GraphQL实践6——Netflix Dgs Graphql N+1问题

原创

F嘉阳

发布于 2022-10-28 20:53:46

5700

发布于 2022-10-28 20:53:46

文章被收录于专栏：graphqlgraphql

N+1问题介绍

对于上一篇文章样例，如果要获取每个电影的演员名单，要执行如下动作

查询所有电影清单
遍历N个电影，查询对应电影的演员名单

总查询开销为N+1次查询，代价非常大，效率低

优化方案 DataLoader

对于一对一表关联的情况且每个关联对象只有一个值，可以直接使用BatchLoader

但实际上，对于大部分表关联情况，通常为一对多或者多对多，不保证每个关联都有值，此时需要使用MappedBatchLoader维护关联关系

维护多对多实体关系Map

public interface ActorService {
    Map<Integer, List<Actor>> listByFilmId(Collection<Integer> filmIds);
}

@Slf4j
@Service
@RequiredArgsConstructor
public class ActorServiceImpl implements ActorService {

    private final ActorRepository actorRepository;

    private final FilmActorRepository filmActorRepository;

    @Override
    public Map<Integer, List<Actor>> listByFilmId(Collection<Integer> filmIds) {
        List<FilmActor> filmActors = filmActorRepository.listByFilmId(filmIds);
        if (CollectionUtils.isEmpty(filmActors)) {
            return Collections.emptyMap();
        }
        Map<Integer, List<FilmActor>> filmActorMap = filmActors.stream().collect(Collectors.groupingBy(FilmActor::getFilmId));
        List<Integer> actorIds = filmActors.stream().map(FilmActor::getActorId).distinct().toList();
        List<Actor> actors = actorRepository.listByIds(actorIds);
        Map<Integer, Actor> actorsMap = Stream.ofNullable(actors).flatMap(Collection::stream).collect(Collectors.toMap(Actor::getActorId, Function.identity(), (e1, e2) -> e1));
        Map<Integer, List<Actor>> result = Maps.newHashMapWithExpectedSize(filmIds.size());
        filmActorMap.forEach((k, v) -> {
            List<Actor> actorList = Stream.ofNullable(filmActorMap.get(k)).flatMap(Collection::stream).map(e -> actorsMap.get(e.getActorId())).toList();
            result.put(k, actorList);
        });
        return result;
    }
}

Dgs MappedBatchLoader 配置

@DgsDataLoader(name = "actors")
@RequiredArgsConstructor
public class FilmActorsDataLoader implements MappedBatchLoader<Integer, List<Actor>> {

    private final ActorService actorService;

    @Override
    public CompletionStage<Map<Integer, List<Actor>>> load(Set<Integer> keys) {
        return CompletableFuture.supplyAsync(() -> actorService.listByFilmId(keys));
    }
}