
for more information.
In Beam YAML windowing specifications can also be placed directly on any
transform rather than requiring an explicit WindowInto
transform.
8.3.1. Fixed-time windows
The following example code shows how to apply Window
to divide a PCollection
into fixed windows, each 60 seconds in length:
8.3.2. Sliding time windows
The following example code shows how to apply Window
to divide a PCollection
into sliding time windows. Each window is 30 seconds in length, and a new window
begins every five seconds:
8.3.3. Session windows
The following example code shows how to apply Window
to divide a PCollection
into session windows, where each session must be separated by a time gap of at
least 10 minutes (600 seconds):
Note that the sessions are per-key — each key in the collection will have its own session groupings depending on the data distribution.
8.3.4. Single global window
If your PCollection
is bounded (the size is fixed), you can assign all the
elements to a single global window. The following example code shows how to set
a single global window for a PCollection
:
8.4. Watermarks and late data
In any data processing system, there is a certain amount of lag between the time a data event occurs (the “event time”, determined by the timestamp on the data element itself) and the time the actual data element gets processed at any stage in your pipeline (the “processing time”, determined by the clock on the system processing the element). In addition, there are no guarantees that data events will appear in your pipeline in the same order that they were generated.
For example, let’s say we have a PCollection
that’s using fixed-time
windowing, with windows that are five minutes long. For each window, Beam must
collect all the data with an event time timestamp in the given window range
(between 0:00 and 4:59 in the first window, for instance). Data with timestamps
outside that range (data from 5:00 or later) belong to a different window.
However, data isn’t always guaranteed to arrive in a pipeline in time order, or to always arrive at predictable intervals. Beam tracks a watermark, which is the system’s notion of when all data in a certain window can be expected to have arrived in the pipeline. Once the watermark progresses past the end of a window, any further element that arrives with a timestamp in that window is considered late data.
From our example, suppose we have a simple watermark that assumes approximately 30s of lag time between the data timestamps (the event time) and the time the data appears in the pipeline (the processing time), then Beam would close the first window at 5:30. If a data record arrives at 5:34, but with a timestamp that would put it in the 0:00-4:59 window (say, 3:38), then that record is late data.
Note: For simplicity, we’ve assumed that we’re using a very straightforward
watermark that estimates the lag time. In practice, your PCollection
’s data
source determines the watermark, and watermarks can be more precise or complex.
Beam’s default windowing configuration tries to determine when all data has
arrived (based on the type of data source) and then advances the watermark past
the end of the window. This default configuration does not allow late data.
Triggers allow you to modify and refine the windowing strategy for
a PCollection
. You can use triggers to decide when each individual window
aggregates and reports its results, including how the window emits late
elements.
8.4.1. Managing late data
You can allow late data by invoking the .withAllowedLateness
operation when
you set your PCollection
’s windowing strategy. The following code example
demonstrates a windowing strategy that will allow late data up to two days after
the end of a window.
When you set .withAllowedLateness
on a PCollection
, that allowed lateness
propagates forward to any subsequent PCollection
derived from the first
PCollection
you applied allowed lateness to. If you want to change the allowed
lateness later in your pipeline, you must do so explicitly by applying
Window.configure().withAllowedLateness()
.
8.5. Adding timestamps to a PCollection’s elements
An unbounded source provides a timestamp for each element. Depending on your unbounded source, you may need to configure how the timestamp is extracted from the raw data stream.
However, bounded sources (such as a file from TextIO
) do not provide
timestamps. If you need timestamps, you must add them to your PCollection
’s
elements.
You can assign new timestamps to the elements of a PCollection
by applying a
ParDo transform that outputs new elements with timestamps that you
set.
An example might be if your pipeline reads log records from an input file, and
each log record includes a timestamp field; since your pipeline reads the
records in from a file, the file source doesn’t assign timestamps automatically.
You can parse the timestamp field from each record and use a ParDo
transform
with a DoFn
to attach the timestamps to each element in your PCollection
.
PCollection<LogEntry> unstampedLogs = ...;
PCollection<LogEntry> stampedLogs =
unstampedLogs.apply(ParDo.of(new DoFn<LogEntry, LogEntry>() {
public void processElement(@Element LogEntry element, OutputReceiver<LogEntry> out) {
// Extract the timestamp from log entry we're currently processing.
Instant logTimeStamp = extractTimeStampFromLogEntry(element);
// Use OutputReceiver.outputWithTimestamp (rather than
// OutputReceiver.output) to emit the entry with timestamp attached.
out.outputWithTimestamp(element, logTimeStamp);
}
}));
class AddTimestampDoFn(beam.DoFn):
def process(self, element):
# Extract the numeric Unix seconds-since-epoch timestamp to be
# associated with the current log entry.
unix_timestamp = extract_timestamp_from_log_entry(element)
# Wrap and emit the current entry and new timestamp in a
# TimestampedValue.
yield beam.window.TimestampedValue(element, unix_timestamp)
timestamped_items = items | 'timestamp' >> beam.ParDo(AddTimestampDoFn())
// AddTimestampDoFn extracts an event time from a LogEntry.
func AddTimestampDoFn(element LogEntry, emit func(beam.EventTime, LogEntry)) {
et := extractEventTime(element)
// Defining an emitter with beam.EventTime as the first parameter
// allows the DoFn to set the event time for the emitted element.
emit(mtime.FromTime(et), element)
}
// Use the DoFn with ParDo as normal.
stampedLogs := beam.ParDo(s, AddTimestampDoFn, unstampedLogs)
9. Triggers
Note: The Trigger API in the Beam SDK for Go is currently experimental and subject to change.
When collecting and grouping data into windows, Beam uses triggers to determine when to emit the aggregated results of each window (referred to as a pane). If you use Beam’s default windowing configuration and default trigger, Beam outputs the aggregated result when it estimates all data has arrived, and discards all subsequent data for that window.
You can set triggers for your PCollection
s to change this default behavior.
Beam provides a number of pre-built triggers that you can set:
- Event time triggers. These triggers operate on the event time, as indicated by the timestamp on each data element. Beam’s default trigger is event time-based.
- Processing time triggers. These triggers operate on the processing time – the time when the data element is processed at any given stage in the pipeline.
- Data-driven triggers. These triggers operate by examining the data as it arrives in each window, and firing when that data meets a certain property. Currently, data-driven triggers only support firing after a certain number of data elements.
- Composite triggers. These triggers combine multiple triggers in various ways.
At a high level, triggers provide two additional capabilities compared to simply outputting at the end of a window:
- Triggers allow Beam to emit early results, before all the data in a given window has arrived. For example, emitting after a certain amount of time elapses, or after a certain number of elements arrives.
- Triggers allow processing of late data by triggering after the event time watermark passes the end of the window.
These capabilities allow you to control the flow of your data and balance between different factors depending on your use case:
- Completeness: How important is it to have all of your data before you compute your result?
- Latency: How long do you want to wait for data? For example, do you wait until you think you have all data? Do you process data as it arrives?
- Cost: How much compute power/money are you willing to spend to lower the latency?
For example, a system that requires time-sensitive updates might use a strict time-based trigger that emits a window every N seconds, valuing promptness over data completeness. A system that values data completeness more than the exact timing of results might choose to use Beam’s default trigger, which fires at the end of the window.
You can also set a trigger for an unbounded PCollection
that uses a single
global window for its windowing function. This can be useful when
you want your pipeline to provide periodic updates on an unbounded data set —
for example, a running average of all data provided to the present time, updated
every N seconds or every N elements.
