code: def _get_servers_deque(servers, database):
      
      key = (servers, database)
      if key not in _servers_deques:
          _servers_deques[key] = deque(servers)
      return _servers_deques[key]
docs: Returns deque of servers for given tuple of servers and
      database name.
      This deque have active server at the begining, if first server
      is not accessible at the moment the deque will be rotated,
      second server will be moved to the first position, thirt to the
      second position etc, and previously first server will be moved
      to the last position.
      This allows to remember last successful server between calls
      to connect function.