Developing I/O connectors for Python
IMPORTANT: Please use
Splittable DoFn to develop your new I/O. For more details, please read
the new I/O connector overview.
To connect to a data store that isn’t supported by Beam’s existing I/O connectors, you must create a custom I/O connector that usually consist of a source and a sink. All Beam sources and sinks are composite transforms; however, the implementation of your custom I/O depends on your use case. Before you start, read the new I/O connector overview for an overview of developing a new I/O connector, the available implementation options, and how to choose the right option for your use case.
This guide covers using the Source and FileBasedSink interfaces for Python. The Java SDK offers the same functionality, but uses a slightly different API. See Developing I/O connectors for Java for information specific to the Java SDK.
Basic code requirements
Beam runners use the classes you provide to read and/or write data using
multiple worker instances in parallel. As such, the code you provide for
FileBasedSink subclasses must meet some basic requirements:
FileBasedSinksubclass must be serializable. The service may create multiple instances of your
FileBasedSinksubclass to be sent to multiple remote workers to facilitate reading or writing in parallel. The way the source and sink objects are serialized is runner specific.
FileBasedSinksubclass must be effectively immutable. You should only use mutable state in your
FileBasedSinksubclass if you are using lazy evaluation of expensive computations that you need to implement the source.
Thread-Safety: Your code must be thread-safe. The Beam SDK for Python provides the
RangeTrackerclass to make this easier.
Testability: It is critical to exhaustively unit-test all of your
FileBasedSinksubclasses. A minor implementation error can lead to data corruption or data loss (such as skipping or duplicating records) that can be hard to detect. You can use test harnesses and utility methods available in the source_test_utils module to develop tests for your source.
In addition, see the PTransform style guide for Beam’s transform style guidance.
Implementing the Source interface
To create a new data source for your pipeline, you’ll need to provide the format-specific logic that tells the service how to read data from your input source, and how to split your data source into multiple parts so that multiple worker instances can read your data in parallel.
Supply the logic for your new source by creating the following classes:
- A subclass of
BoundedSourceis a source that reads a finite amount of input records. The class describes the data you want to read, including the data’s location and parameters (such as how much data to read).
- A subclass of
RangeTrackeris a thread-safe object used to manage a range for a given position type.
- One or more user-facing wrapper composite transforms (
PTransform) that wrap read operations. PTransform wrappers discusses why you should avoid exposing your sources, and walks through how to create a wrapper.
You can find these classes in the apache_beam.io.iobase module.
Implementing the BoundedSource subclass
BoundedSource represents a finite data set from which the service reads, possibly in parallel.
BoundedSource contains a set of methods that the service uses to split the data set for reading by multiple remote workers.
To implement a
BoundedSource, your subclass must override the following methods:
estimate_size: Services use this method to estimate the total size of your data, in bytes. This estimate is in terms of external storage size, before performing decompression or other processing.
split: Service use this method to split your finite data into bundles of a given size.
get_range_tracker: Services use this method to get the
RangeTrackerfor a given position range, and use the information to report progress and perform dynamic splitting of sources.
read: This method returns an iterator that reads data from the source, with respect to the boundaries defined by the given
Implementing the RangeTracker subclass
RangeTracker is a thread-safe object used to manage the current range and current position of the reader of a
BoundedSource and protect concurrent access to them.
To implement a
RangeTracker, you should first familiarize yourself with the following definitions:
Position-based sources - A position-based source can be described by a range of positions of an ordered type, and the records read by the source can be described by positions of that type. For example, for a record within a file, the position can be the starting byte offset of the record. The position type for the record in this case is
The main requirement for position-based sources is associativity: Reading records in position range ‘[A, B)’ and records in position range ‘[B, C)’ should give the same records as reading records in position range ‘[A, C)', where ‘A’ <= ‘B’ <= ‘C’. This property ensures that no matter how many arbitrary sub-ranges a range of positions is split into, the total set of records they describe stays the same.