9.1. Event time triggers
The AfterWatermark
trigger operates on event time. The AfterWatermark
trigger emits the contents of a window after the
watermark passes the end of the window, based on the
timestamps attached to the data elements. The watermark is a global progress
metric, and is Beam’s notion of input completeness within your pipeline at any
given point. AfterWatermark.pastEndOfWindow()
AfterWatermark
trigger.AfterEndOfWindow
only fires when the
watermark passes the end of the window.
In addition, you can configure triggers that fire if your pipeline receives data before or after the end of the window.
The following example shows a billing scenario, and uses both early and late firings:
// Create a bill at the end of the month.
AfterWatermark.pastEndOfWindow()
// During the month, get near real-time estimates.
.withEarlyFirings(
AfterProcessingTime
.pastFirstElementInPane()
.plusDuration(Duration.standardMinutes(1))
// Fire on any late data so the bill can be corrected.
.withLateFirings(AfterPane.elementCountAtLeast(1))
9.1.1. Default trigger
The default trigger for a PCollection
is based on event time, and emits the
results of the window when the Beam’s watermark passes the end of the window,
and then fires each time late data arrives.
However, if you are using both the default windowing configuration and the default trigger, the default trigger emits exactly once, and late data is discarded. This is because the default windowing configuration has an allowed lateness value of 0. See the Handling Late Data section for information about modifying this behavior.
9.2. Processing time triggers
The AfterProcessingTime
trigger operates on processing time. For example,
the AfterProcessingTime.pastFirstElementInPane()
AfterProcessingTime
trigger.AfterProcessingTime()
trigger emits a window
after a certain amount of processing time has passed since data was received.
The processing time is determined by the system clock, rather than the data
element’s timestamp.
The AfterProcessingTime
trigger is useful for triggering early results from a
window, particularly a window with a large time frame such as a single global
window.
9.3. Data-driven triggers
Beam provides one data-driven trigger,
AfterPane.elementCountAtLeast()
AfterCount
trigger.AfterCount()
. This trigger works on an element
count; it fires after the current pane has collected at least N elements. This
allows a window to emit early results (before all the data has accumulated),
which can be particularly useful if you are using a single global window.
It is important to note that if, for example, you specify
.elementCountAtLeast(50)
AfterCount(50)
trigger.AfterCount(50)
and only 32 elements arrive,
those 32 elements sit around forever. If the 32 elements are important to you,
consider using composite triggers to combine multiple
conditions. This allows you to specify multiple firing conditions such as “fire
either when I receive 50 elements, or every 1 second”.
9.4. Setting a trigger
When you set a windowing function for a PCollection
by using the
Window
WindowInto
beam.WindowInto
transform, you can also specify a trigger.
You set the trigger(s) for a PCollection
by invoking the method
.triggering()
on the result of your Window.into()
transform. This code
sample sets a time-based trigger for a PCollection
, which emits results one
minute after the first element in that window has been processed. The last line
in the code sample, .discardingFiredPanes()
, sets the window’s accumulation
mode.
You set the trigger(s) for a PCollection
by setting the trigger
parameter
when you use the WindowInto
transform. This code sample sets a time-based
trigger for a PCollection
, which emits results one minute after the first
element in that window has been processed. The accumulation_mode
parameter
sets the window’s accumulation mode.
You set the trigger(s) for a PCollection
by passing in the beam.Trigger
parameter
when you use the beam.WindowInto
transform. This code sample sets a time-based
trigger for a PCollection
, which emits results one minute after the first
element in that window has been processed.
The beam.AccumulationMode
parameter sets the window’s accumulation mode.
9.4.1. Window accumulation modes
When you specify a trigger, you must also set the window’s accumulation mode. When a trigger fires, it emits the current contents of the window as a pane. Since a trigger can fire multiple times, the accumulation mode determines whether the system accumulates the window panes as the trigger fires, or discards them.
To set a window to accumulate the panes that are produced when the trigger
fires, invoke.accumulatingFiredPanes()
when you set the trigger. To set a
window to discard fired panes, invoke .discardingFiredPanes()
.
To set a window to accumulate the panes that are produced when the trigger
fires, set the accumulation_mode
parameter to ACCUMULATING
when you set the
trigger. To set a window to discard fired panes, set accumulation_mode
to
DISCARDING
.
To set a window to accumulate the panes that are produced when the trigger
fires, set the beam.AccumulationMode
parameter to beam.PanesAccumulate()
when you set the
trigger. To set a window to discard fired panes, set beam.AccumulationMode
to
beam.PanesDiscard()
.
Let’s look an example that uses a PCollection
with fixed-time windowing and a
data-based trigger. This is something you might do if, for example, each window
represented a ten-minute running average, but you wanted to display the current
value of the average in a UI more frequently than every ten minutes. We’ll
assume the following conditions:
- The
PCollection
uses 10-minute fixed-time windows. - The
PCollection
has a repeating trigger that fires every time 3 elements arrive.
The following diagram shows data events for key X as they arrive in the PCollection and are assigned to windows. To keep the diagram a bit simpler, we’ll assume that the events all arrive in the pipeline in order.
9.4.1.1. Accumulating mode
If our trigger is set to accumulating mode, the trigger emits the following values each time it fires. Keep in mind that the trigger fires every time three elements arrive:
First trigger firing: [5, 8, 3]
Second trigger firing: [5, 8, 3, 15, 19, 23]
Third trigger firing: [5, 8, 3, 15, 19, 23, 9, 13, 10]
9.4.1.2. Discarding mode
If our trigger is set to discarding mode, the trigger emits the following values on each firing:
First trigger firing: [5, 8, 3]
Second trigger firing: [15, 19, 23]
Third trigger firing: [9, 13, 10]
9.4.2. Handling late data
If you want your pipeline to process data that arrives after the watermark passes the end of the window, you can apply an allowed lateness when you set your windowing configuration. This gives your trigger the opportunity to react to the late data. If allowed lateness is set, the default trigger will emit new results immediately whenever late data arrives.
You set the allowed lateness by using .withAllowedLateness()
allowed_lateness
beam.AllowedLateness()
when you set your windowing function:
This allowed lateness propagates to all PCollection
s derived as a result of
applying transforms to the original PCollection
. If you want to change the
allowed lateness later in your pipeline, you can apply
Window.configure().withAllowedLateness()
allowed_lateness
beam.AllowedLateness()
again, explicitly.
9.5. Composite triggers
You can combine multiple triggers to form composite triggers, and can specify a trigger to emit results repeatedly, at most once, or under other custom conditions.
9.5.1. Composite trigger types
Beam includes the following composite triggers:
- You can add additional early firings or late firings to
AfterWatermark.pastEndOfWindow
via.withEarlyFirings
and.withLateFirings
. Repeatedly.forever
specifies a trigger that executes forever. Any time the trigger’s conditions are met, it causes a window to emit results and then resets and starts over. It can be useful to combineRepeatedly.forever
with.orFinally
to specify a condition that causes the repeating trigger to stop.AfterEach.inOrder
combines multiple triggers to fire in a specific sequence. Each time a trigger in the sequence emits a window, the sequence advances to the next trigger.AfterFirst
takes multiple triggers and emits the first time any of its argument triggers is satisfied. This is equivalent to a logical OR operation for multiple triggers.AfterAll
takes multiple triggers and emits when all of its argument triggers are satisfied. This is equivalent to a logical AND operation for multiple triggers.orFinally
can serve as a final condition to cause any trigger to fire one final time and never fire again.
