gistfile1.hrl

ring_trans(Fun, Args) ->
    gen_server2:call(?MODULE, {ring_trans, Fun, Args}).
...

handle_call({ring_trans, Fun, Args}, _From, State) ->
    {ok, Ring} = get_my_ring(),
    case catch Fun(Ring, Args) of
        {new_ring, NewRing} ->
            set_my_ring(NewRing),
            {reply, {ok, NewRing}, State};
        _ ->
            {reply, not_changed, State}
    end.