CODEWITHSUNDEEP
javarx-java
Shvalb
Shvalb

Reputation: 1933

RxJava - Non Deterministic behaviour when calling Observables methods

I have a problem with the following code:

public void foo(List<Player> players_list) {

    JsonArray playersArr = new JsonArray();

    rx.Observable.from(players_list) // assume players_list is a list contains 2 players
    .concatMap(player -> {

    return getUser(player.getUserId()) // get entity 'User' from DB
    .flatMap(userObj -> {
        User user = ...;

        playersArr.add(new JsonObject()
            .putString("uid", player.getExtUserId())
        );

            return rx.Observable.just(playersArr);
        });
    }).subscribe(playersObj -> {

    }, (exception) -> {
        log.error("", exception);

    }, () -> {

        // at this point I expect 'playersArr' to consist 2 entries!
    });
}

when running this code it seems that its output is non-deterministic, meaning - most of the times I get valid results of 2 entries in JsonArray, but sometimes I get 1 entry.

I'm trying to figure out why??

EDIT:
I tried switching from .flatMap --> .concatMap and it seems to solve the problem but Im not sure it's really a good solution.

Upvotes: 1

Views: 150

Answers (1)

dwursteisen
dwursteisen

Reputation: 11515

I will asume that playerArrs is not be thread safe, if your Observable is executed in a asynchronous context, maybe playerArrs may skip a add call.

Your Observable have some side effect, has you update an object that exist outside the Observable. To avoid this, you can update your code in a way that your Observable will build the JsonArray

public void foo(List<Player> players_list) {

    rx.Observable.from(players_list) // assume players_list is a list contains 2 players
                 .concatMap(player -> getUser(player.getUserId()) // get entity 'User' from DB
                 .map(userObj -> new JsonObject().putString("uid", player.getExtUserId())
                .reduce(new JsonArray(), (seed, acu) -> seed.add(acu))) // build the JsonArray
                .subscribe(playersObj -> { }, 
                           (exception) -> { log.error("", exception); }, 
                          () -> {
    // at this point I expect 'playersArr' to consist 2 entries!
                 });
}

I'm not sure of my code, but it should be close to what you need, I think.

Upvotes: 2

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