Project Reactor conditional execution

Question

I have an object to save (to MongoDB), but before it I need to check if some conditions are true.

Object contains IDs to other objects. It looks like

"object": {
   "id": "123",
   "subobject1": { "id": "1" },
   "subobject2": { "id": "2" }
}

Subobjects contain only id, other info is located in other collection, so I have to check is the information exist.

In block-style I can do something like

    if (!languageRepository.exists(Example.of(wordSet.getNativeLanguage())).block()) {
        throw new RuntimeException("Native language doesn't exist");
    }

    if (!languageRepository.exists(Example.of(wordSet.getTargetLanguage())).block()) {
        throw new RuntimeException("Target language doesn't exist");
    }

and only then I can save my object

return wordSetRepository.save(wordSet);

How can I do it in "reactive" style without blocking?

languageRepository.exists, Example.of, wordSet.getTargetLanguage() - what does it all mean? — Alexei Kaigorodov, Apr 16 '18 at 15:21
We go to languageRepository to check if nativeLanguage and targetLanguage (subobject1 and subobject2) exist there. If they exist in languageRepository, we can save this object (wordSet) to wordSetRepository — Вадим Парафенюк, Apr 16 '18 at 15:27
wordSet will be save with only id of nativeLanguage and targetLanguage, not full objects. Full objects are in languageRepository. — Вадим Парафенюк, Apr 16 '18 at 15:28
I mean, what kind of objects are they? using `block()` hints the `languageRepository.exists` is a `Mono`. What are the others? — Alexei Kaigorodov, Apr 16 '18 at 18:34
targetLanguage and nativeLanguage are of type Language (just some fields and id). WordSet contains some fields (include targetLanguage and nativeLanguage) and id. — Вадим Парафенюк, Apr 16 '18 at 19:16
in If I check if this objects are already saved in languageRepository (exists(...).block()). If no - exception. If they exist -> save wordSet (but instead of Languages I save only id of languages in wordset). — Вадим Парафенюк, Apr 16 '18 at 19:18
When I want to get wordset I use aggregation to fetch Languages by id. But when I save WordSet I want to save only id of languages (so existence of Languages in other repository have to be guaranteed) — Вадим Парафенюк, Apr 16 '18 at 19:20

score 14 · Accepted Answer · answered Apr 17 '18 at 11:08

If you want to propagate distinct errors for the native vs target language error cases, you'll need to perform async filtering inside a flatMap:

objectFlux.flatMap(o ->
    Mono.just(o)
        .filterWhen(languageRepository.exists(...)) //native
        .switchIfEmpty(Mono.error(new RuntimeException("Native language doesn't exist"))
        .filterWhen(languageRepository.exists(...)) //target
        .switchIfEmpty(Mono.error(new RuntimeException("Target language doesn't exist"))
    )
    .flatMap(wordSetRepository::save);

The async filtering inside the flatMap ensures that if the test doesn't pass, the inner sequence is empty. This in turn allows us to detect the case and propagate the adequate error. If both tests pass, the original o is propagated in the main sequence.

The second flatMap takes it from there, only receiving the elements that passed both filters and saving them in DB.

Note that the first element to not pass the filters will interrupt the whole sequence (but it was the same in the blocking code since an exception was thrown).

Thanks! One more question: method should have objectFlux as input parameter? I mean that this method is located in service-layer and it takes just "Object" (no Mono — Вадим Парафенюк, Apr 18 '18 at 09:30
both are fine really, typical use of `Mono.just` is to act as the conversion method at the boundary where imperative legacy code meets upgraded reactive code. — Simon Baslé, Apr 23 '18 at 08:41

Project Reactor conditional execution

1 Answers1