9.5.2. Composition with AfterWatermark
Some of the most useful composite triggers fire a single time when Beam estimates that all the data has arrived (i.e. when the watermark passes the end of the window) combined with either, or both, of the following:
Speculative firings that precede the watermark passing the end of the window to allow faster processing of partial results.
Late firings that happen after the watermark passes the end of the window, to allow for handling late-arriving data
You can express this pattern using AfterWatermark
. For example, the following
example trigger code fires on the following conditions:
On Beam’s estimate that all the data has arrived (the watermark passes the end of the window)
Any time late data arrives, after a ten-minute delay
- After two days, we assume no more data of interest will arrive, and the trigger stops executing
9.5.3. Other composite triggers
You can also build other sorts of composite triggers. The following example code shows a simple composite trigger that fires whenever the pane has at least 100 elements, or after a minute.
10. Metrics
In the Beam model, metrics provide some insight into the current state of a user pipeline, potentially while the pipeline is running. There could be different reasons for that, for instance:
- Check the number of errors encountered while running a specific step in the pipeline;
- Monitor the number of RPCs made to backend service;
- Retrieve an accurate count of the number of elements that have been processed;
- …and so on.
10.1. The main concepts of Beam metrics
- Named. Each metric has a name which consists of a namespace and an actual name. The namespace can be used to differentiate between multiple metrics with the same name and also allows querying for all metrics within a specific namespace.
- Scoped. Each metric is reported against a specific step in the pipeline, indicating what code was running when the metric was incremented.
- Dynamically Created. Metrics may be created during runtime without pre-declaring them, in much the same way a logger could be created. This makes it easier to produce metrics in utility code and have them usefully reported.
- Degrade Gracefully. If a runner doesn’t support some part of reporting metrics, the fallback behavior is to drop the metric updates rather than failing the pipeline. If a runner doesn’t support some part of querying metrics, the runner will not return the associated data.
Reported metrics are implicitly scoped to the transform within the pipeline that reported them. This allows reporting the same metric name in multiple places and identifying the value each transform reported, as well as aggregating the metric across the entire pipeline.
Note: It is runner-dependent whether metrics are accessible during pipeline execution or only after jobs have completed.
10.2. Types of metrics
There are three types of metrics that are supported for the moment: Counter
, Distribution
and
Gauge
.
In the Beam SDK for Go, a context.Context
provided by the framework must be passed to the metric
or the metric value will not be recorded. The framework will automatically provide a valid
context.Context
to ProcessElement
and similar methods when it’s the first parameter.
Counter: A metric that reports a single long value and can be incremented or decremented.
Distribution: A metric that reports information about the distribution of reported values.
Gauge: A metric that reports the latest value out of reported values. Since metrics are collected from many workers the value may not be the absolute last, but one of the latest values.
10.3. Querying metrics
PipelineResult
has a method metrics()
which returns a MetricResults
object that allows
accessing metrics. The main method available in MetricResults
allows querying for all metrics
matching a given filter.
beam.PipelineResult
has a method Metrics()
which returns a metrics.Results
object that allows
accessing metrics. The main method available in metrics.Results
allows querying for all metrics
matching a given filter. It takes in a predicate with a SingleResult
parameter type, which can
be used for custom filters.
PipelineResult
has a metrics
method that returns a MetricResults
object. The MetricResults
object lets you
access metrics. The main method available in the MetricResults
object, query
, lets you
query all metrics that match a given filter. The query
method takes in a MetricsFilter
object that you can
use to filter by several different criteria. Querying a MetricResults
object returns
a dictionary of lists of MetricResult
objects, with the dictionary organizing them by type,
for example, Counter
, Distribution
, and Gauge
. The MetricResult
object contains a result
function
that gets the value of the metric and contains a key
property. The key
property contains information about
the namespace and the name of the metric.
public interface PipelineResult {
MetricResults metrics();
}
public abstract class MetricResults {
public abstract MetricQueryResults queryMetrics(@Nullable MetricsFilter filter);
}
public interface MetricQueryResults {
Iterable<MetricResult<Long>> getCounters();
Iterable<MetricResult<DistributionResult>> getDistributions();
Iterable<MetricResult<GaugeResult>> getGauges();
}
public interface MetricResult<T> {
MetricName getName();
String getStep();
T getCommitted();
T getAttempted();
}
class PipelineResult:
def metrics(self) -> MetricResults:
"""Returns a the metric results from the pipeline."""
class MetricResults:
def query(self, filter: MetricsFilter) -> Dict[str, List[MetricResult]]:
"""Filters the results against the specified filter."""
class MetricResult:
def result(self):
"""Returns the value of the metric."""
10.4. Using metrics in pipeline
Below, there is a simple example of how to use a Counter
metric in a user pipeline.
// creating a pipeline with custom metrics DoFn
pipeline
.apply(...)
.apply(ParDo.of(new MyMetricsDoFn()));
pipelineResult = pipeline.run().waitUntilFinish(...);
// request the metric called "counter1" in namespace called "namespace"
MetricQueryResults metrics =
pipelineResult
.metrics()
.queryMetrics(
MetricsFilter.builder()
.addNameFilter(MetricNameFilter.named("namespace", "counter1"))
.build());
// print the metric value - there should be only one line because there is only one metric
// called "counter1" in the namespace called "namespace"
for (MetricResult<Long> counter: metrics.getCounters()) {
System.out.println(counter.getName() + ":" + counter.getAttempted());
}
public class MyMetricsDoFn extends DoFn<Integer, Integer> {
private final Counter counter = Metrics.counter( "namespace", "counter1");
@ProcessElement
public void processElement(ProcessContext context) {
// count the elements
counter.inc();
context.output(context.element());
}
}
func addMetricDoFnToPipeline(s beam.Scope, input beam.PCollection) beam.PCollection {
return beam.ParDo(s, &MyMetricsDoFn{}, input)
}
func executePipelineAndGetMetrics(ctx context.Context, p *beam.Pipeline) (metrics.QueryResults, error) {
pr, err := beam.Run(ctx, runner, p)
if err != nil {
return metrics.QueryResults{}, err
}
// Request the metric called "counter1" in namespace called "namespace"
ms := pr.Metrics().Query(func(r beam.MetricResult) bool {
return r.Namespace() == "namespace" && r.Name() == "counter1"
})
// Print the metric value - there should be only one line because there is
// only one metric called "counter1" in the namespace called "namespace"
for _, c := range ms.Counters() {
fmt.Println(c.Namespace(), "-", c.Name(), ":", c.Committed)
}
return ms, nil
}
type MyMetricsDoFn struct {
counter beam.Counter
}
func init() {
beam.RegisterType(reflect.TypeOf((*MyMetricsDoFn)(nil)))
}
func (fn *MyMetricsDoFn) Setup() {
// While metrics can be defined in package scope or dynamically
// it's most efficient to include them in the DoFn.
fn.counter = beam.NewCounter("namespace", "counter1")
}
func (fn *MyMetricsDoFn) ProcessElement(ctx context.Context, v beam.V, emit func(beam.V)) {
// count the elements
fn.counter.Inc(ctx, 1)
emit(v)
}
class MyMetricsDoFn(beam.DoFn):
def __init__(self):
self.counter = metrics.Metrics.counter("namespace", "counter1")
def process(self, element):
counter.inc()
yield element
pipeline = beam.Pipeline()
pipeline | beam.ParDo(MyMetricsDoFn())
result = pipeline.run().wait_until_finish()
metrics = result.metrics().query(
metrics.MetricsFilter.with_namespace("namespace").with_name("counter1"))
for metric in metrics["counters"]:
print(metric)
10.5. Export metrics
Beam metrics can be exported to external sinks. If a metrics sink is set up in the configuration, the runner will push metrics to it at a default 5s period. The configuration is held in the MetricsOptions class. It contains push period configuration and also sink specific options such as type and URL. As for now only the REST HTTP and the Graphite sinks are supported and only Flink and Spark runners support metrics export.
