code: function defRoute(session, msg, context, cb) {
      const list = context.getServersByType(msg.serverType);
      if (!list || !list.length) {
          cb(new Error(`can not find server info for type:${msg.serverType}`));
          return;
      }
      const uid = session ? (session.uid || '') : '';
      const index = Math.abs(crc.crc32(uid.toString())) % list.length;
      utils.invokeCallback(cb, null, list[index].id);
  }
docs: Calculate route info and return an appropriate server id.
  
  @param session {Object} session object for current rpc request
  @param msg {Object} rpc message. {serverType, service, method, args, opts}
  @param context {Object} context of client
  @param cb(err, serverId)