-spec mem_data_fallback(Local,
                        classy:cluster_id(),
                        FallbackData) -> {ok, sync_data()} |
                                         {error,
                                          _} when FallbackData ::
                                                      [{ok,
                                                        node(),
                                                        classy:site(),
                                                        classy:cluster_id(),
                                                        classy:site_metadata()} |
                                                       {error,
                                                        _}],
                                                  Local ::
                                                      classy:site().