code: void hubHeartbeat(UpdateServerHeartbeat updateServer,
                      UpdateRackHeartbeat updateRack,
                      UpdatePodSystem updatePod,
                      long sequence)
    {
      if (startUpdate(updateRack, updatePod)) {
        getHeartbeat().hubHeartbeat(updateServer, updateRack, updatePod, sequence);
      }
    }
docs: Sends a hub heartbeat message if the update has changed, or it's time
  for a periodic heartbeat.
  
  The hub heartbeat message include all information for the current cluster
  from a hub server.