How do I limit the events currently being processed in a flatMap process?

Question

Given the following piece of code

public static void main(String[] args) {

    long start = System.currentTimeMillis();

    Flux.<Long>generate(s -> s.next(System.currentTimeMillis() - start))
            .flatMap(DemoApp::delayedAction)
            .doOnNext(l -> System.out.println(l + " -- " + (System.currentTimeMillis() - start)))
            .blockLast(Duration.ofSeconds(3));
}

private static Publisher<? extends Long> delayedAction(Long l) {
    return Mono.just(l).delayElement(Duration.ofSeconds(1));
}

One can see from the output that a large number of events are "processed" in delayedAction concurrently. In this example 256 events get generated within a few milliseconds and then wait about a second until they get emitted again.

I want to limit this number to e.g. 10, how can I do this?

The solution should be independent of what happens inside delayedAction

Background

What is really happening in delayed Action is: I do HTTP requests and starting an unlimited (or very large) amount of requests doesn't sound like a good idea.

Answer 1

There is already a method for this: Flux.flatMap(Function> mapper, int concurrency)

From its documentation:

The concurrency argument allows to control how many Publisher can be subscribed to and merged in parallel.