How to handle sse connection closed? - spring-boot

Question

I have an endpoint streamed as in the sample code block. When streaming, I call an async method through streamHelper.getStreamSuspendCount(). I am stopping this async method in changing state. But I can not access this async method when the browser is closed and the session is terminated. I am stopping the async method in session scope when changing state. But I can not access this async method when the browser is closed and the session is terminated. How can I access this scope when Session is closed?
#RequestMapping(value = "/stream/{columnId}/suspendCount", produces = MediaType.TEXT_EVENT_STREAM_VALUE)
#ResponseBody
public Flux<Integer> suspendCount(#PathVariable String columnId) {
ColumnObject columnObject = streamHelper.findColumnObjectInListById(columnId);
return streamHelper.getStreamSuspendCount(columnObject);
}
getStreamSuspendCount(ColumnObject columnObject) {
...
//async flux
Flux<?> newFlux = beSubscribeFlow.get(i);
Disposable disposable = newFlux.subscribe();
beDisposeFlow.add(disposable); // my session scope variable. if change state, i will kill disposable (dispose()).
...
return Flux.fromStream(Stream.generate(() -> columnObject.getPendingObject().size())).distinctUntilChanged()
.doOnNext(i -> {
System.out.println(i);
}));
}

Answer 1

I think part of the problem is that you are attempting to get a Disposable that you want to call at the end of the session. But in doing so, you are subscribing to the sequence yourself. Spring Framework will also subscribe to the Flux returned by getStreamSuspendCount, and it is THAT subscription that needs to be cancelled for the SSE client to get notified.
Now how to achieve this? What you need is a sort of "valve" that will cancel its source upon receiving an external signal. This is what takeUntilOther(Publisher<?>) does.
So now you need a Publisher<?> that you can tie to the session lifecycle (more specifically the session close event): as soon as it emits, takeUntilOther will cancel its source.
2 options there:
the session close event is exposed in a listener-like API: use Mono.create
you really need to manually trigger the cancel: use MonoProcessor.create() and when the time comes, push any value through it
Here are simplified examples with made up APIs to clarify:
Create
return theFluxForSSE.takeUntilOther(Mono.create(sink ->
sessionEvent.registerListenerForClose(closeEvent -> sink.success(closeEvent))
));
MonoProcessor
MonoProcessor<String> processor = MonoProcessor.create();
beDisposeFlow.add(processor); // make it available to your session scope?
return theFluxForSSE.takeUntilOther(processor); //Spring will subscribe to this
Let's simulate the session close with a scheduled task:
Executors.newSingleThreadScheduledExecutor().schedule(() ->
processor.onNext("STOP") // that's the key part: manually sending data through the processor to signal takeUntilOther
, 2, TimeUnit.SECONDS);
Here is a simulated unit test example that you can run to better understand what happens:
#Test
public void simulation() {
Flux<Long> theFluxForSSE = Flux.interval(Duration.ofMillis(100));
MonoProcessor<String> processor = MonoProcessor.create();
Executors.newSingleThreadScheduledExecutor().schedule(() -> processor.onNext("STOP"), 2, TimeUnit.SECONDS);
theFluxForSSE.takeUntilOther(processor.log())
.log()
.blockLast();
}