@Override
  public String toString() {
    return "OutgoingMessageEnvelope [systemStream=" + systemStream + ", keySerializerName=" + keySerializerName + ", messageSerializerName=" + messageSerializerName + ", partitionKey=" + partitionKey + ", key=" + key + ", message=" + message + "]";
  }
Constructs a new OutgoingMessageEnvelope from specified components.
@param systemStream Object representing the appropriate stream of which this envelope will be sent on.
@param message A deserialized message to be sent in this envelope.
samza-api/src/main/java/org/apache/samza/system/OutgoingMessageEnvelope.java