Fix ReactiveDiscoveryClient probe() method default implementation

[trajano]

Describe the bug

the .probe method is defined as

default void probe() {
    getServices();
}

and getServices() also returns a Flux, since Flux don't do anything without a subscribe this is practically a no-op

Perhaps the interface should be defined as returning Mono<Void> as in

default Mono<Void> probe() {
    return getServices()
      .then();
}

[OlgaMaciaszek]

Hi @trajano, thanks for reporting it. This is right, however, it's actually done on purpose. The assumption is that the concrete implementations should override it, for example: https://github.com/spring-cloud/spring-cloud-consul/blob/main/spring-cloud-consul-discovery/src/main/java/org/springframework/cloud/consul/discovery/ConsulDiscoveryClient.java#L104 .

[trajano]

The ConsulDiscoveryClient is not a ReactiveDiscoveryClient though.

[OlgaMaciaszek]

Right, that was probably not the best example. There are implementations of ReactiveDiscoveryClient in various projects, such as SimpleReactiveDiscoveryClient and SimpleReactiveDiscoveryClient and that is where appropriate implementation should be added.

[trajano]

Okay but if something like https://github.com/spring-cloud/spring-cloud-commons/blob/main/spring-cloud-commons/src/main/java/org/springframework/cloud/client/discovery/simple/reactive/SimpleReactiveDiscoveryClient.java did implement it with the current signature, what would it mean?

Because it's return value is not a Mono<void> but a void

[OlgaMaciaszek]

Thanks, @trajano - that's right. Makes sense. Will mark it as a bug.

[trajano]

That change would likely trigger a lot of problems especially for those that made custom implementations already.
I would recommend at least just mark the existing method as deprecated first and have a default implementation like

default Mono<Void> reactiveProbe() {
  return Mono.fromRunnable(this::probe);
}