Subscription to UnicastProcessor never triggers - spring

Question

I wish to batch and process items as they come along so i created a UnicastProcessor and subscribed to it like this
UnicastProcessor<String> processor = UnicastProcessor.create()
processor
.bufferTimeout(10, Duration.ofMillis(500))
.subscribe(new Subscriber<List<String>>() {
#Override
public void onSubscribe(Subscription subscription) {
System.out.println("OnSubscribe");
}
#Override
public void onNext(List<String> strings) {
System.out.println("OnNext");
}
#Override
public void onError(Throwable throwable) {
System.out.println("OnError");
}
#Override
public void onComplete() {
System.out.println("OnComplete");
}
});
And then for testing purposes i created a new thread and started adding items in a loop
new Thread(() -> {
int limit = 100
i = 0
while(i < limit) {
++i
processor.sink().next("Hello $i")
}
System.out.println("Published all")
}).start()
After running this (and letting the main thread sleep for 5 seconds) i can see that all item have been published, but the subscriber does not trigger on any of the events so i can't process any of the published items.
What am I doing wrong here?

Answer 1

Reactive Streams specification is the answer!
The total number of onNext´s signalled by a Publisher to a Subscriber
MUST be less than or equal to the total number of elements requested
by that Subscriber´s Subscription at all times. [Rule 1.1]
In your example, you just simply provide a subscriber who does nothing in any sense. In turn, Reactive Streams specification, directly says that nothing will happen (there will be no onNext invocation) if you have not called Subscription#request method
A Subscriber MUST signal demand via Subscription.request(long n) to
receive onNext signals. [Rule 2.1]
Thus, to fix your problem, one of the possible solutions is changing the code in the following way:
UnicastProcessor<String> processor = UnicastProcessor.create()
processor
.bufferTimeout(10, Duration.ofMillis(500))
.subscribe(new Subscriber<List<String>>() {
#Override
public void onSubscribe(Subscription subscription) {
System.out.println("OnSubscribe");
subscription.request(Long.MAX_VALUE);
}
#Override
public void onNext(List<String> strings) {
System.out.println("OnNext");
}
#Override
public void onError(Throwable throwable) {
System.out.println("OnError");
}
#Override
public void onComplete() {
System.out.println("OnComplete");
}
});
Note, in this example demand in size Long.MAX_VALUE means an unbounded demand so that all messages will be directly pushed to the given Subscriber [Rule 3.17]
Use UnicatProcessor correctly
On the one hand, your example will work correctly with mentioned fixes. However, on the other hand, each invocation of FluxProcessor#sink() (yeah sink is FluxProcessor's method) will lead to a redundant calling of UnicastProcessor's onSubscribe method, which under the hood cause a few atomic reads and writes which might be avoided if create FluxSink once and safely use it as many tame as needed. For example:
UnicastProcessor<String> processor = UnicastProcessor.create()
FluxSink<String> sink = processor.serialize().sink();
...
new Thread(() -> {
int limit = 100
i = 0
while(i < limit) {
++i
sink.next("Hello $i")
}
System.out.println("Published all")
}).start()
Note, in this example, I executed an additional method serialize which provide thread-safe sink and ensure that the calling of FluxSink#next concurrently will not cause a violation of the ReactiveStreams spec.