Also Beam metrics are exported to inner Spark and Flink dashboards to be consulted in their respective UI.
11. State and Timers
Beam’s windowing and triggering facilities provide a powerful abstraction for grouping and aggregating unbounded input data based on timestamps. However there are aggregation use cases for which developers may require a higher degree of control than provided by windows and triggers. Beam provides an API for manually managing per-key state, allowing for fine-grained control over aggregations.
Beam’s state API models state per key. To use the state API, you start out with a keyed PCollection
, which in Java
is modeled as a PCollection<KV<K, V>>
. A ParDo
processing this PCollection
can now declare state variables. Inside
the ParDo
these state variables can be used to write or update state for the current key or to read previous state
written for that key. State is always fully scoped only to the current processing key.
Windowing can still be used together with stateful processing. All state for a key is scoped to the current window. This means that the first time a key is seen for a given window any state reads will return empty, and that a runner can garbage collect state when a window is completed. It’s also often useful to use Beam’s windowed aggregations prior to the stateful operator. For example, using a combiner to preaggregate data, and then storing aggregated data inside of state. Merging windows are not currently supported when using state and timers.
Sometimes stateful processing is used to implement state-machine style processing inside a DoFn
. When doing this,
care must be taken to remember that the elements in input PCollection have no guaranteed order and to ensure that the
program logic is resilient to this. Unit tests written using the DirectRunner will shuffle the order of element
processing, and are recommended to test for correctness.
In Java, DoFn declares states to be accessed by creating final StateSpec
member variables representing each state. Each
state must be named using the StateId
annotation; this name is unique to a ParDo in the graph and has no relation
to other nodes in the graph. A DoFn
can declare multiple state variables.
In Python, DoFn declares states to be accessed by creating StateSpec
class member variables representing each state. Each
StateSpec
is initialized with a name, this name is unique to a ParDo in the graph and has no relation
to other nodes in the graph. A DoFn
can declare multiple state variables.
In Go, DoFn declares states to be accessed by creating state struct member variables representing each state. Each
state variable is initialized with a key, this key is unique to a ParDo in the graph and has no relation
to other nodes in the graph. If no name is supplied, the key defaults to the member variable’s name.
A DoFn
can declare multiple state variables.
Note: The Beam SDK for Typescript does not yet support a State and Timer API, but it is possible to use these features from cross-language pipelines (see below).
11.1. Types of state
Beam provides several types of state:
ValueState
A ValueState is a scalar state value. For each key in the input, a ValueState will store a typed value that can be
read and modified inside the DoFn’s @ProcessElement
or @OnTimer
methods. If the type of the ValueState has a coder
registered, then Beam will automatically infer the coder for the state value. Otherwise, a coder can be explicitly
specified when creating the ValueState. For example, the following ParDo creates a single state variable that
accumulates the number of elements seen.
Note: ValueState
is called ReadModifyWriteState
in the Python SDK.
PCollection<KV<String, ValueT>> perUser = readPerUser();
perUser.apply(ParDo.of(new DoFn<KV<String, ValueT>, OutputT>() {
@StateId("state") private final StateSpec<ValueState<Integer>> numElements = StateSpecs.value();
@ProcessElement public void process(@StateId("state") ValueState<Integer> state) {
// Read the number element seen so far for this user key.
// state.read() returns null if it was never set. The below code allows us to have a default value of 0.
int currentValue = MoreObjects.firstNonNull(state.read(), 0);
// Update the state.
state.write(currentValue + 1);
}
}));
// valueStateFn keeps track of the number of elements seen.
type valueStateFn struct {
Val state.Value[int]
}
func (s *valueStateFn) ProcessElement(p state.Provider, book string, word string, emitWords func(string)) error {
// Get the value stored in our state
val, ok, err := s.Val.Read(p)
if err != nil {
return err
}
if !ok {
s.Val.Write(p, 1)
} else {
s.Val.Write(p, val+1)
}
if val > 10000 {
// Example of clearing and starting again with an empty bag
s.Val.Clear(p)
}
return nil
}
Beam also allows explicitly specifying a coder for ValueState
values. For example:
class ReadModifyWriteStateDoFn(DoFn):
STATE_SPEC = ReadModifyWriteStateSpec('num_elements', VarIntCoder())
def process(self, element, state=DoFn.StateParam(STATE_SPEC)):
# Read the number element seen so far for this user key.
current_value = state.read() or 0
state.write(current_value+1)
_ = (p | 'Read per user' >> ReadPerUser()
| 'state pardo' >> beam.ParDo(ReadModifyWriteStateDoFn()))
const pcoll = root.apply(
beam.create([
{ key: "a", value: 1 },
{ key: "b", value: 10 },
{ key: "a", value: 100 },
])
);
const result: PCollection<number> = await pcoll
.apply(
withCoderInternal(
new KVCoder(new StrUtf8Coder(), new VarIntCoder())
)
)
.applyAsync(
pythonTransform(
// Construct a new Transform from source.
"__constructor__",
[
pythonCallable(`
# Define a DoFn to be used below.
class ReadModifyWriteStateDoFn(beam.DoFn):
STATE_SPEC = beam.transforms.userstate.ReadModifyWriteStateSpec(
'num_elements', beam.coders.VarIntCoder())
def process(self, element, state=beam.DoFn.StateParam(STATE_SPEC)):
current_value = state.read() or 0
state.write(current_value + 1)
yield current_value + 1
class MyPythonTransform(beam.PTransform):
def expand(self, pcoll):
return pcoll | beam.ParDo(ReadModifyWriteStateDoFn())
`),
],
// Keyword arguments to pass to the transform, if any.
{},
// Output type if it cannot be inferred
{ requestedOutputCoders: { output: new VarIntCoder() } }
)
);
CombiningState
CombiningState
allows you to create a state object that is updated using a Beam combiner. For example, the previous
ValueState
example could be rewritten to use CombiningState
PCollection<KV<String, ValueT>> perUser = readPerUser();
perUser.apply(ParDo.of(new DoFn<KV<String, ValueT>, OutputT>() {
@StateId("state") private final StateSpec<CombiningState<Integer, int[], Integer>> numElements =
StateSpecs.combining(Sum.ofIntegers());
@ProcessElement public void process(@StateId("state") ValueState<Integer> state) {
state.add(1);
}
}));
// combiningStateFn keeps track of the number of elements seen.
type combiningStateFn struct {
// types are the types of the accumulator, input, and output respectively
Val state.Combining[int, int, int]
}
func (s *combiningStateFn) ProcessElement(p state.Provider, book string, word string, emitWords func(string)) error {
// Get the value stored in our state
val, _, err := s.Val.Read(p)
if err != nil {
return err
}
s.Val.Add(p, 1)
if val > 10000 {
// Example of clearing and starting again with an empty bag
s.Val.Clear(p)
}
return nil
}
func combineState(s beam.Scope, input beam.PCollection) beam.PCollection {
// ...