The other important property is how the source’s range relates to positions of records in the source. In many sources each record can be identified by a unique starting position. In this case:
- All records returned by a source ‘[A, B)’ must have starting positions in this range.
- All but the last record should end within this range. The last record may or may not extend past the end of the range.
- Records must not overlap.
Such sources should define “read ‘[A, B)'” as “read from the first record starting at or after ‘A’, up to but not including the first record starting at or after ‘B’".
Some examples of such sources include reading lines or CSV from a text file, reading keys and values from a database, etc.
The concept of split points allows to extend the definitions for dealing with sources where some records cannot be identified by a unique starting position.
Split points - A split point describes a record that is the first one returned when reading the range from and including position A up to infinity (i.e. [A, infinity)).
Some sources may have records that are not directly addressable. For example, imagine a file format consisting of a sequence of compressed blocks. Each block can be assigned an offset, but records within the block cannot be directly addressed without decompressing the block. Let us refer to this hypothetical format as CBF (Compressed Blocks Format).
Many such formats can still satisfy the associativity property. For example, in CBF, reading [A, B) can mean “read all the records in all blocks whose starting offset is in [A, B)".
To support such complex formats, Beam introduces the notion of split points. A record is a split point if there exists a position A such that the record is the first one to be returned when reading the range [A, infinity). In CBF, the only split points would be the first records in each block.
Split points allow us to define the meaning of a record’s position and a source’s range in the following cases:
- For a record that is at a split point, its position is defined to be the largest A such that reading a source with the range [A, infinity) returns this record.
- Positions of other records are only required to be non-decreasing.
- Reading the source [A, B) must return records starting from the first split point at or after A, up to but not including the first split point at or after B. In particular, this means that the first record returned by a source MUST always be a split point.
- Positions of split points must be unique.
As a result, for any decomposition of the full range of the source into position ranges, the total set of records will be the full set of records in the source, and each record will be read exactly once.
Consumed positions - Consumed positions refer to records that have been read.
As the source is being read, and records read from it are being passed to the downstream transforms in the pipeline, we say that positions in the source are being consumed. When a reader has read a record (or promised to a caller that a record will be returned), positions up to and including the record’s start position are considered consumed.
Dynamic splitting can happen only at unconsumed positions. If the reader just returned a record at offset 42 in a file, dynamic splitting can happen only at offset 43 or beyond. Otherwise, that record could be read twice (by the current reader and the reader of the new task).
To implement a
RangeTracker, your subclass must override the following methods:
start_position: Returns the starting position of the current range, inclusive.
stop_position: Returns the ending position of the current range, exclusive.
try_claim: This method is used to determine if a record at a split point is within the range. This method should modify the internal state of the
RangeTrackerby updating the last-consumed position to the given starting
positionof the record being read by the source. The method returns true if the given position falls within the current range.
set_current_position: This method updates the last-consumed position to the given starting position of a record being read by a source. You can invoke this method for records that do not start at split points, and this should modify the internal state of the
RangeTracker. If the record starts at a split point, you must invoke
try_claiminstead of this method.
position_at_fraction: Given a fraction within the range [0.0, 1.0), this method will return the position at the given fraction compared to the position range [
try_split: This method attempts to split the current range into two parts around a suggested position. It is allowed to split at a different position, but in most cases it will split at the suggested position.
This method splits the current range [
self.stop_position) into a “primary” part [
split_position), and a “residual” part [
self.stop_position), assuming that
split_position has not been consumed yet.
split_position has already been consumed, the method returns
None. Otherwise, it updates the current range to be the primary and returns a tuple (
split_fraction should be the fraction of size of range [
split_position) compared to the original (before split) range [
fraction_consumed: Returns the approximate fraction of consumed positions in the source.
Note: Methods of class
iobase.RangeTracker may be invoked by multiple threads, hence this class must be made thread-safe, for example, by using a single lock object.
Convenience Source base classes
The Beam SDK for Python contains some convenient abstract base classes to help you easily create new sources.
FileBasedSource is a framework for developing sources for new file types. You can derive your
BoundedSource class from the FileBasedSource class.
