Downsteam Demand Does Not Propagate Upstream After Encoutering SubscribeOn

Question

I have a Spring application that is triggered with an ApplicationReadyEvent as:

  @EventListener(ApplicationReadyEvent.class)
  public void poll() throws InterruptedException {
    Flux.just(1, 2, 3)
        .doOnNext(System.out::println)
        .log()
        .subscribeOn(Schedulers.boundedElastic())
        .log()
        .flatMap(Mono::just)
        .subscribe();
    System.out.println("Thread exiting: " + Thread.currentThread().getName());
    Thread.sleep(Integer.MAX_VALUE);
  }

Here are the logs:

{"timeStamp":"2021-04-21T15:16:31.288+05:30","message":"onSubscribe(FluxSubscribeOn.SubscribeOnSubscriber)","logger":"reactor.Flux.SubscribeOn.2","thread":"main","level":"INFO"}
{"timeStamp":"2021-04-21T15:16:31.289+05:30","message":"request(256)","logger":"reactor.Flux.SubscribeOn.2","thread":"main","level":"INFO"}
Thread exiting: main

The request does seem to propagate above subscribeOn. However, if I don't use subscribeOn, it appears to be working normally as expected.

  @EventListener(ApplicationReadyEvent.class)
  public void poll() throws InterruptedException {
    Flux.just(1, 2, 3)
        .doOnNext(System.out::println)
        .log()
        .flatMap(Mono::just)
        .subscribe();
    System.out.println("Thread exiting: " + Thread.currentThread().getName());
    Thread.sleep(Integer.MAX_VALUE);
  }

Logs:

{"timeStamp":"2021-04-21T15:20:35.370+05:30","message":"| onSubscribe([Fuseable] FluxPeekFuseable.PeekFuseableSubscriber)","logger":"reactor.Flux.PeekFuseable.1","thread":"main","level":"INFO"}
{"timeStamp":"2021-04-21T15:20:35.371+05:30","message":"| request(256)","logger":"reactor.Flux.PeekFuseable.1","thread":"main","level":"INFO"}
1
{"timeStamp":"2021-04-21T15:20:35.371+05:30","message":"| onNext(1)","logger":"reactor.Flux.PeekFuseable.1","thread":"main","level":"INFO"}
2
{"timeStamp":"2021-04-21T15:20:35.371+05:30","message":"| onNext(2)","logger":"reactor.Flux.PeekFuseable.1","thread":"main","level":"INFO"}
3
{"timeStamp":"2021-04-21T15:20:35.372+05:30","message":"| onNext(3)","logger":"reactor.Flux.PeekFuseable.1","thread":"main","level":"INFO"}
{"timeStamp":"2021-04-21T15:20:35.372+05:30","message":"| onComplete()","logger":"reactor.Flux.PeekFuseable.1","thread":"main","level":"INFO"}
Thread exiting: main

What is the reason behind such behaviour?

Answer 1

This problem was caused by a badly implemented forwarding ScheduledExecutorService class that I was using instead of Reactor's default service as:

Schedulers.addExecutorServiceDecorator(
        "forwardingScheduledExecutorService",
        (scheduler, scheduledExecutorService) ->
            new ForwardingScheduledExecutorService(scheduledExecutorService));