// CombineFn param can be a simple fn like this or a structural CombineFn
cFn := state.MakeCombiningState[int, int, int]("stateKey", func(a, b int) int {
return a + b
})
combined := beam.ParDo(s, combiningStateFn{Val: cFn}, input)
// ...
BagState
A common use case for state is to accumulate multiple elements. BagState
allows for accumulating an unordered set
of elements. This allows for addition of elements to the collection without requiring the reading of the entire
collection first, which is an efficiency gain. In addition, runners that support paged reads can allow individual
bags larger than available memory.
PCollection<KV<String, ValueT>> perUser = readPerUser();
perUser.apply(ParDo.of(new DoFn<KV<String, ValueT>, OutputT>() {
@StateId("state") private final StateSpec<BagState<ValueT>> numElements = StateSpecs.bag();
@ProcessElement public void process(
@Element KV<String, ValueT> element,
@StateId("state") BagState<ValueT> state) {
// Add the current element to the bag for this key.
state.add(element.getValue());
if (shouldFetch()) {
// Occasionally we fetch and process the values.
Iterable<ValueT> values = state.read();
processValues(values);
state.clear(); // Clear the state for this key.
}
}
}));
class BagStateDoFn(DoFn):
ALL_ELEMENTS = BagStateSpec('buffer', coders.VarIntCoder())
def process(self, element_pair, state=DoFn.StateParam(ALL_ELEMENTS)):
state.add(element_pair[1])
if should_fetch():
all_elements = list(state.read())
process_values(all_elements)
state.clear()
_ = (p | 'Read per user' >> ReadPerUser()
| 'Bag state pardo' >> beam.ParDo(BagStateDoFn()))
// bagStateFn only emits words that haven't been seen
type bagStateFn struct {
Bag state.Bag[string]
}
func (s *bagStateFn) ProcessElement(p state.Provider, book, word string, emitWords func(string)) error {
// Get all values we've written to this bag state in this window.
vals, ok, err := s.Bag.Read(p)
if err != nil {
return err
}
if !ok || !contains(vals, word) {
emitWords(word)
s.Bag.Add(p, word)
}
if len(vals) > 10000 {
// Example of clearing and starting again with an empty bag
s.Bag.Clear(p)
}
return nil
}
11.2. Deferred state reads
When a DoFn
contains multiple state specifications, reading each one in order can be slow. Calling the read()
function
on a state can cause the runner to perform a blocking read. Performing multiple blocking reads in sequence adds latency
to element processing. If you know that a state will always be read, you can annotate it as @AlwaysFetched, and then the
runner can prefetch all of the states necessary. For example:
PCollection<KV<String, ValueT>> perUser = readPerUser();
perUser.apply(ParDo.of(new DoFn<KV<String, ValueT>, OutputT>() {
@StateId("state1") private final StateSpec<ValueState<Integer>> state1 = StateSpecs.value();
@StateId("state2") private final StateSpec<ValueState<String>> state2 = StateSpecs.value();
@StateId("state3") private final StateSpec<BagState<ValueT>> state3 = StateSpecs.bag();
@ProcessElement public void process(
@AlwaysFetched @StateId("state1") ValueState<Integer> state1,
@AlwaysFetched @StateId("state2") ValueState<String> state2,
@AlwaysFetched @StateId("state3") BagState<ValueT> state3) {
state1.read();
state2.read();
state3.read();
}
}));
If however there are code paths in which the states are not fetched, then annotating with @AlwaysFetched will add unnecessary fetching for those paths. In this case, the readLater method allows the runner to know that the state will be read in the future, allowing multiple state reads to be batched together.
PCollection<KV<String, ValueT>> perUser = readPerUser();
perUser.apply(ParDo.of(new DoFn<KV<String, ValueT>, OutputT>() {
@StateId("state1") private final StateSpec<ValueState<Integer>> state1 = StateSpecs.value();
@StateId("state2") private final StateSpec<ValueState<String>> state2 = StateSpecs.value();
@StateId("state3") private final StateSpec<BagState<ValueT>> state3 = StateSpecs.bag();
@ProcessElement public void process(
@StateId("state1") ValueState<Integer> state1,
@StateId("state2") ValueState<String> state2,
@StateId("state3") BagState<ValueT> state3) {
if (/* should read state */) {
state1.readLater();
state2.readLater();
state3.readLater();
}
// The runner can now batch all three states into a single read, reducing latency.
processState1(state1.read());
processState2(state2.read());
processState3(state3.read());
}
}));
11.3. Timers
Beam provides a per-key timer callback API. This allows for delayed processing of data stored using the state API. Timers can be set to callback at either an event-time or a processing-time timestamp. Every timer is identified with a TimerId. A given timer for a key can only be set for a single timestamp. Calling set on a timer overwrites the previous firing time for that key’s timer.
11.3.1. Event-time timers
Event-time timers fire when the input watermark for the DoFn passes the time at which the timer is set, meaning that the runner believes that there are no more elements to be processed with timestamps before the timer timestamp. This allows for event-time aggregations.
PCollection<KV<String, ValueT>> perUser = readPerUser();
perUser.apply(ParDo.of(new DoFn<KV<String, ValueT>, OutputT>() {
@StateId("state") private final StateSpec<ValueState<Integer>> state = StateSpecs.value();
@TimerId("timer") private final TimerSpec timer = TimerSpecs.timer(TimeDomain.EVENT_TIME);
@ProcessElement public void process(
@Element KV<String, ValueT> element,
@Timestamp Instant elementTs,
@StateId("state") ValueState<Integer> state,
@TimerId("timer") Timer timer) {
...
// Set an event-time timer to the element timestamp.
timer.set(elementTs);
}
@OnTimer("timer") public void onTimer() {
//Process timer.
}
}));
class EventTimerDoFn(DoFn):
ALL_ELEMENTS = BagStateSpec('buffer', coders.VarIntCoder())
TIMER = TimerSpec('timer', TimeDomain.WATERMARK)
def process(self,
element_pair,
t = DoFn.TimestampParam,
buffer = DoFn.StateParam(ALL_ELEMENTS),
timer = DoFn.TimerParam(TIMER)):
buffer.add(element_pair[1])
# Set an event-time timer to the element timestamp.
timer.set(t)
@on_timer(TIMER)
def expiry_callback(self, buffer = DoFn.StateParam(ALL_ELEMENTS)):
state.clear()
_ = (p | 'Read per user' >> ReadPerUser()
| 'EventTime timer pardo' >> beam.ParDo(EventTimerDoFn()))
type eventTimerDoFn struct {
State state.Value[int64]
Timer timers.EventTime
}
func (fn *eventTimerDoFn) ProcessElement(ts beam.EventTime, sp state.Provider, tp timers.Provider, book, word string, emitWords func(string)) {
// ...
// Set an event-time timer to the element timestamp.
fn.Timer.Set(tp, ts.ToTime())
// ...
}
func (fn *eventTimerDoFn) OnTimer(sp state.Provider, tp timers.Provider, w beam.Window, key string, timer timers.Context, emitWords func(string)) {
switch timer.Family {
case fn.Timer.Family:
// process callback for this timer
}
}
func AddEventTimeDoFn(s beam.Scope, in beam.PCollection) beam.PCollection {
return beam.ParDo(s, &eventTimerDoFn{
// Timers are given family names so their callbacks can be handled independantly.
