code: def _push_async_request(self, msg_id, request, reply_cb, inform_cb,
                              user_data, timeout_handle):
          
          assert get_thread_ident() == self.ioloop_thread_id
          self._async_queue[msg_id] = (
              request, reply_cb, inform_cb, user_data, timeout_handle)
          if request.name in self._async_id_stack:
              self._async_id_stack[request.name].append(msg_id)
          else:
              self._async_id_stack[request.name] = [msg_id]
docs: Store reply / inform callbacks for request we've sent.