code: void clusterHeartbeat(String clusterId,
                          UpdateServerHeartbeat updateSelf,
                          UpdateRackHeartbeat updateRack,
                          UpdatePodSystem updatePod,
                          long sequence)
    {
      if (startUpdate(updateRack, updatePod)) {
        getHeartbeat().clusterHeartbeat(clusterId, updateSelf, updateRack, updatePod, sequence);
      }
    }
docs: Sends a cluster heartbeat message if the update has changed, or it's time
  for a periodic heartbeat.
  
  The cluster heartbeat message include all information for a foreign cluster.