Timer: timers.InEventTime("processWatermark"),
State: state.MakeValueState[int64]("latest"),
}, in)
}
11.3.2. Processing-time timers
Processing-time timers fire when the real wall-clock time passes. This is often used to create larger batches of data before processing. It can also be used to schedule events that should occur at a specific time. Just like with event-time timers, processing-time timers are per key - each key has a separate copy of the timer.
While processing-time timers can be set to an absolute timestamp, it is very common to set them to an offset relative
to the current time. In Java, the Timer.offset
and Timer.setRelative
methods can be used to accomplish this.
PCollection<KV<String, ValueT>> perUser = readPerUser();
perUser.apply(ParDo.of(new DoFn<KV<String, ValueT>, OutputT>() {
@TimerId("timer") private final TimerSpec timer = TimerSpecs.timer(TimeDomain.PROCESSING_TIME);
@ProcessElement public void process(@TimerId("timer") Timer timer) {
...
// Set a timer to go off 30 seconds in the future.
timer.offset(Duration.standardSeconds(30)).setRelative();
}
@OnTimer("timer") public void onTimer() {
//Process timer.
}
}));
class ProcessingTimerDoFn(DoFn):
ALL_ELEMENTS = BagStateSpec('buffer', coders.VarIntCoder())
TIMER = TimerSpec('timer', TimeDomain.REAL_TIME)
def process(self,
element_pair,
buffer = DoFn.StateParam(ALL_ELEMENTS),
timer = DoFn.TimerParam(TIMER)):
buffer.add(element_pair[1])
# Set a timer to go off 30 seconds in the future.
timer.set(Timestamp.now() + Duration(seconds=30))
@on_timer(TIMER)
def expiry_callback(self, buffer = DoFn.StateParam(ALL_ELEMENTS)):
# Process timer.
state.clear()
_ = (p | 'Read per user' >> ReadPerUser()
| 'ProcessingTime timer pardo' >> beam.ParDo(ProcessingTimerDoFn()))
type processingTimerDoFn struct {
Timer timers.ProcessingTime
}
func (fn *processingTimerDoFn) ProcessElement(sp state.Provider, tp timers.Provider, book, word string, emitWords func(string)) {
// ...
// Set a timer to go off 30 seconds in the future.
fn.Timer.Set(tp, time.Now().Add(30*time.Second))
// ...
}
func (fn *processingTimerDoFn) OnTimer(sp state.Provider, tp timers.Provider, w beam.Window, key string, timer timers.Context, emitWords func(string)) {
switch timer.Family {
case fn.Timer.Family:
// process callback for this timer
}
}
func AddProcessingTimeDoFn(s beam.Scope, in beam.PCollection) beam.PCollection {
return beam.ParDo(s, &processingTimerDoFn{
// Timers are given family names so their callbacks can be handled independantly.
Timer: timers.InProcessingTime("timer"),
}, in)
}
11.3.3. Dynamic timer tags
Beam also supports dynamically setting a timer tag using TimerMap
in the Java SDK. This allows for setting multiple different timers
in a DoFn
and allowing for the timer tags to be dynamically chosen - e.g. based on data in the input elements. A
timer with a specific tag can only be set to a single timestamp, so setting the timer again has the effect of
overwriting the previous expiration time for the timer with that tag. Each TimerMap
is identified with a timer family
id, and timers in different timer families are independent.
In the Python SDK, a dynamic timer tag can be specified while calling set()
or clear()
. By default, the timer
tag is an empty string if not specified.
PCollection<KV<String, ValueT>> perUser = readPerUser();
perUser.apply(ParDo.of(new DoFn<KV<String, ValueT>, OutputT>() {
@TimerFamily("actionTimers") private final TimerSpec timer =
TimerSpecs.timerMap(TimeDomain.EVENT_TIME);
@ProcessElement public void process(
@Element KV<String, ValueT> element,
@Timestamp Instant elementTs,
@TimerFamily("actionTimers") TimerMap timers) {
timers.set(element.getValue().getActionType(), elementTs);
}
@OnTimerFamily("actionTimers") public void onTimer(@TimerId String timerId) {
LOG.info("Timer fired with id " + timerId);
}
}));
class TimerDoFn(DoFn):
ALL_ELEMENTS = BagStateSpec('buffer', coders.VarIntCoder())
TIMER = TimerSpec('timer', TimeDomain.REAL_TIME)
def process(self,
element_pair,
buffer = DoFn.StateParam(ALL_ELEMENTS),
timer = DoFn.TimerParam(TIMER)):
buffer.add(element_pair[1])
# Set a timer to go off 30 seconds in the future with dynamic timer tag 'first_timer'.
# And set a timer to go off 60 seconds in the future with dynamic timer tag 'second_timer'.
timer.set(Timestamp.now() + Duration(seconds=30), dynamic_timer_tag='first_timer')
timer.set(Timestamp.now() + Duration(seconds=60), dynamic_timer_tag='second_timer')
# Note that a timer can also be explicitly cleared if previously set with a dynamic timer tag:
# timer.clear(dynamic_timer_tag=...)
@on_timer(TIMER)
def expiry_callback(self, buffer = DoFn.StateParam(ALL_ELEMENTS), timer_tag=DoFn.DynamicTimerTagParam):
# Process timer, the dynamic timer tag associated with expiring timer can be read back with DoFn.DynamicTimerTagParam.
buffer.clear()
yield (timer_tag, 'fired')
_ = (p | 'Read per user' >> ReadPerUser()
| 'ProcessingTime timer pardo' >> beam.ParDo(TimerDoFn()))
type hasAction interface {
Action() string
}
type dynamicTagsDoFn[V hasAction] struct {
Timer timers.EventTime
}
func (fn *dynamicTagsDoFn[V]) ProcessElement(ts beam.EventTime, tp timers.Provider, key string, value V, emitWords func(string)) {
// ...
// Set a timer to go off 30 seconds in the future.
fn.Timer.Set(tp, ts.ToTime(), timers.WithTag(value.Action()))
// ...
}
func (fn *dynamicTagsDoFn[V]) OnTimer(tp timers.Provider, w beam.Window, key string, timer timers.Context, emitWords func(string)) {
switch timer.Family {
case fn.Timer.Family:
tag := timer.Tag // Do something with fired tag
_ = tag
}
}
func AddDynamicTimerTagsDoFn[V hasAction](s beam.Scope, in beam.PCollection) beam.PCollection {
return beam.ParDo(s, &dynamicTagsDoFn[V]{
Timer: timers.InEventTime("actionTimers"),
}, in)
}
11.3.4. Timer output timestamps
By default, event-time timers will hold the output watermark of the ParDo
to the timestamp of the timer. This means
that if a timer is set to 12pm, any windowed aggregations or event-time timers later in the pipeline graph that finish
after 12pm will not expire. The timestamp of the timer is also the default output timestamp for the timer callback. This
means that any elements output from the onTimer method will have a timestamp equal to the timestamp of the timer firing.
For processing-time timers, the default output timestamp and watermark hold is the value of the input watermark at the
time the timer was set.
In some cases, a DoFn needs to output timestamps earlier than the timer expiration time, and therefore also needs to hold its output watermark to those timestamps. For example, consider the following pipeline that temporarily batches records into state, and sets a timer to drain the state. This code may appear correct, but will not work properly.
