org.apache.flink.streaming.api.functions.AssignerWithPunctuatedWatermarks Maven / Gradle / Ivy
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*
* http://www.apache.org/licenses/LICENSE-2.0
*
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package org.apache.flink.streaming.api.functions;
import org.apache.flink.streaming.api.watermark.Watermark;
import javax.annotation.Nullable;
/**
* The {@code AssignerWithPunctuatedWatermarks} 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.
*
* Use this class if certain special elements act as markers that signify event time progress,
* and when you want to emit watermarks specifically at certain events. The system will generate a
* new watermark, if the probed value is non-null and has a timestamp larger than that of the
* previous watermark (to preserve the contract of ascending watermarks).
*
*
For use cases that should periodically emit watermarks based on element timestamps, use the
* {@link AssignerWithPeriodicWatermarks} instead.
*
*
The following example illustrates how to use this timestamp extractor and watermark generator.
* It assumes elements carry a timestamp that describes when they were created, and that some
* elements carry a flag, marking them as the end of a sequence such that no elements with smaller
* timestamps can come anymore.
*
*
{@code
* public class WatermarkOnFlagAssigner implements AssignerWithPunctuatedWatermarks {
*
* public long extractTimestamp(MyElement element, long previousElementTimestamp) {
* return element.getSequenceTimestamp();
* }
*
* public Watermark checkAndGetNextWatermark(MyElement lastElement, long extractedTimestamp) {
* return lastElement.isEndOfSequence() ? new Watermark(extractedTimestamp) : null;
* }
* }
* }
*
* Timestamps and watermarks are defined as {@code longs} that represent the milliseconds since
* the Epoch (midnight, January 1, 1970 UTC). A watermark with a certain value {@code t} indicates
* that no elements with event timestamps {@code x}, where {@code x} is lower or equal to {@code t},
* will occur any more.
*
* @param The type of the elements to which this assigner assigns timestamps.
* @see org.apache.flink.streaming.api.watermark.Watermark
*/
@Deprecated
public interface AssignerWithPunctuatedWatermarks extends TimestampAssigner {
/**
* Asks this implementation if it wants to emit a watermark. This method is called right after
* the {@link #extractTimestamp(Object, long)} method.
*
* The returned watermark will be emitted only if it is non-null and its timestamp is larger
* than that of the previously emitted watermark (to preserve the contract of ascending
* watermarks). If a null value is returned, or the timestamp of the returned watermark is
* smaller than that of the last emitted one, then no new watermark will be generated.
*
*
For an example how to use this method, see the documentation of {@link
* AssignerWithPunctuatedWatermarks this class}.
*
* @return {@code Null}, if no watermark should be emitted, or the next watermark to emit.
*/
@Nullable
Watermark checkAndGetNextWatermark(T lastElement, long extractedTimestamp);
}