Concat Vs Merge

Concat will not have interleave, which means it will wait until the first source finish before moving on to the second.

Merge will have interleave as it will just dispatch whichever one finish first without waiting

Concat

We can use either of these syntaxes

public static Flux<User> concatWithNoInterleave(Flux<User> flux1, Flux<User> flux2) {  
  return flux1.concatWith(flux2); // wait for flux 1 finish before using flux2  
}  
  
public static Flux<User> concatNoInterleave(Flux<User> flux1, Flux<User> flux2) {  
  return Flux.concat(flux1, flux2); // wait for flux 1 finish before using flux2  
}

For example:

public static void mergeFluxMain(BiFunction<Flux<User>, Flux<User>, Flux<User>> callback) throws InterruptedException {  
  CountDownLatch countDownLatch = new CountDownLatch(1);  
  Flux<User> flux1 = Flux.just(  
      new User("austin"),  
      new User("test"),  
      new User("david"),  
      new User("don")  
  ).delayElements(Duration.ofMillis(1000));  
  
  Flux<User> flux2 = Flux.just(  
      new User("partir"),  
      new User("peter")  
  );  
  
  callback.apply(flux1, flux2)  
      .doOnComplete(countDownLatch::countDown)  
      .subscribe(user -> System.out.println(user.username));  
  
  countDownLatch.await();  
}

public static void main(String[] args) throws InterruptedException {  
	//    mergeFluxMain(MergeApp::concatWithNoInterleave);
	// Same as
	mergeFluxMain(MergeApp::concatNoInterleave);
}

This will result into

austin
test
david
don
partir
peter

Because it will wait for flux1 to finish first before emitting to flux2.

Merge

Merge can be use with in both these syntaxes:

public static Flux<User> mergeFluxWithInterleave(Flux<User> flux1, Flux<User> flux2) {  
  return flux1.mergeWith(flux2); // Does not wait for flux1, first come first serve  
}

public static Flux<User> mergeInterleave(Flux<User> flux1, Flux<User> flux2) {  
  return Flux.merge(flux1, flux2); // Does not wait for flux1, first come first serve  
}

Given the same mergeFluxMain function above, we can call using the following:

  public static void main(String[] args) throws InterruptedException {  
	// mergeFluxMain(MergeApp::mergeFluxWithInterleave);  
	// Same as
    mergeFluxMain(MergeApp::mergeInterleave);  
  }

Which will have

partir
peter
austin
test
david
don

In this case, it will execute whichever stream finishes first, therefore results into interleaving between 2 streams.