At what point does the subscription take place? (spring webflux)

Question

At what point does the spring webflux do the subscription? Everywhere I have read that there must be a subscription otherwise no change happens. In my short time with Spring Webflux, I have never seen a subscribe() neither in the controller or services.

My doubt is also when using flatMap(), map(),... etc.. at what point does the subscription take place?

What I have read does not really resolve my doubts.

public Flux method(){
    ....
    myFlux.flatMap(data -> {
          ....

    }).flatMap(e -> { .... });
}

I know this is an asynchronous issue, but each flatMap runs at the same time?...and so sometimes some data I have noticed is null.

Arnhem

Answer 1

It's the framework (spring-webflux) that subscribes to the returned Mono or Flux. For example if you use Netty (that's the default), then subscription happens here based on my debugging: https://github.com/reactor/reactor-netty/blob/db27625064fc78f8374c1ef0af3160ec3ae979f4/reactor-netty-http/src/main/java/reactor/netty/http/server/HttpServer.java#L962

Also, this article might be of help to understand what happens when: https://spring.io/blog/2019/03/06/flight-of-the-flux-1-assembly-vs-subscription

Zeeba

Thanks, yes I had read the last link (and re-read). It is clear to me that one must subscribe but in the case of the Spring WebFlux, does it take care of it at the end? According to the example I posted, when it subscribes, it performs a flatMap and when it finishes, does it continue with the other one and so on? – Arnhem

There is one subscribe made by the framework and then that subscription is propagated through the whole chain up until it reaches the source. After that the source propagates down the onNext, onComplete (or onError) signals through the opeator chain one after the other. – Inhume

Answer 2

You need to call a .subscribe() or block() function after your flatmap. Here's an example.

Assuming that myFlux is of type Flux, the following will execute the subscription based on the example above myFlux.subscribe(System.out::println);

Here's an explanation on a separate StackOverflow thread.

But in your method function, you are returning a Flux object - so it's up to the consumer of the method() function how it wants to subscribe to the Flux. You shouldn't be trying to subscribe to the Flux from within

Tagmemics

Answer 3

The answer is: it depends.

For example, if this is a Spring Controller method, then it is the framework itself that subscribes to the Mono or Flux.

If it is a method that is triggered from time to time by a Scheduler, then you must explicitly subscribe to the Mono or Flux, otherwise, no processing will take place.

This means that if your application only exposes a REST API and no processing need to be triggered in any other way, then it is very likely that you will never need to explicitly subscribe to a Mono or Flux because Spring will take care of that by you.

Dexter