public abstract class Window<T> extends PTransform<PCollection<T>,PCollection<T>>
Window logically divides up or groups the elements of a PCollection into finite
windows according to a WindowFn. The output of Window contains the same elements
as input, but they have been logically assigned to windows. The next GroupByKeys, including one within composite
transforms, will group by the combination of keys and windows.
See GroupByKey for more information about how grouping
with windows works.
Windowing a PCollection divides the elements into windows based on the associated
event time for each element. This is especially useful for PCollections with
unbounded size, since it allows operating on a sub-group of the elements placed into a related
window. For PCollections with a bounded size (aka. conventional batch mode),
by default, all data is implicitly in a single window, unless Window is applied.
For example, a simple form of windowing divides up the data into fixed-width time intervals,
using FixedWindows. The following example demonstrates how to use Window in a
pipeline that counts the number of occurrences of strings each minute:
PCollection<String> items = ...;
PCollection<String> windowed_items = items.apply(
Window.<String>into(FixedWindows.of(Duration.standardMinutes(1))));
PCollection<KV<String, Long>> windowed_counts = windowed_items.apply(
Count.<String>perElement());
Let (data, timestamp) denote a data element along with its timestamp. Then, if the input to this pipeline consists of {("foo", 15s), ("bar", 30s), ("foo", 45s), ("foo", 1m30s)}, the output will be {(KV("foo", 2), 1m), (KV("bar", 1), 1m), (KV("foo", 1), 2m)}
Several predefined WindowFns are provided:
FixedWindows partitions the timestamps into fixed-width intervals.
SlidingWindows places data into overlapping fixed-width intervals.
Sessions groups data into sessions where each item in a window is separated from
the next by no more than a specified gap.
Additionally, custom WindowFns can be created, by creating new subclasses of WindowFn.
triggering(Trigger) allows specifying a trigger to control when (in processing
time) results for the given window can be produced. If unspecified, the default behavior is to
trigger first when the watermark passes the end of the window, and then trigger again every time
there is late arriving data.
Elements are added to the current window pane as they arrive. When the root trigger fires, output is produced based on the elements in the current pane.
Depending on the trigger, this can be used both to output partial results early during the processing of the whole window, and to deal with late arriving in batches.
Continuing the earlier example, if we wanted to emit the values that were available when the watermark passed the end of the window, and then output any late arriving elements once-per (actual hour) hour until we have finished processing the next 24-hours of data. (The use of watermark time to stop processing tends to be more robust if the data source is slow for a few days, etc.)
PCollection<String> items = ...;
PCollection<String> windowed_items = items.apply(
Window.<String>into(FixedWindows.of(Duration.standardMinutes(1)))
.triggering(
AfterWatermark.pastEndOfWindow()
.withLateFirings(AfterProcessingTime
.pastFirstElementInPane().plusDelayOf(Duration.standardHours(1))))
.withAllowedLateness(Duration.standardDays(1)));
PCollection<KV<String, Long>> windowed_counts = windowed_items.apply(
Count.<String>perElement());
On the other hand, if we wanted to get early results every minute of processing time (for which there were new elements in the given window) we could do the following:
PCollection<String> windowed_items = items.apply(
Window.<String>into(FixedWindows.of(Duration.standardMinutes(1)))
.triggering(
AfterWatermark.pastEndOfWindow()
.withEarlyFirings(AfterProcessingTime
.pastFirstElementInPane().plusDelayOf(Duration.standardMinutes(1))))
.withAllowedLateness(Duration.ZERO));
After a GroupByKey the trigger is set to a trigger that
will preserve the intent of the upstream trigger. See Trigger.getContinuationTrigger() for
more information.
See Trigger for details on the available triggers.
| Modifier and Type | Class and Description |
|---|---|
static class |
Window.Assign<T>
A Primitive
PTransform that assigns windows to elements based on a WindowFn. |
static class |
Window.ClosingBehavior
Specifies the conditions under which a final pane will be created when a window is permanently
closed.
|
static class |
Window.OnTimeBehavior
Specifies the conditions under which an on-time pane will be created when a window is closed.
|
name| Constructor and Description |
|---|
Window() |
| Modifier and Type | Method and Description |
|---|---|
Window<T> |
accumulatingFiredPanes()
Returns a new
Window PTransform that uses the registered WindowFn and
Triggering behavior, and that accumulates elements in a pane after they are triggered. |
static <T> Window<T> |
configure()
Returns a new builder for a
Window transform for setting windowing parameters other
than the windowing function. |
Window<T> |
discardingFiredPanes()
Returns a new
Window PTransform that uses the registered WindowFn and
Triggering behavior, and that discards elements in a pane after they are triggered. |
PCollection<T> |
expand(PCollection<T> input)
Override this method to specify how this
PTransform should be expanded on the given
InputT. |
protected java.lang.String |
getKindString()
Returns the name to use by default for this
PTransform (not including the names of any
enclosing PTransforms). |
WindowingStrategy<?,?> |
getOutputStrategyInternal(WindowingStrategy<?,?> inputStrategy)
Get the output strategy of this
Window PTransform. |
abstract WindowFn<? super T,?> |
getWindowFn() |
static <T> Window<T> |
into(WindowFn<? super T,?> fn)
|
void |
populateDisplayData(DisplayData.Builder builder)
Register display data for the given transform or component.
