YEMEAC

    private Mono<UserDto> validateUpdate(Long partyId, UserDto request) {
        return partyClient.getIndividual(partyId) 
                .flatMap(r -> securityClient.getByUserName(request.getUsername()))
                .switchIfEmpty(Mono.just(UserModel.builder().withId(partyId).build())) //dummy to have something to compare with and kee the reactive flow "flowing"
                .flatMap(r -> {
                    if (!r.getId().equals(partyId)) {
                        throw new UserAlreadyExistsException("Username already taken");
                    }
                    return Mono.just(request);
                }

YEMEAC

  curl -X PUT \
    http://127.0.0.1:8500/v1/catalog/register \
    -H 'cache-control: no-cache' \
    -H 'content-type: application/json' \
    -H 'postman-token: 9aa3163f-31b9-8c8f-f9cd-6feaca81d4ce' \
    -d '{
    "Datacenter": "dc1",
    "Node": "externalServices",
    "Address": "172.30.50.59",
    "Service": {

YEMEAC

   private Mono<UserDto> validateUserNameAvailable(UserDto user) {
        Mono<Boolean> result = securityClient.getByUserName(user.getUserName()).hasElement();
        return result.flatMap(value -> {
            if (value) {
                throw new UserAlreadyExistsException(
                        String.format("User {} already exists", user.getUserName()));
            }
            return Mono.just(user);
        });
    }