switchIfEmpty in Reactor java is not executing

I'm new with Reactor on java and I experience a problem with switchIfEmpty.

My objective is to check an id if valid to perform a findById from a cache variable, and if not exist, i get data from the db repository to update cache variable and perfom some operations. check the code below.

My problem is that when we get Mono.empty, the code inside switchIfEmpty(Mono.defer(()) is never executed in run time (in the two calls of switchIfEmpty in the code below) but it is working in a UT test

// Verification of the cache
    Mono<Param> ParamMono = Optional.ofNullable(data.get(id)).map(Mono::just).orElseGet(Mono::empty);
    return ParamMono.flatMap(item -> {
        return this.updateParamDB(item, nouveauLibelle);
    })
    .switchIfEmpty(Mono.defer(() -> {
        // Vérification de la BD si élement non existant en cache.
        return paramRepository.findById(id).flatMap(item ->{
            this.updateParamDB(item, nouveauLibelle);
            return Mono.just(Param).subscribeOn(scheduler);
        })
        .switchIfEmpty(Mono.defer(()->{
            // Création de l'objet en si non existant.
            if(StringUtils.isNoneBlank(nouveauLibelle)){
                data.put(id, Param);
                return paramRepository.save(Param);
            }
        return Mono.just(Param).subscribeOn(scheduler);

        }));
    }));

Any idea please?

To determine if there is a subscriber for your flow, you can use both implicit and explicit approaches. Explicitly, you can check by using the subscribe method, while implicitly, you can utilize flatMap/map. It's important to note that invoking onNext alone will not affect the flow or establish a subscription.