PCollection<KV<String, ValueT>> perUser = readPerUser();
perUser.apply(ParDo.of(new DoFn<KV<String, ValueT>, OutputT>() {
@StateId("elementBag") private final StateSpec<BagState<ValueT>> elementBag = StateSpecs.bag();
@StateId("timerSet") private final StateSpec<ValueState<Boolean>> timerSet = StateSpecs.value();
@TimerId("outputState") private final TimerSpec timer = TimerSpecs.timer(TimeDomain.PROCESSING_TIME);
@ProcessElement public void process(
@Element KV<String, ValueT> element,
@StateId("elementBag") BagState<ValueT> elementBag,
@StateId("timerSet") ValueState<Boolean> timerSet,
@TimerId("outputState") Timer timer) {
// Add the current element to the bag for this key.
elementBag.add(element.getValue());
if (!MoreObjects.firstNonNull(timerSet.read(), false)) {
// If the timer is not current set, then set it to go off in a minute.
timer.offset(Duration.standardMinutes(1)).setRelative();
timerSet.write(true);
}
}
@OnTimer("outputState") public void onTimer(
@StateId("elementBag") BagState<ValueT> elementBag,
@StateId("timerSet") ValueState<Boolean> timerSet,
OutputReceiver<ValueT> output) {
for (ValueT bufferedElement : elementBag.read()) {
// Output each element.
output.outputWithTimestamp(bufferedElement, bufferedElement.timestamp());
}
elementBag.clear();
// Note that the timer has now fired.
timerSet.clear();
}
}));
type badTimerOutputTimestampsFn[V any] struct {
ElementBag state.Bag[V]
TimerSet state.Value[bool]
OutputState timers.ProcessingTime
}
func (fn *badTimerOutputTimestampsFn[V]) ProcessElement(sp state.Provider, tp timers.Provider, key string, value V, emit func(string)) error {
// Add the current element to the bag for this key.
if err := fn.ElementBag.Add(sp, value); err != nil {
return err
}
set, _, err := fn.TimerSet.Read(sp)
if err != nil {
return err
}
if !set {
fn.OutputState.Set(tp, time.Now().Add(1*time.Minute))
fn.TimerSet.Write(sp, true)
}
return nil
}
func (fn *badTimerOutputTimestampsFn[V]) OnTimer(sp state.Provider, tp timers.Provider, w beam.Window, key string, timer timers.Context, emit func(string)) error {
switch timer.Family {
case fn.OutputState.Family:
vs, _, err := fn.ElementBag.Read(sp)
if err != nil {
return err
}
for _, v := range vs {
// Output each element
emit(fmt.Sprintf("%v", v))
}
fn.ElementBag.Clear(sp)
// Note that the timer has now fired.
fn.TimerSet.Clear(sp)
}
return nil
}
The problem with this code is that the ParDo is buffering elements, however nothing is preventing the watermark from advancing past the timestamp of those elements, so all those elements might be dropped as late data. In order to prevent this from happening, an output timestamp needs to be set on the timer to prevent the watermark from advancing past the timestamp of the minimum element. The following code demonstrates this.
PCollection<KV<String, ValueT>> perUser = readPerUser();
perUser.apply(ParDo.of(new DoFn<KV<String, ValueT>, OutputT>() {
// The bag of elements accumulated.
@StateId("elementBag") private final StateSpec<BagState<ValueT>> elementBag = StateSpecs.bag();
// The timestamp of the timer set.
@StateId("timerTimestamp") private final StateSpec<ValueState<Long>> timerTimestamp = StateSpecs.value();
// The minimum timestamp stored in the bag.
@StateId("minTimestampInBag") private final StateSpec<CombiningState<Long, long[], Long>>
minTimestampInBag = StateSpecs.combining(Min.ofLongs());
@TimerId("outputState") private final TimerSpec timer = TimerSpecs.timer(TimeDomain.PROCESSING_TIME);
@ProcessElement public void process(
@Element KV<String, ValueT> element,
@StateId("elementBag") BagState<ValueT> elementBag,
@AlwaysFetched @StateId("timerTimestamp") ValueState<Long> timerTimestamp,
@AlwaysFetched @StateId("minTimestampInBag") CombiningState<Long, long[], Long> minTimestamp,
@TimerId("outputState") Timer timer) {
// Add the current element to the bag for this key.
elementBag.add(element.getValue());
// Keep track of the minimum element timestamp currently stored in the bag.
minTimestamp.add(element.getValue().timestamp());
// If the timer is already set, then reset it at the same time but with an updated output timestamp (otherwise
// we would keep resetting the timer to the future). If there is no timer set, then set one to expire in a minute.
Long timerTimestampMs = timerTimestamp.read();
Instant timerToSet = (timerTimestamp.isEmpty().read())
? Instant.now().plus(Duration.standardMinutes(1)) : new Instant(timerTimestampMs);
// Setting the outputTimestamp to the minimum timestamp in the bag holds the watermark to that timestamp until the
// timer fires. This allows outputting all the elements with their timestamp.
timer.withOutputTimestamp(minTimestamp.read()).s et(timerToSet).
timerTimestamp.write(timerToSet.getMillis());
}
@OnTimer("outputState") public void onTimer(
@StateId("elementBag") BagState<ValueT> elementBag,
@StateId("timerTimestamp") ValueState<Long> timerTimestamp,
OutputReceiver<ValueT> output) {
for (ValueT bufferedElement : elementBag.read()) {
// Output each element.
output.outputWithTimestamp(bufferedElement, bufferedElement.timestamp());
}
// Note that the timer has now fired.
timerTimestamp.clear();
}
}));
type element[V any] struct {
Timestamp int64
Value V
}
type goodTimerOutputTimestampsFn[V any] struct {
ElementBag state.Bag[element[V]] // The bag of elements accumulated.
TimerTimerstamp state.Value[int64] // The timestamp of the timer set.
MinTimestampInBag state.Combining[int64, int64, int64] // The minimum timestamp stored in the bag.
OutputState timers.ProcessingTime // The timestamp of the timer.
}
func (fn *goodTimerOutputTimestampsFn[V]) ProcessElement(et beam.EventTime, sp state.Provider, tp timers.Provider, key string, value V, emit func(beam.EventTime, string)) error {
// ...
// Add the current element to the bag for this key, and preserve the event time.
if err := fn.ElementBag.Add(sp, element[V]{Timestamp: et.Milliseconds(), Value: value}); err != nil {
return err
}
// Keep track of the minimum element timestamp currently stored in the bag.
fn.MinTimestampInBag.Add(sp, et.Milliseconds())
// If the timer is already set, then reset it at the same time but with an updated output timestamp (otherwise
// we would keep resetting the timer to the future). If there is no timer set, then set one to expire in a minute.
ts, ok, _ := fn.TimerTimerstamp.Read(sp)
var tsToSet time.Time
if ok {
tsToSet = time.UnixMilli(ts)
} else {
tsToSet = time.Now().Add(1 * time.Minute)
}
minTs, _, _ := fn.MinTimestampInBag.Read(sp)
outputTs := time.UnixMilli(minTs)
// Setting the outputTimestamp to the minimum timestamp in the bag holds the watermark to that timestamp until the
// timer fires. This allows outputting all the elements with their timestamp.
