Package org.apache.beam.sdk.transforms.windowing

Class Window<T>

java.lang.Object
org.apache.beam.sdk.transforms.PTransform<PCollection<T>,PCollection<T>>
org.apache.beam.sdk.transforms.windowing.Window<T>
All Implemented Interfaces:
Serializable, HasDisplayData
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

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.

Triggers

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.

See Also:

  • Constructor Details

    • Window

      public Window()

  • Method Details

    • into

      public static <T> Window<T> into(WindowFn<? super T,?> fn)
      Creates a 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.

    • configure

      public static <T> Window<T> configure()
      Returns a new builder for a Window transform for setting windowing parameters other than the windowing function.

    • getWindowFn

      public abstract @Nullable WindowFn<? super T,?> getWindowFn()

    • triggering

      public Window<T> triggering(Trigger trigger)
      Sets a non-default trigger for this 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().

    • discardingFiredPanes

      public 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.

      Does not modify this transform. The resulting PTransform is sufficiently specified to be applied, but more properties can still be specified.

    • accumulatingFiredPanes

      public 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.

      Does not modify this transform. The resulting PTransform is sufficiently specified to be applied, but more properties can still be specified.

    • withAllowedLateness

      public 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. 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.

    • withTimestampCombiner

      public Window<T> withTimestampCombiner(TimestampCombiner timestampCombiner)
      Override the default TimestampCombiner, to control the output timestamp of values output from a GroupByKey operation.

    • withAllowedLateness

      public Window<T> withAllowedLateness(Duration allowedLateness, Window.ClosingBehavior behavior)
      Override the amount of lateness allowed for data elements in the pipeline. 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.

    • withOnTimeBehavior

      public Window<T> withOnTimeBehavior(Window.OnTimeBehavior behavior)
      Override the default Window.OnTimeBehavior, to control whether to output an empty on-time pane.

    • getOutputStrategyInternal

      public WindowingStrategy<?,?> getOutputStrategyInternal(WindowingStrategy<?,?> inputStrategy)
      Get the output strategy of this Window PTransform. For internal use only.

    • expand

      public PCollection<T> expand(PCollection<T> input)
      Description copied from class: PTransform
      Override this method to specify how this PTransform 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).

      Specified by:
      expand in class PTransform<PCollection<T>,PCollection<T>>

    • populateDisplayData

      public void populateDisplayData(DisplayData.Builder builder)
      Description copied from class: PTransform
      Register display data for the given transform or component.

      populateDisplayData(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.

      Specified by:
      populateDisplayData in interface HasDisplayData
      Overrides:
      populateDisplayData in class PTransform<PCollection<T>,PCollection<T>>
      Parameters:
      builder - The builder to populate with display data.
      See Also:

    • getKindString

      protected String getKindString()
      Description copied from class: PTransform
      Returns the name to use by default for this PTransform (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.

      Overrides:
      getKindString in class PTransform<PCollection<T>,PCollection<T>>

    • remerge

      public 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.