code: protected void emit(StreamMessage message, Object messageKey)
      {
          KeyHistory newHistory = null;
          if (this.recordHistory)
          {
              newHistory = createKeyRecorededHistory(this.executingKeyHistory, messageKey);
          }
          else
          {
              newHistory = createKeyRecorededHistory(this.executingKeyHistory);
          }
  
          message.getHeader().setHistory(newHistory);
  
          getCollector().emit(this.getExecutingTuple(), new Values("", message));
      }
docs: Use anchor function(child message failed. notify fail to parent message.), MessageKey(Use key history's value).<br>
  Send message to downstream component.<br>
  Use following situation.
  <ol>
  <li>Use this class's key history function.</li>
  <li>Use storm's fault detect function.</li>
  </ol>
  
  @param message sending message
  @param messageKey MessageKey(Use key history's value)