Project Reactor - subscribe on parallel scheduler doesn't work

Question

I'm looking at examples and reading documentation and I've found some problems while trying to subscribe on Flux in a parallel manner.

I have a 3 functions, as below.

private val log = LoggerFactory.getLogger("main")
private val sequence = Flux.just(1, 2)

fun a() {
    sequence.subscribeOn(Schedulers.parallel()).subscribe { log.info("*** {}", it) }
    sequence.subscribe { log.info(">>> {}", it) }
}

fun b() {
    sequence.subscribe { log.info(">>> {}", it) }
}

fun c() {
    sequence.subscribeOn(Schedulers.parallel()).subscribe { log.info("*** {}", it) }
}

Now, when I run each method separately I have a proper output from functions a() and b(), but output from c() is empty. Is that to be expected, is it by design? If so, why is that happening?

Of course, you're using `just` directly without using `defer` — EpicPandaForce, Aug 18 '18 at 08:03
I'm just following core reference docs and early examples while trying to experiment a little bit. Since you clearly know explanation, please, make it into an answer. A `defer` doesn't tell me much at this point. — wst, Aug 18 '18 at 12:20
Actually I'm basing this on RxJava, but if you return `Flux.just` from a `Flux.defer`, you'll be able to schedule it on a different thread, while `just` runs immediately so its execution can't be moved to different thread. I was looking for a construct like `Observable.fromCallable` but it doesn't seem to exist, so I think `Flux.defer { Flux.just` would work. — EpicPandaForce, Aug 18 '18 at 14:05
I wrapped as suggested `Flux.defer { Flux.just(1, 2) }`. Method `c()` still doesn't produce output. — wst, Aug 19 '18 at 23:19

score 5 · Accepted Answer · answered Aug 20 '18 at 09:53

Flux.just(...) captures value(s) and thus is optimized to execute immediately in the subscribing Thread.

When you use subscribeOn, you change that subscribing Thread from main to something else, making the just truly asynchronous.

In a(), without a subscribeOn that second just would block the main thread just enough that the test doesn't finish before the asynchronous alternative completes.

In c(), there is no such blocking of the main thread. As a consequence, the test terminates before the asynchronous just has had time to emit anything, and that is why you see no output.

To make that more visible, add a Thread.sleep(10) and you'll see some output.

Project Reactor - subscribe on parallel scheduler doesn't work

1 Answers1