org.apache.flink.streaming.api.functions

Interface AssignerWithPeriodicWatermarks<T>

Type Parameters:

T - The type of the elements to which this assigner assigns timestamps.

All Superinterfaces:

Function, Serializable, TimestampAssigner<T>

All Known Implementing Classes:

AscendingTimestampExtractor, IngestionTimeExtractor

public interface AssignerWithPeriodicWatermarks<T>
extends TimestampAssigner<T>

The AssignerWithPeriodicWatermarks assigns event time timestamps to elements, and generates low watermarks that signal event time progress within the stream. These timestamps and watermarks are used by functions and operators that operate on event time, for example event time windows.

This class is used to generate watermarks in a periodical interval. At most every i milliseconds (configured via ExecutionConfig.getAutoWatermarkInterval(), the system will call the getCurrentWatermark() method to probe for the next watermark value. The system will generate a new watermark, if the probed value is larger than zero and larger than the previous watermark.

The system may call the getCurrentWatermark() method less often than every i milliseconds, of no new elements arrived since the last call to the method.

Timestamps and watermarks are defined as longs that represent the milliseconds since the Epoch (midnight, January 1, 1970 UTC). A watermark with a certain value t indicates that no elements with event timestamps x, where x is lower or equal to t, will occur any more.

See Also:

Watermark

Method Summary

Methods

Modifier and Type	Method and Description
Watermark	getCurrentWatermark() Returns the current watermark.

Methods inherited from interface org.apache.flink.streaming.api.functions.TimestampAssigner

extractTimestamp

Method Detail

getCurrentWatermark

Watermark getCurrentWatermark()

Returns the current watermark. This method is periodically called by the system to retrieve the current watermark. The method may return null to indicate that no new Watermark is available.

The returned watermark will be emitted only if it is non-null and larger than the previously emitted watermark. If the current watermark is still identical to the previous one, no progress in event time has happened since the previous call to this method.

If this method returns a value that is smaller than the previously returned watermark, then the implementation does not properly handle the event stream timestamps. In that case, the returned watermark will not be emitted (to preserve the contract of ascending watermarks), and the violation will be logged and registered in the metrics.

The interval in which this method is called and Watermarks are generated depends on ExecutionConfig.getAutoWatermarkInterval().

See Also:

Watermark, ExecutionConfig.getAutoWatermarkInterval()