Flux.subscribe finishes before the last element in processed - spring

Question

Strange behavior of Spring + Flux. I have Python server code (using Flask, but that's not important, treat it as pseudo-code) which is streaming response:
def generate():
for row in range(0,10):
time.sleep(1)
yield json.dumps({"count": row}) + '\n'
return Response(generate(), mimetype='application/json')
With that, I simulate processing some tasks from the list and sending me results as soon as they are ready, instead of waiting for everything to be done, mostly to avoid keeping that everything in memory first of the server and then of the client. Now I want to consume that with Spring WebClient:
Flux<Count> alerts = webClient
.post()
.uri("/testStream")
.accept(MediaType.APPLICATION_JSON)
.retrieve()
.bodyToFlux( Count.class )
.log();
alerts.subscribe(a -> log.debug("Received count: " + a.count));
Mono<Void> mono = Mono.when(alerts);
mono.block();
log.debug("All done in method");
Here is what I'm getting in log:
2019-07-03 18:45:08.330 DEBUG 16256 --- [ctor-http-nio-4] c.k.c.restapi.rest.Controller : Received count: 8
2019-07-03 18:45:09.323 INFO 16256 --- [ctor-http-nio-2] reactor.Flux.MonoFlatMapMany.4 : onNext(com.ksftech.chainfacts.restapi.rest.Controller$Count#55d09f83)
2019-07-03 18:45:09.324 INFO 16256 --- [ctor-http-nio-2] reactor.Flux.MonoFlatMapMany.4 : onComplete()
2019-07-03 18:45:09.325 DEBUG 16256 --- [io-28088-exec-4] c.k.c.restapi.rest.Controller : All done in method
2019-07-03 18:45:09.331 INFO 16256 --- [ctor-http-nio-4] reactor.Flux.MonoFlatMapMany.4 : onNext(com.ksftech.chainfacts.restapi.rest.Controller$Count#da447dd)
2019-07-03 18:45:09.332 DEBUG 16256 --- [ctor-http-nio-4] c.k.c.restapi.rest.Controller : Received count: 9
2019-07-03 18:45:09.333 INFO 16256 --- [ctor-http-nio-4] reactor.Flux.MonoFlatMapMany.4 : onComplete()
Notice how last object is processed by subscribe after mono.block returns. I understand that Reactor is asynchronous, and once it sees no more objects, it releases Mono and calls my code in subscribe in parallel. Then it is a mercy of scheduler to see what runs first.
I came up with quite ugly kludge of having subscribe with completeConsumer, and using good old wait/notify. Then it works fine. But is there more elegant way of making sure my method waits until all elements of Flux are processed?

Answer 1

OK, I have studied this area and realized that Reactor is for asynchronous execution. If I need it synchronously, I have to use synchronization. And to have a code which executes after everything has been fed to subscribe, I need to use doOnComplete:
public class FluxResult {
public boolean success = true;
public Exception ex = null;
public void error() {success = false;}
public void error(Exception e) {success = false; ex = e;}
public synchronized void waitForFluxCompletion() throws InterruptedException {
wait();
}
public synchronized void notifyAboutFluxCompletion() {
notify();
}
}
.... // do something which returns Flux
myflux
.doFirst(() -> {
// initialization
})
.doOnError(e -> {
log.error("Exception", e);
})
.doOnComplete(() -> {
try {
// finalization. If we were accumulating objects, now flush them
}
catch (Exception e) {
log.error("Exception", e);
flux_res.error(e);
}
finally {
flux_res.notifyAboutFluxCompletion();
}
})
.subscribe(str -> {
// something which must be executed for each item
});
And then wait for object to be signaled:
flux_res.waitForFluxCompletion();
if (!flux_res.success) {
if (flux_res.ex != null) {