To create a source for a new file type, you need to create a sub-class of
FileBasedSource. Sub-classes of
FileBasedSource must implement the method
See AvroSource for an example implementation of
Reading from a new Source
The following example,
CountingSource, demonstrates an implementation of
BoundedSource and uses the SDK-provided
class CountingSource(iobase.BoundedSource): def __init__(self, count): self.records_read = Metrics.counter(self.__class__, 'recordsRead') self._count = count def estimate_size(self): return self._count def get_range_tracker(self, start_position, stop_position): if start_position is None: start_position = 0 if stop_position is None: stop_position = self._count return OffsetRangeTracker(start_position, stop_position) def read(self, range_tracker): for i in range(range_tracker.start_position(), range_tracker.stop_position()): if not range_tracker.try_claim(i): return self.records_read.inc() yield i def split(self, desired_bundle_size, start_position=None, stop_position=None): if start_position is None: start_position = 0 if stop_position is None: stop_position = self._count bundle_start = start_position while bundle_start < stop_position: bundle_stop = min(stop_position, bundle_start + desired_bundle_size) yield iobase.SourceBundle( weight=(bundle_stop - bundle_start), source=self, start_position=bundle_start, stop_position=bundle_stop) bundle_start = bundle_stop
To read data from the source in your pipeline, use the
Note: When you create a source that end-users are going to use, we
recommended that you do not expose the code for the source itself as
demonstrated in the example above. Use a wrapping
PTransform wrappers discusses why you should avoid
exposing your sources, and walks through how to create a wrapper.
Using the FileBasedSink abstraction
If your data source uses files, you can implement the FileBasedSink
abstraction to create a file-based sink. For other sinks, use
GroupByKey, and other transforms offered by the Beam SDK for Python. See the
developing I/O connectors overview
for more details.
When using the
FileBasedSink interface, you must provide the format-specific
logic that tells the runner how to write bounded data from your pipeline’s
PCollections to an output sink. The runner writes bundles of data in parallel
using multiple workers.
Supply the logic for your file-based sink by implementing the following classes:
A subclass of the abstract base class
FileBasedSinkdescribes a location or resource that your pipeline can write to in parallel. To avoid exposing your sink to end-users, use the
_prefix when creating your
A user-facing wrapper
PTransformthat, as part of the logic, calls
Writeand passes your
FileBasedSinkas a parameter. A user should not need to call
FileBasedSink abstract base class implements code that is common to Beam
sinks that interact with files, including:
- Setting file headers and footers
- Sequential record writing
- Setting the output MIME type
FileBasedSink and its subclasses support writing files to any Beam-supported
FileSystem implementations. See the following Beam-provided
implementation for an example:
When you create a source or sink that end-users will use, avoid exposing your
source or sink code. To avoid exposing your sources and sinks to end-users, your
new classes should use the
_ prefix. Then, implement a user-facing
PTransform.`By exposing your source or sink as a transform, your
implementation is hidden and can be arbitrarily complex or simple. The greatest
benefit of not exposing implementation details is that later on, you can add
additional functionality without breaking the existing implementation for users.
For example, if your users’ pipelines read from your source using
beam.io.Read and you want to insert a reshard into the pipeline, all
users would need to add the reshard themselves (using the
transform). To solve this, we recommended that you expose the source as a
PTransform that performs both the read operation and the reshard.
See Beam’s PTransform style guide
for additional information about wrapping with a
The following examples change the source and sink from the above sections so
that they are not exposed to end-users. For the source, rename
_CountingSource. Then, create the wrapper
Finally, read from the source:
For the sink, rename
_SimpleKVSink. Then, create the wrapper
class WriteToKVSink(PTransform): def __init__(self, simplekv, url, final_table_name): self._simplekv = simplekv super(WriteToKVSink, self).__init__() self._url = url self._final_table_name = final_table_name def expand(self, pcoll): return pcoll | iobase.Write( _SimpleKVSink(self._simplekv, self._url, self._final_table_name))
Finally, write to the sink:
Last updated on 2020/10/29
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