Difference Between Flux.create and Flux.generate

Question

What is the difference between Flux.create and Flux.generate? I am looking--ideally with an example use case--to understand when I should use one or the other.

Answer 1

In short:

Flux::create doesn't react to changes in the state of the app while Flux::generate does.

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The long version

Flux::create

You will use it when you want to calculate multiple (0...infinity) values which are not influenced by the state of your app and the state of your pipeline (your pipeline == the chain of operations which comes after Flux::create == downstream).

Why? Because the method which you sent to Flux::create keeps calculating elements (or none). The downstream will determine how many elements (elements == next signals) it wants and if he can't keep up, those elements which are already emitted will be removed/buffered in some strategy (by default they will be buffered until the downstream will ask for more).

The first and easiest use case is for emitting values which you, theoretically, could sum to a collection and only then take each element and do something with it:

Flux articlesFlux = Flux.create((FluxSink sink) -> {
    /* get all the latest article from a server and
       emit them one by one to downstream. */
    List articals = getArticalsFromServer();
    articals.forEach(sink::next);
});

As you can see, Flux.create is used for interaction between blocking method (getArticalsFromServer) to asynchronous code.

I'm sure there are other use cases for Flux.create.

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Flux::generate

Flux.generate((SynchronousSink synchronousSink) -> {
        synchronousSink.next(1);
    })
    .doOnNext(number -> System.out.println(number))
    .doOnNext(number -> System.out.println(number + 4))
    .subscribe();

The output will be 1 5 1 5 1 5................forever

In each invocation of the method you sent to Flux::generate, synchronousSink can only emits: onSubscribe onNext? (onError | onComplete)?.

It means that Flux::generate will calculate and emit values on demand. When should you use it? In cases where it's too expensive to calculate elements which may not be used downstream or the events which you emit are influenced by the state of the app or from the state of your pipeline (your pipeline == the chain of operations which comes after Flux::create == downstream).

For example, if you are building a torrent application then you are receiving blocks of data in real time. You could use Flux::generate to give tasks (blocks to download) to multiple threads and you will calculate the block you want to download inside Flux::generate only when some thread is asking. So you will emit only blocks you don't have. The same algorithm with Flux::create will fail because Flux::create will emit all the blocks we don't have and if some blocks failed to be downloaded then we have a problem. because Flux::create doesn't react to changes in the state of the app while Flux::generate does.