@Override
  public String toString() {
    return "IncomingMessageEnvelope [systemStreamPartition=" + systemStreamPartition + ", offset=" + offset +
        ", key=" + key + ", message=" + message + ", eventTime=" + eventTime +
        ", arrivalTime=" + arrivalTime + "]";
  }
This method is deprecated in favor of WatermarkManager.buildEndOfStreamEnvelope(SystemStreamPartition ssp).

@param ssp The SSP that is at end-of-stream.
@return an IncomingMessageEnvelope corresponding to end-of-stream for that SSP.
samza-api/src/main/java/org/apache/samza/system/IncomingMessageEnvelope.java