Spring Webclient Flux fire and forget while keeping the flux

What I would like to achieve

• For each element of a Flux of MyPojo Flux<MyPojo>, send a http request with the json payload to a third party web service, fire and forget style, while being able to return the Flux for follow up processing.

Information

• the third party web server, which I have no control over, takes 5 seconds constant time to process one request.

• The third party server does not offer any bulk or list API, it is one by one

• The third party server is known to be very reliable, I am allowed to hit it as hard as possible

• I do not care that much about the response, especially, there is no value in waiting 5 seconds for a response I will do nothing with.

What I have tried

• Using this code:

    private Flux<MyPojo> sendMyPojoFireAndForget(Flux<MyPojo> myPojoFlux) {
        return myPojoFlux.flatMap(oneMyPojo -> webClient.post().uri("http://example.com/path").bodyValue(oneMyPojo).exchangeToMono(clientResponse -> Mono.just(oneMyPojo)));
    }

However, this is not fire and forget. With proper logs on my side and seeing some logs from the third party, it seems I am waiting for the response.

I also tried

webClient.post()
            .uri("http://example.com/path")
            .bodyValue(oneMyPojo)
            .retrieve()
            .bodyToMono(Void.class)
            .subscribe();

But this will make me "lose" the Flux of MyPojo and end up with a Flux of Void instead

Question:

• How to send each element of the Flux to this external webserver fire and forget style, while retaining the entire flux for downstream processing?

You can use multiple hooks for an observable (such as a Mono or a Flux): in this case, you can use the doOnNext() before subscribing:

@Test
void test() {
    Flux<MyPojo> myPojoFlux = Flux.just(
            new MyPojo(1),
            new MyPojo(2),
            new MyPojo(3),
            new MyPojo(4),
            new MyPojo(5)
    );

    myPojoFlux
      .doOnNext(this::sendPostRequest)
      .subscribe(this::doSomethingElse);
}

private void doSomethingElse(MyPojo myPojo) {
    System.out.println(myPojo);
}

private void sendPostRequest(MyPojo oneMyPojo) {
    webClient.get()
            .uri(...)
            .bodyValue(oneMyPojo)
            .retrieve()
            .bodyToMono(String.class)
            .subscribe(resp -> System.out.println(oneMyPojo + " " + resp));
}

Since the client works asynchronously if you don't block , the doSomethingElse method will be called regardless of the response of sendPostRequest