BatchLoader<String, Data> batchLoader =new BatchLoader<>() {
            @Override
            public CompletionStage<List<Data>> load(List<String> keys) {
                Mono<List<Data>> monoOfData = loadData(keys);
                CompletableFuture<List<Data>> cfOfData = monoOfData.toFuture();
                return cfOfData;
            }
        };