type Subscription {
  postAdded: Post!
}

@Component
public class PostSubscription implements GraphQLSubscriptionResolver {

    private final Publisher<Post> postPublisher;

    public PostSubscription(Publisher<Post> postPublisher) {
        this.postPublisher = postPublisher;
    }

    public Publisher<Post> postAdded() {
        return postPublisher;
    }
}

@Component
public class PostMutation implements GraphQLMutationResolver {

    private final Publisher<Post> postPublisher;
    private final PostService postService;

    public PostMutation(PostService postService, Publisher<Post> postPublisher) {
        this.postService = postService;
        this.postPublisher = postPublisher;
    }

    public Post createPost(CreatePostInput input) {
        Post newPost = postService.create(input);
        postPublisher.publish(newPost); // пушим в подписчиков
        return newPost;
    }
}

Таким образом фронтенд автоматически получает новые посты без повторных запросов.

public Publisher<Post> postAdded() {
    return Flux.from(postGrpcStub.subscribePosts());
}

type User @key(fields: "id") {
  id: ID!
  name: String!
}

type Post {
  id: ID!
  title: String!
  author: User @provides(fields: "name")
}

@Component
public class PostResolver implements GraphQLResolver<Post> {

    private final UserGrpc.UserBlockingStub userStub;

    public PostResolver(UserGrpc.UserBlockingStub userStub) {
        this.userStub = userStub;
    }

    public User author(Post post) {
        UserRequest req = UserRequest.newBuilder().setId(post.getAuthorId()).build();
        UserResponse resp = userStub.getUser(req);
        return mapToGraphQLUser(resp);
    }
}

Gateway собирает всю федеративную схему и возвращает фронтенду единый API.

[Frontend SPA / Mobile] --GraphQL--> [GraphQL Gateway] --gRPC--> [UserService]
                                                   |--> [PostService]
                                                   |--> [CommentService]
                                                   |--> [External REST API]

import java.util.Optional;

public class Task011225 {
    public static void main(String[] args) {
        Optional<String> emptyOpt = Optional.empty();
        Optional<String> valueOpt = Optional.of("hello");
        Optional<String> nullOpt = Optional.ofNullable(null);
        
        System.out.println(valueOpt.get());
        System.out.println(nullOpt.isPresent());
        System.out.println(emptyOpt.isPresent());
        System.out.println(emptyOpt.get());
    }
}