final long getInputWatermark() {
    return this.currentWatermark;
  }
Handling of the input watermark and returns the output watermark.
In-memory operator can override this to fire event-time triggers. Output operators need to override it
so it can propagate watermarks over the wire. By default it simply returns the input watermark.
@param inputWatermark  input watermark
@param collector message collector
@param coordinator task coordinator
@return output watermark, or null if the output watermark should not be updated.
samza-core/src/main/java/org/apache/samza/operators/impl/OperatorImpl.java