docs: def _start_socket(self):
          """Start the depth cache socket
  
          :return:
          """
          if self._bm is None:
              self._bm = BinanceSocketManager(self._client)
  
          self._conn_key = self._bm.start_depth_socket(self._symbol, self._depth_event)
          if not self._bm.is_alive():
              self._bm.start()
  
          # wait for some socket responses
          while not len(self._depth_message_buffer):
              time.sleep(1)