code: void updateRack(HeartbeatImpl heartbeat,
                    UpdateRackHeartbeat updateRack)
    {
      for (UpdateServerHeartbeat serverUpdate : updateRack.getServers()) {
        if (serverUpdate == null) {
          continue;
        }
   
        update(serverUpdate);
        // updateTargetServers();
   
        ServerHeartbeat peerServer = findServer(serverUpdate.getAddress(),
                                                serverUpdate.getPort());
   
        if (peerServer.isSelf()) {
          continue;
        }
        
        heartbeat.updateServer(peerServer, serverUpdate);
        /*
        String externalId = serverUpdate.getExternalId();
  
        heartbeat.updateExternal(peerServer, externalId);
  
        if (peerServer.onHeartbeatUpdate(serverUpdate)) {
          if (peerServer.isUp()) {
            onServerStart(peerServer);
          }
          else {
            onServerStop(peerServer);
          }
        }
        */
      }
  
      update();
    }
docs: Update the rack with a heartbeat message.