docs: function(self, agent, msg, cb) {
    logger.debug('[%s] receive addServer signal: %j', self.app.serverId, msg);
    if(!msg || !msg.server) {
      logger.warn('monitorwatcher addServer receive empty message: %j', msg);
      utils.invokeCallback(cb, Constants.SIGNAL.FAIL);
      return;
    }
    addServers(self, [msg.server]);
    utils.invokeCallback(cb, Constants.SIGNAL.OK);
  }