What is the correct way of managing transactionality in RxJava Services?

Question

I have recently started experimenting with RxJava and came across a presentation by a Netflix engineer that suggested moving our business APIs to Observable APIs, for instance:

public interface VideoService {
    Observable<VideoBasicInfo> createVideoBasicInfo(VideoBasicInfo videoBasic);
    Observable<VideoBasicInfo> getVideoBasicInfo(Integer videoId);
    Observable<VideoRating> getVideoRating(Integer videoId);
}

However I haven't found any place that explained how transactionality should be managed in this services. At first I just annotated my service implementation with @Transactional

@Service
@Transactional
public class VideoServiceImpl implements VideoService{

    @Autowired
    private VideoBasicInfoRepository basicInfoRepo;
    @Autowired
    private VideoRatingRepository ratingRepo;

    public Observable<VideoBasicInfo> createVideoBasicInfo(VideoBasicInfo videoBasic){
        return Observable.create( s -> {
            s.onNext(basicInfoRepo.save(videBasic));
        });
    }

What we would want is that the execution of all the code inside the Object.create lambda (s -> { // This code }) happened in a transaction. HOWEVER, what actually happens is that:

1. The call to createVideoBasicInfo() executes in a transactional way, returning the cold observable.
2. The save() executes as an atomic transaction.

Obviously it makes sense since the Spring proxy applies to the serviceImpl methods. I have thought of ways to do what I really expect such as starting a programmatic transaction:

return Observable.create( s -> {
    VideoBasicInfo savedBasic = transactionTemplate.execute( status -> {
        VideoBasicInfo basicInfo = basicInfoRepo.save(videoBasicInfo);
        return basicInfo;
    });
    s.onNext(savedBasic);
});

Is this the recommended way of managing transactions when working with reactive APIs?

Answer 1

Spring Data JpaRepository method signatures are already marked @Transactional, so if you are using just one then you don't need to do anything special:

public interface PersonRepository extends JpaRepository<Person, Integer> {
}

---

@RunWith(SpringJUnit4ClassRunner.class)
@SpringApplicationConfiguration(classes = {RepositoryConfiguration.class})
public class PersonRepositoryTest {
    private PersonRepository personRepository;

    @Autowired
    public void setPersonRepository(PersonRepository PersonRepository) {
        this.personRepository = PersonRepository;
    }

    @Test
    public void testReactiveSavePerson() {
        Person person = new Person("Jane", "Doe");
        assertNull(person.getId()); //null before save

        //save person
        Observable.create(s -> {
            s.onNext(personRepository.save(person));
        }).subscribe();

        //fetch from DB
        Person fetchedPerson = personRepository.findOne(person.getId());

        //should not be null
        assertNotNull(fetchedPerson);

        //should equal
        assertEquals(person.getId(), fetchedPerson.getId());
        assertEquals(person.getFirstName(), fetchedPerson.getFirstName());
    }
}

If you need to combine multiple repositories into one transaction, you could use something like the class below:

@Component()
public class ObservableTxFactory {
    public final <T> Observable<T> create(Observable.OnSubscribe<T> f) {
        return new ObservableTx<>(this, f);
    }

    @Transactional
    public void call(Observable.OnSubscribe onSubscribe, Subscriber subscriber) {
        onSubscribe.call(subscriber);
    }

    private static class ObservableTx<T> extends Observable<T> {

        public ObservableTx(ObservableTxFactory observableTxFactory, OnSubscribe<T> f) {
            super(new OnSubscribeDecorator<>(observableTxFactory, f));
        }
    }

    private static class OnSubscribeDecorator<T> implements Observable.OnSubscribe<T> {

        private final ObservableTxFactory observableTxFactory;
        private final Observable.OnSubscribe<T> onSubscribe;

        OnSubscribeDecorator(final ObservableTxFactory observableTxFactory, final Observable.OnSubscribe<T> s) {
            this.onSubscribe = s;
            this.observableTxFactory = observableTxFactory;
        }

        @Override
        public void call(Subscriber<? super T> subscriber) {
            observableTxFactory.call(onSubscribe, subscriber);
        }
    }
}

The factory bean needs to be defined as well:

@Bean
ObservableTxFactory observableTxFactory() {
    return new ObservableTxFactory();
}

---

Service:

@Service
public class PersonService {
    @Autowired
    PersonRepository personRepository;
    @Autowired
    ObservableTxFactory observableTxFactory;

    public Observable<Person> createPerson(String firstName, String lastName) {
        return observableTxFactory.create(s -> {
            Person p = new Person(firstName, lastName);
            s.onNext(personRepository.save(p));
        });
    }
}

---

Test:

@RunWith(SpringJUnit4ClassRunner.class)
@SpringApplicationConfiguration(classes = {RepositoryConfiguration.class})
public class PersonServiceTest {
    @Autowired
    PersonRepository personRepository;
    @Autowired
    ObservableTxFactory observableTxFactory;

    @Test
    public void testPersonService() {
        final PersonService service = new PersonService();
        service.personRepository = personRepository;
        service.observableTxFactory = observableTxFactory;

        final Observable<Person> personObservable = service.createPerson("John", "Doe");
        personObservable.subscribe();

        //fetch from DB
        final Person fetchedPerson = StreamSupport.stream(personRepository.findAll().spliterator(), false)
                .filter(p -> p.getFirstName().equals("John") && p.getLastName().equals("Doe"))
                .findFirst()
                .get();

        //should not be null
        assertNotNull(fetchedPerson);
    }

}

Screenshot showing proxy: