docs: function rdRoute(client, serverType, msg, cb) {
      const servers = client._station.serversMap[serverType];
      if (!servers || !servers.length) {
          utils.invokeCallback(cb, new Error(`rpc servers not exist with serverType: ${serverType}`));
          return;
      }
      const index = Math.floor(Math.random() * servers.length);
      utils.invokeCallback(cb, null, servers[index]);
  }