Amount of flatMap executions depends on observer pull, while single is needed - reactive-programming

Question

I have a push notification mechanism (PublishSubject) which triggers http request logic (flatMap). Basic scenario is that whenever a push arrives, single http call is made and results propagated to multiple observers.
I've written a simple demo for the case but flatMap executes for each registered observer, while I would like it to be triggered just once on each push.
PublishSubject<Integer> subject = PublishSubject.create();
Observable<String> obs = subject.asObservable().flatMap(integer -> {
// this code runs for each observer, which is twice in this case
return Observable.just(String.valueOf(integer));
});
Observer mock = mock(Observer.class);
Observer mock1 = mock(Observer.class);
obs.subscribe(mock);
obs.subscribe(mock1);
subject.onNext(1);
Could you suggest a fix?
Thanks
P.S. Right now I'm using cache(1) to fix the issue but I'm not sure if it's ok to do it this way. Moreover, I can't just quite understand why a single stream of execution would depend on amount of observers attached. Can you comment on that?

Answer 1

You are already sort of using it, you need either publish() + connect() or publish().refCount() if you want to make the value shareable by multiple observables. The first case lets you control when to actually make the Observable go hot, while the second will go live as soon as you subscribe the first time. RxJS also has share which wraps publish().refCount() not sure if RxJava has that as well.
PublishSubject<Integer> subject = PublishSubject.create();
Observable<String> obs = subject.asObservable().flatMap(integer -> {
// this code runs for each observer, which is twice in this case
return Observable.just(String.valueOf(integer));
}).publish().refCount();
Observer mock = mock(Observer.class);
Observer mock1 = mock(Observer.class);
obs.subscribe(mock);
obs.subscribe(mock1);
subject.onNext(1);