CompletableFuture to make multiple DB calls and get the response back

Question

How to make Async call to DB and save the details and get the response entity back? Assume I am hitting to two different Data Centers. How can I get the entity back as response?

CompletableFuture.supplyAsync(() -> abcDc1Repository.save(localEntity));
CompletableFuture.supplyAsync(() -> abcDc2Repository.save(localEntity));

Answer 1

You can use CompletableFuture.allOf()

    CompletableFuture<OrderEntity> firstFuture = CompletableFuture.supplyAsync(
            () -> firstRepository.save(firstEntity)
    );
    CompletableFuture<OrderEntity> secondFuture = CompletableFuture.supplyAsync(
            () -> secondRepository.save(secondEntity)
    );

    CompletableFuture.allOf(firstFuture, secondFuture).join();

    OrderEntity firstEntity = firstFuture.get();
    OrderEntity secondEntity = secondFuture.get();

You don't need a result, if you save new entity. Hibernate will set id to the passed entity.

From SimpleJpaRepository

    @Transactional
    @Override
    public <S extends T> S save(S entity) {
        if (entityInformation.isNew(entity)) {
            em.persist(entity);
            return entity;
        } else {
            return em.merge(entity);
        }
    }

Notes

Keep in mind that CompletableFuture.supplyAsync() uses common fork-join pool. So your operation will wait for stream.parallel() operations in any point of the application.

Also you shouldn't have a common transaction (a persistent context, with @Transactional) for these two calls, because persistent context is not thread safe.