code: void updateRack(UpdateRackHeartbeat updateRack)
    {
      ClusterHeartbeat cluster = findCluster(updateRack.getClusterId());
  
      if (cluster == null) {
        return;
      }
      
      RackHeartbeat rack;
      
      if (cluster != _serverSelf.getCluster()) {
        rack = cluster.createRack("external");
        ClusterTarget target = createClusterTarget(cluster);
      }
      else {
        rack = cluster.findRack(updateRack.getId());
      }
      
      if (rack == null) {
        return;
      }
      
      rack.updateRack(this, updateRack);
      
      updateHeartbeats();
    }
docs: Process an update message for a rack of servers.