|
static <T> org.apache.beam.sdk.transforms.windowing.Window.Remerge<T> |
remerge()
Creates a
Window PTransform that does not change assigned windows, but will
cause windows to be merged again as part of the next GroupByKey. |
Window<T> |
triggering(Trigger trigger)
Sets a non-default trigger for this
Window PTransform. |
Window<T> |
withAllowedLateness(Duration allowedLateness)
Override the amount of lateness allowed for data elements in the output
PCollection and
downstream PCollections until explicitly set again. |
Window<T> |
withAllowedLateness(Duration allowedLateness,
Window.ClosingBehavior behavior)
Override the amount of lateness allowed for data elements in the pipeline.
|
Window<T> |
withOnTimeBehavior(Window.OnTimeBehavior behavior)
(Experimental) Override the default
Window.OnTimeBehavior, to control whether to
output an empty on-time pane. |
Window<T> |
withTimestampCombiner(TimestampCombiner timestampCombiner)
(Experimental) Override the default
TimestampCombiner, to control the
output timestamp of values output from a GroupByKey operation. |
getAdditionalInputs, getDefaultOutputCoder, getDefaultOutputCoder, getDefaultOutputCoder, getName, toString, validatepublic static <T> Window<T> into(WindowFn<? super T,?> fn)
Window PTransform that uses the given WindowFn to window the
data.
The resulting PTransform's types have been bound, with both the input and output
being a PCollection<T>, inferred from the types of the argument WindowFn. It is
ready to be applied, or further properties can be set on it first.
public static <T> Window<T> configure()
Window transform for setting windowing parameters other
than the windowing function.@Experimental(value=TRIGGER) public Window<T> triggering(Trigger trigger)
Window PTransform. Elements that are
assigned to a specific window will be output when the trigger fires.
Trigger has more details on the available
triggers.
Must also specify allowed lateness using withAllowedLateness(org.joda.time.Duration) and accumulation mode
using either discardingFiredPanes() or accumulatingFiredPanes().
@Experimental(value=TRIGGER) public Window<T> discardingFiredPanes()
Window PTransform that uses the registered WindowFn and
Triggering behavior, and that discards elements in a pane after they are triggered.
Does not modify this transform. The resulting PTransform is sufficiently specified
to be applied, but more properties can still be specified.
@Experimental(value=TRIGGER) public Window<T> accumulatingFiredPanes()
Window PTransform that uses the registered WindowFn and
Triggering behavior, and that accumulates elements in a pane after they are triggered.
Does not modify this transform. The resulting PTransform is sufficiently specified
to be applied, but more properties can still be specified.
@Experimental(value=TRIGGER) public Window<T> withAllowedLateness(Duration allowedLateness)
PCollection and
downstream PCollections until explicitly set again. Like the other
properties on this Window operation, this will be applied at the next GroupByKey. Any elements that are later than this as decided by the system-maintained
watermark will be dropped.
This value also determines how long state will be kept around for old windows. Once no elements will be added to a window (because this duration has passed) any state associated with the window will be cleaned up.
Depending on the trigger this may not produce a pane with PaneInfo.isLast. See
Window.ClosingBehavior.FIRE_IF_NON_EMPTY for more details.
@Experimental(value=OUTPUT_TIME) public Window<T> withTimestampCombiner(TimestampCombiner timestampCombiner)
TimestampCombiner, to control the
output timestamp of values output from a GroupByKey operation.@Experimental(value=TRIGGER) public Window<T> withAllowedLateness(Duration allowedLateness, Window.ClosingBehavior behavior)
Window operation, this will be applied at the next GroupByKey. Any elements that are later than this as decided by the system-maintained
watermark will be dropped.
This value also determines how long state will be kept around for old windows. Once no elements will be added to a window (because this duration has passed) any state associated with the window will be cleaned up.
@Experimental(value=TRIGGER) public Window<T> withOnTimeBehavior(Window.OnTimeBehavior behavior)
Window.OnTimeBehavior, to control whether to
output an empty on-time pane.public WindowingStrategy<?,?> getOutputStrategyInternal(WindowingStrategy<?,?> inputStrategy)
Window PTransform. For internal use only.public PCollection<T> expand(PCollection<T> input)
PTransformPTransform should be expanded on the given
InputT.
NOTE: This method should not be called directly. Instead apply the PTransform should
be applied to the InputT using the apply method.
Composite transforms, which are defined in terms of other transforms, should return the output of one of the composed transforms. Non-composite transforms, which do not apply any transforms internally, should return a new unbound output and register evaluators (via backend-specific registration methods).
expand in class PTransform<PCollection<T>,PCollection<T>>public void populateDisplayData(DisplayData.Builder builder)
PTransformpopulateDisplayData(DisplayData.Builder) is invoked by Pipeline runners to collect
display data via DisplayData.from(HasDisplayData). Implementations may call super.populateDisplayData(builder) in order to register display data in the current namespace,
but should otherwise use subcomponent.populateDisplayData(builder) to use the namespace
of the subcomponent.
By default, does not register any display data. Implementors may override this method to provide their own display data.
populateDisplayData in interface HasDisplayDatapopulateDisplayData in class PTransform<PCollection<T>,PCollection<T>>builder - The builder to populate with display data.HasDisplayDataprotected java.lang.String getKindString()
PTransformPTransform (not including the names of any
enclosing PTransforms).
By default, returns the base name of this PTransform's class.
The caller is responsible for ensuring that names of applied PTransforms are unique,
e.g., by adding a uniquifying suffix when needed.
getKindString in class PTransform<PCollection<T>,PCollection<T>>public static <T> org.apache.beam.sdk.transforms.windowing.Window.Remerge<T> remerge()
Window PTransform that does not change assigned windows, but will
cause windows to be merged again as part of the next GroupByKey.