How to use delayElements() with Flux merge? - spring-webflux

Question

I am following a tutorial and I believe my code is same with the instructor's one but I did not understand why delayElements() is not working.
Here is the caller method:
public static void main(String[] args) {
FluxAndMonoGeneratorService fluxAndMonoGeneratorService = new FluxAndMonoGeneratorService();
fluxAndMonoGeneratorService.explore_merge()
.doOnComplete(() -> System.out.println("Completed !"))
.onErrorReturn("asdasd")
.subscribe(System.out::println);
}
If I write the method without delay elements as:
public Flux<String> explore_merge() {
Flux<String> abcFlux = Flux.just("A", "B", "C");
Flux<String> defFlux = Flux.just("D", "E", "F");
return Flux.merge(abcFlux, defFlux);
}
Then the output in the console is(as expected):
00:53:19.443 [main] DEBUG reactor.util.Loggers$LoggerFactory - Using Slf4j logging framework
A
B
C
D
E
F
Completed !
BUILD SUCCESSFUL in 1s
But I want to use delayElements() to test merge() method as:
public Flux<String> explore_merge() {
Flux<String> abcFlux = Flux.just("A", "B", "C").delayElements(Duration.ofMillis(151));
Flux<String> defFlux = Flux.just("D", "E", "F").delayElements(Duration.ofMillis(100));
return Flux.merge(abcFlux, defFlux);
}
And nothing happens, not onComplete nor onErrorReturn and the output is just nothing:
0:55:22: Executing ':reactive-programming-using-reactor:FluxAndMonoGeneratorService.main()'...
> Task :reactive-programming-using-reactor:generateLombokConfig UP-TO-DATE
> Task :reactive-programming-using-reactor:compileJava
> Task :reactive-programming-using-reactor:processResources NO-SOURCE
> Task :reactive-programming-using-reactor:classes
> Task :reactive-programming-using-reactor:FluxAndMonoGeneratorService.main()
00:55:23.715 [main] DEBUG reactor.util.Loggers$LoggerFactory - Using Slf4j logging framework
BUILD SUCCESSFUL in 1s
What is the reason for this ? (I mean at least onError I am expecting but nothing...)
note: mergeWith() is also not working with this delayElements()

Answer 1

subscribe is not blocking operation and delayElements will be scheduled on another thread (by default parallel Scheduler). As result your program exits before elements are emitted. Here is a test
#Test
void mergeWithDelayElements() {
Flux<String> abcFlux = Flux.just("A", "B", "C").delayElements(Duration.ofMillis(151));
Flux<String> defFlux = Flux.just("D", "E", "F").delayElements(Duration.ofMillis(100));
StepVerifier.create(Flux.merge(abcFlux, defFlux))
.expectNext("D", "A", "E", "B", "F", "C")
.verifyComplete();
}