@Override
  public boolean equals(Object obj) {
    if (this == obj)
      return true;
    if (obj == null)
      return false;
    if (getClass() != obj.getClass())
      return false;
    if (!super.equals(obj))
      return false;

    final WatermarkMessage other = (WatermarkMessage) obj;
    if (timestamp != other.timestamp) {
      return false;
    }
    return true;
  }
The WatermarkMessage is a control message that is sent out to next stage
 with a watermark timestamp and the task that produces the watermark.
samza-api/src/main/java/org/apache/samza/system/WatermarkMessage.java