fn.OutputState.Set(tp, tsToSet, timers.WithOutputTimestamp(outputTs))
fn.TimerTimerstamp.Write(sp, tsToSet.UnixMilli())
return nil
}
func (fn *goodTimerOutputTimestampsFn[V]) OnTimer(sp state.Provider, tp timers.Provider, w beam.Window, key string, timer timers.Context, emit func(beam.EventTime, string)) error {
switch timer.Family {
case fn.OutputState.Family:
vs, _, err := fn.ElementBag.Read(sp)
if err != nil {
return err
}
for _, v := range vs {
// Output each element with their timestamp
emit(beam.EventTime(v.Timestamp), fmt.Sprintf("%v", v.Value))
}
fn.ElementBag.Clear(sp)
// Note that the timer has now fired.
fn.TimerTimerstamp.Clear(sp)
}
return nil
}
func AddTimedOutputBatching[V any](s beam.Scope, in beam.PCollection) beam.PCollection {
return beam.ParDo(s, &goodTimerOutputTimestampsFn[V]{
ElementBag: state.MakeBagState[element[V]]("elementBag"),
TimerTimerstamp: state.MakeValueState[int64]("timerTimestamp"),
MinTimestampInBag: state.MakeCombiningState[int64, int64, int64]("minTimestampInBag", func(a, b int64) int64 {
if a < b {
return a
}
return b
}),
OutputState: timers.InProcessingTime("outputState"),
}, in)
}
11.4. Garbage collecting state
Per-key state needs to be garbage collected, or eventually the increasing size of state may negatively impact performance. There are two common strategies for garbage collecting state.
11.4.1. Using windows for garbage collection
All state and timers for a key is scoped to the window it is in. This means that depending on the timestamp of the input element the ParDo will see different values for the state depending on the window that element falls into. In addition, once the input watermark passes the end of the window, the runner should garbage collect all state for that window. (note: if allowed lateness is set to a positive value for the window, the runner must wait for the watermark to pass the end of the window plus the allowed lateness before garbage collecting state). This can be used as a garbage-collection strategy.
For example, given the following:
PCollection<KV<String, ValueT>> perUser = readPerUser();
perUser.apply(Window.into(CalendarWindows.days(1)
.withTimeZone(DateTimeZone.forID("America/Los_Angeles"))));
.apply(ParDo.of(new DoFn<KV<String, ValueT>, OutputT>() {
@StateId("state") private final StateSpec<ValueState<Integer>> state = StateSpecs.value();
...
@ProcessElement public void process(@Timestamp Instant ts, @StateId("state") ValueState<Integer> state) {
// The state is scoped to a calendar day window. That means that if the input timestamp ts is after
// midnight PST, then a new copy of the state will be seen for the next day.
}
}));
This ParDo
stores state per day. Once the pipeline is done processing data for a given day, all the state for that
day is garbage collected.
11.4.1. Using timers For garbage collection
In some cases, it is difficult to find a windowing strategy that models the desired garbage-collection strategy. For example, a common desire is to garbage collect state for a key once no activity has been seen on the key for some time. This can be done by updating a timer that garbage collects state. For example
PCollection<KV<String, ValueT>> perUser = readPerUser();
perUser.apply(ParDo.of(new DoFn<KV<String, ValueT>, OutputT>() {
// The state for the key.
@StateId("state") private final StateSpec<ValueState<ValueT>> state = StateSpecs.value();
// The maximum element timestamp seen so far.
@StateId("maxTimestampSeen") private final StateSpec<CombiningState<Long, long[], Long>>
maxTimestamp = StateSpecs.combining(Max.ofLongs());
@TimerId("gcTimer") private final TimerSpec gcTimer = TimerSpecs.timer(TimeDomain.EVENT_TIME);
@ProcessElement public void process(
@Element KV<String, ValueT> element,
@Timestamp Instant ts,
@StateId("state") ValueState<ValueT> state,
@StateId("maxTimestampSeen") CombiningState<Long, long[], Long> maxTimestamp,
@TimerId("gcTimer") gcTimer) {
updateState(state, element);
maxTimestamp.add(ts.getMillis());
// Set the timer to be one hour after the maximum timestamp seen. This will keep overwriting the same timer, so
// as long as there is activity on this key the state will stay active. Once the key goes inactive for one hour's
// worth of event time (as measured by the watermark), then the gc timer will fire.
Instant expirationTime = new Instant(maxTimestamp.read()).plus(Duration.standardHours(1));
timer.set(expirationTime);
}
@OnTimer("gcTimer") public void onTimer(
@StateId("state") ValueState<ValueT> state,
@StateId("maxTimestampSeen") CombiningState<Long, long[], Long> maxTimestamp) {
// Clear all state for the key.
state.clear();
maxTimestamp.clear();
}
}
class UserDoFn(DoFn):
ALL_ELEMENTS = BagStateSpec('state', coders.VarIntCoder())
MAX_TIMESTAMP = CombiningValueStateSpec('max_timestamp_seen', max)
TIMER = TimerSpec('gc-timer', TimeDomain.WATERMARK)
def process(self,
element,
t = DoFn.TimestampParam,
state = DoFn.StateParam(ALL_ELEMENTS),
max_timestamp = DoFn.StateParam(MAX_TIMESTAMP),
timer = DoFn.TimerParam(TIMER)):
update_state(state, element)
max_timestamp.add(t.micros)
# Set the timer to be one hour after the maximum timestamp seen. This will keep overwriting the same timer, so
# as long as there is activity on this key the state will stay active. Once the key goes inactive for one hour's
# worth of event time (as measured by the watermark), then the gc timer will fire.
expiration_time = Timestamp(micros=max_timestamp.read()) + Duration(seconds=60*60)
timer.set(expiration_time)
@on_timer(TIMER)
def expiry_callback(self,
state = DoFn.StateParam(ALL_ELEMENTS),
max_timestamp = DoFn.StateParam(MAX_TIMESTAMP)):
state.clear()
max_timestamp.clear()
_ = (p | 'Read per user' >> ReadPerUser()
| 'User DoFn' >> beam.ParDo(UserDoFn()))
type timerGarbageCollectionFn[V any] struct {
State state.Value[V] // The state for the key.
MaxTimestampInBag state.Combining[int64, int64, int64] // The maximum element timestamp seen so far.
GcTimer timers.EventTime // The timestamp of the timer.
}
func (fn *timerGarbageCollectionFn[V]) ProcessElement(et beam.EventTime, sp state.Provider, tp timers.Provider, key string, value V, emit func(beam.EventTime, string)) {
updateState(sp, fn.State, key, value)
fn.MaxTimestampInBag.Add(sp, et.Milliseconds())
// Set the timer to be one hour after the maximum timestamp seen. This will keep overwriting the same timer, so
// as long as there is activity on this key the state will stay active. Once the key goes inactive for one hour's
// worth of event time (as measured by the watermark), then the gc timer will fire.
maxTs, _, _ := fn.MaxTimestampInBag.Read(sp)
expirationTime := time.UnixMilli(maxTs).Add(1 * time.Hour)
fn.GcTimer.Set(tp, expirationTime)
}
func (fn *timerGarbageCollectionFn[V]) OnTimer(sp state.Provider, tp timers.Provider, w beam.Window, key string, timer timers.Context, emit func(beam.EventTime, string)) {
switch timer.Family {
case fn.GcTimer.Family:
// Clear all the state for the key
fn.State.Clear(sp)
fn.MaxTimestampInBag.Clear(sp)
}
}
func AddTimerGarbageCollection[V any](s beam.Scope, in beam.PCollection) beam.PCollection {