// TODO: decide how to call this
 public void ackReceived(long messageID) {
   // Remove if received acknowledgment message id contains in retained message list.
   retainedMessageList.remove(messageID);
 }
 /**
  * Record the given message ID as a retained message in the trcker.
  *
  * @param messageID Message ID of the retained message
  */
 public void recordRetainedMessage(long messageID) {
   retainedMessageList.add(messageID);
 }