RxJava2 Observable backpressure

Question

recently I realized that I don't understand how RxJava2 backpressure works.

I made small test and I expect that it should fail with MissingBackpressureException exception:

@Test
public void testBackpressureWillFail() {
    Observable.<Integer>create(e -> {
        for (int i = 0; i < 10000; i++) {
            System.out.println("Emit: " + i);
            e.onNext(i);
        }
        e.onComplete();
    })
    .subscribeOn(Schedulers.newThread())
    .observeOn(Schedulers.computation())
    .doOnNext(i -> {
        Thread.sleep(100);
        System.out.println("Processed:" + i);
    })
    .blockingSubscribe();
}

System out shows next:

Emit: 0
Emit: 1
Emit: 2
...
Emit: 10000

Processed:0
Processed:1
Processed:2
...
Processed:10000

Why it doesn't produce MissingBackpressureException.

I expect that e.onNext(i); will put item into buffer of ObservableObserveOn and after it's size is greater than static final int BUFFER_SIZE = Math.max(16,Integer.getInteger("rx2.buffer-size",128).intValue());

It should throw MissingBackpressureException which doesn't happen. Does the buffer automatically grow? If not where are items stored?

Answer 1

That's because backpressure moved out to Flowableonly with RxJava2, see here.
If you will switch to Flowable with BackpressureStrategy.MISSING you will get the exception.
That also means that in your case you indeed have buffer that automatically grows, from observerOn docs:

Modifies an ObservableSource to perform its emissions and notifications on a specified Scheduler, asynchronously with an unbounded buffer...