Spring Reactor Merge vs Concat

Question

I´m playing with Spring reactor, and I cannot see any differences between concat and merge operator

Here's my example

    @Test
    public void merge() {
        Flux<String> flux1 = Flux.just("hello").doOnNext(value -> {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        Flux<String> flux2 = Flux.just("reactive").doOnNext(value -> {
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        Flux<String> flux3 = Flux.just("world");
        Flux.merge(flux1, flux2, flux3)
                .map(String::toUpperCase)
                .subscribe(System.out::println);
    }

    @Test
    public void concat() {
        Flux<String> flux1 = Flux.just("hello").doOnNext(value -> {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        Flux<String> flux2 = Flux.just("reactive").doOnNext(value -> {
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        Flux<String> flux3 = Flux.just("world");
        Flux.concat(flux1, flux2, flux3)
                .map(String::toUpperCase)
                .subscribe(System.out::println);    
}

Both behave exactly the same. Can someone explain the difference between the two operations?

Answer 1

The essential difference between merge and concat is that in merge, both streams are live. In case of concat, first stream is terminated and then the other stream is concatenated to it.

Concat

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Merge