docs: function rrRoute(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;
      }
      let index;
      if (!client.rrParam) {
          client.rrParam = {};
      }
      if (client.rrParam[serverType]) {
          index = client.rrParam[serverType];
      } else {
          index = 0;
      }
      utils.invokeCallback(cb, null, servers[index % servers.length]);
      if (index++ === Number.MAX_VALUE) {
          index = 0;
      }
      client.rrParam[serverType] = index;
  }