The flatMap operator

The flatMap operator works in a similar way to the map, but works asynchronously. This means that it should return an instance that can return a value in the future, such as Flux or Mono. The following example code shows how it can be used:

Flux.fromArray(arrayOf(1, 2, 3))
        .flatMap { Mono.just(it).delayElement(Duration.ofSeconds(1)) }
        .subscribe { println(it) }

The output of this example looks as follows:

 1
 2
 3

Mono is similar to Flux, but it can emit one or zero elements. In this example, we use the delayElement function, which is why each element is received by a subscriber with a one-second delay.

The following diagram shows how it works:

This shows that each flatMap operator passes each value to the downstream asynchronously, with a one-second delay.