Add Vertx Future, RxJava3 API to Vertx DataLoader Helper

[hantsy]

There are several helpers in Vertx 4.1.0 to build GraphQL DataLoader / BatchLoaders.

public class AuthorsDataLoader implements VertxBatchLoader<String, Author> {
    final AuthorService authorService;
    @Override
    public CompletionStage<List<Author>> load(List<String> keys, BatchLoaderEnvironment environment) {
        return null;
    }
}

But there is no helpers to use Future or Flowable when building a DataLoader.

Hope there are some interfaces to use Future or Flowable directly.

public class AuthorsDataLoader implements VertxFavoredBatchLoader<String, Author> {
    final AuthorService authorService;
    @Override
    public Future<List<Author>> load(List<String> keys, BatchLoaderEnvironment environment) {
        return null;
    }
}

or

public class AuthorsDataLoader implements VertxFavoredBatchLoader<String, Author> {
    final AuthorService authorService;
    @Override
    public Flowable<List<Author>> load(List<String> keys, BatchLoaderEnvironment environment) {
        return null;
    }
}

[tsegismont]

In your example, AuthorsDataLoader implements VertxBatchLoader but it shouldn't.

Vert.x Web GraphQL already has helpers for futurized API.

Consider a GraphQL server that exposes blog posts data. The server can retrieve all blog posts along with their comments.

To avoid sending a query fetching comments for each post, you could create a DataLoader:

DataLoader<Integer, JsonArray> commentDataLoader = DataLoader.newMappedDataLoader(
  VertxMappedBatchLoader.create((postIds, batchLoaderEnvironment) -> findComments(postIds, batchLoaderEnvironment))
);

The findComments method returns a Future:

private Future<Map<Integer, JsonArray>> findComments(Set<Integer> postIds, BatchLoaderEnvironment env) {
  Collector<Row, ?, Map<Integer, JsonArray>> collector = groupingBy(
    row -> row.getInteger("post_id"),
    mapping(this::toComment, collectingAndThen(toList(), JsonArray::new))
  );
  return pgClient.preparedQuery("select * from comments where post_id = any($1)")
    .collecting(collector)
    .execute(Tuple.of(postIds.toArray(new Integer[0])))
    .map(SqlResult::value);
}

You can find the code here.

Concerning the RxJava integration, you may use the RxJavaJdk8Interop libraray as explained in the doc.

So I think what we could add some documentation about VertxBatchLoader to avoid confusions.