java 49 lines · 8 steps

Batching GraphQL queries with DataLoaders in Spring

Register batch loaders that collapse per-item GraphQL fetches into single bulk queries, killing the N+1 problem.

Explained by highlit
Spring Advanced graphql batching n+1-problem reactive dataloader
1@Configuration
2public class DataLoaderConfig {
3 
4 @Bean
5 public BatchLoaderRegistry batchLoaderRegistry() {
6 return new DefaultBatchLoaderRegistry();
7 }
8 
9 @Bean
10 public ApplicationRunner registerLoaders(BatchLoaderRegistry registry,
11 AuthorRepository authorRepository,
12 CommentRepository commentRepository) {
13 return args -> {
14 registry.forTypePair(Long.class, Author.class)
15 .registerMappedBatchLoader((authorIds, env) ->
16 Mono.fromSupplier(() ->
17 authorRepository.findAllById(authorIds).stream()
18 .collect(Collectors.toMap(Author::getId, Function.identity())))
19 .subscribeOn(Schedulers.boundedElastic()));
20 
21 registry.forName("commentsByPostLoader")
22 .registerMappedBatchLoader((postIds, env) ->
23 Mono.fromSupplier(() ->
24 commentRepository.findByPostIdIn(postIds).stream()
25 .collect(Collectors.groupingBy(Comment::getPostId)))
26 .subscribeOn(Schedulers.boundedElastic()));
27 };
28 }
29}
30 
31@Controller
32class PostController {
33 
34 @SchemaMapping(typeName = "Post", field = "author")
35 public CompletableFuture<Author> author(Post post, DataLoader<Long, Author> loader) {
36 return loader.load(post.getAuthorId());
37 }
38 
39 @SchemaMapping(typeName = "Post", field = "comments")
40 public CompletableFuture<List<Comment>> comments(
41 Post post,
42 @Argument int limit,
43 DataLoader<Long, List<Comment>> commentsByPostLoader) {
44 return commentsByPostLoader.load(post.getId())
45 .thenApply(comments -> comments == null
46 ? List.of()
47 : comments.stream().limit(limit).toList());
48 }
49}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1A BatchLoaderRegistry lets you fetch related entities in one bulk query instead of one per parent, eliminating N+1 database hits.
  2. 2Wrapping blocking repository calls in Mono.fromSupplier on boundedElastic keeps the reactive pipeline non-blocking.
  3. 3Field resolvers return CompletableFuture from a loader so Spring GraphQL can defer and batch the actual data access.

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