ker still may fail between a heartbeat
      and a request and even the fact that it does send hearbeats
      does not necessarily mean that it is operating correctly.
      If it is essential for the client to know
      that all tasks have been performed correctly,
      other verification means are required.
</li><li> Finally, a subscriber will never know
             whether a publisher is still working correctly
             or not, if the publisher does not send data
             periodically. A reli