@Override
  public String toString() {
    return "CoordinatorStreamMessage [keyArray=" + Arrays.toString(keyArray) + ", messageMap=" + messageMap + ", isDelete=" + isDelete + "]";
  }
@return The key for a message. The key's meaning is defined by the type of
        the message.
samza-core/src/main/java/org/apache/samza/coordinator/stream/messages/CoordinatorStreamMessage.java