1 WaterMark生成工作机制

 观察源码

/**
 * A WatermarkGenerator for situations where records are out of order, but you can place an upper
 * bound on how far the events are out of order. An out-of-order bound B means that once an event
 * with timestamp T was encountered, no events older than {@code T - B} will follow any more.
 *
 * <p>The watermarks are generated periodically. The delay introduced by this watermark strategy is
 * the periodic interval length, plus the out-of-orderness bound.
 */
@Public
public class BoundedOutOfOrdernessWatermarks<T> implements WatermarkGenerator<T> {

    /** The maximum timestamp encountered so far. */
    private long maxTimestamp;

    /** The maximum out-of-orderness that this watermark generator assumes. */
    private final long outOfOrdernessMillis;

    /**
     * Creates a new watermark generator with the given out-of-orderness bound.
     *
     * @param maxOutOfOrderness The bound for the out-of-orderness of the event timestamps.
     */
    public BoundedOutOfOrdernessWatermarks(Duration maxOutOfOrderness) {
        checkNotNull(maxOutOfOrderness, "maxOutOfOrderness");
        checkArgument(!maxOutOfOrderness.isNegative(), "maxOutOfOrderness cannot be negative");

        this.outOfOrdernessMillis = maxOutOfOrderness.toMillis();

        // start so that our lowest watermark would be Long.MIN_VALUE.
        this.maxTimestamp = Long.MIN_VALUE + outOfOrdernessMillis + 1;
    }

    // ------------------------------------------------------------------------

    @Override
    public void onEvent(T event, long eventTimestamp, WatermarkOutput output) {
        maxTimestamp = Math.max(maxTimestamp, eventTimestamp);
    }

    @Override
    public void onPeriodicEmit(WatermarkOutput output) {
        output.emitWatermark(new Watermark(maxTimestamp - outOfOrdernessMillis - 1));
    }
}