CODEWITHSUNDEEP
kotlinproject-reactorreactive-streams
wst
wst

Reputation: 4338

Project Reactor - subscribe on parallel scheduler doesn't work

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?

Upvotes: 1

Views: 1274

Answers (1)

Simon Baslé
Simon Baslé

Reputation: 28301

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.

Upvotes: 5

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