code: public void hubHeartbeat(UpdateServerHeartbeat updateServer, 
                             UpdateRackHeartbeat updateRack,
                             UpdatePodSystem updatePod,
                             long sourceTime)
    {
      RackHeartbeat rack = getCluster().findRack(updateRack.getId());
      
      
      if (rack == null) {
        rack = getRack();
      }
      
      updateRack(updateRack);
      
      updateServerStart(updateServer);
      
      // XXX: _podService.onUpdateFromPeer(updatePod);
      
      // rack.update();
      
      updateTargetServers();
  
      PodHeartbeatService podHeartbeat = getPodHeartbeat();
      
      if (podHeartbeat != null && updatePod != null) {
        podHeartbeat.updatePodSystem(updatePod);
      }
      
      _joinState = _joinState.onHubHeartbeat(this);
      
      // updateHubHeartbeat();
      updateHeartbeats();
    }
docs: Heartbeat from a hub server includes the heartbeat status of its rack.