the eventTimestamp in the method signature of WatermarkGenerator#onEvent seems redundant

Question

The following is the definition of WatermarkGenerator,

@Public
public interface WatermarkGenerator<T> {

    /**
     * Called for every event, allows the watermark generator to examine and remember the
     * event timestamps, or to emit a watermark based on the event itself.
     */
    void onEvent(T event, long eventTimestamp, WatermarkOutput output);

    /**
     * Called periodically, and might emit a new watermark, or not.
     *
     * <p>The interval in which this method is called and Watermarks are generated
     * depends on {@link ExecutionConfig#getAutoWatermarkInterval()}.
     */
    void onPeriodicEmit(WatermarkOutput output);
}

For the onEvent method, why there is an redundant argument long eventTimestamp in the method signature, I think the event time should be able to be extracted from the T event(the concrete event type should carry with event time), and it should be equal to eventTimestamp, so I would ask why this redundant agument long eventTimestamp is needed since I can get it from event,what's design consideration here?

Answer 1

The timestamp passed into the onEvent method is the current timestamp in the StreamRecord envelope wrapping your event. This is whatever timestamp has previously been assigned to this event -- e.g., in the case of Kafka this might be the value of the timestamp in the Kafka record header.

While this is typically redundant information, there are situations where it is useful to have access to the timestamp previously assigned upstream.