WebFlux Adding Blocking Logic

Question

I am wondering how to go about doing some validation checks before creating a new object?

@Override
public Mono<Child> create(CreateChildRequest specs) {

    final String parentId = specs.getParentId();
    // TODO: Check if parent exists
    // parentRepository.getById(parentId) -> returns Mono<Parent>

    final Child newChild = new Child(specs.getParentId(), specs.getName());
    return childRepository.insert(newChild)
            .switchIfEmpty(Mono.error(new ResponseStatusException(HttpStatus.BAD_REQUEST, "Failed to create child")));
}

How to add the validation check in a non blocking way?

depends can the Mono<Parent> contain null? but you should do your parentRepository.getById(parentId) and then chain on it to do the checks and if good then childRepository.insert — Toerktumlare
– Toerktumlare, Commented Jul 9, 2019 at 9:51
Yes im looking for way to chain this. Is flatmap the only way? What if i have many logical checks? It will look ugly if i keep nesting codes inside? — user1955934
– user1955934, Commented Jul 9, 2019 at 10:54

arap · Accepted Answer · 2019-07-09 20:05:08Z

If you need to do only simple, non-blocking checks i.e. checking some fields (or in general - not requiring playing another Mono/Flux), you can do it in doOnNext operator and easily extract to another method. Any exception thrown inside this block will be translated to Mono.error

final String parentId = specs.getParentId();

    parentRepository.getById(parentId)
        .switchIfEmpty(Mono.error(new ResponseStatusException(HttpStatus.BAD_REQUEST, "Parent does not exist")))
        .doOnNext(parent -> {
            if (parent.getSomeField() == null) { //this can be easily extracted for readability
                throw new ResponseStatusException(HttpStatus.BAD_REQUEST, "Some field must not be null");
            }
        })
        .then(Mono.just(new Child(specs.getParentId(), specs.getName()))
                .flatMap(childRepository::insert)
                .switchIfEmpty(Mono.error(new ResponseStatusException(HttpStatus.BAD_REQUEST, "Failed to create child")));

If performing checks need to involve another Mono/Flux (for example calling another webservice), it will require using 'subscribing' operators such as flatMap or zip.

    @Override
    public Mono<Child> create(CreateChildRequest specs) {
        final String parentId = specs.getParentId();

        parentRepository.getById(parentId)
            .switchIfEmpty(Mono.error(new ResponseStatusException(HttpStatus.BAD_REQUEST, "Parent does not exist")))
            .flatMap(parent -> validate(parent))
            .then(Mono.just(new Child(specs.getParentId(), specs.getName()))
                    .flatMap(childRepository::insert)
                    .switchIfEmpty(Mono.error(new ResponseStatusException(HttpStatus.BAD_REQUEST, "Failed to create child")));
                }

            }

Mono<Void> validate(Parent parent){
    //some non blocking io logic ending with Mono.empty or Mono.error
}

this is so much harder than imperative and can really look messy if there are lots of validations and logics 😨

Toerktumlare · Accepted Answer · 2019-07-09 11:09:22Z

Maybe something like this, havn't run the code but you get the gist of it.

@Override
public Mono<Child> create(CreateChildRequest specs) {

    final String parentId = specs.getParentId();
    return parentRepository.getById(parentId)
        .doOnSuccess(parent -> {
            verify(parent).doOnSuccess(() -> {
                childRepository.insert(newChild).doOnError(throwable -> {
                    throw new ResponseStatusException(
                        HttpStatus.BAD_REQUEST,
                        "Failed to create child")
                }).doOnError(throwable -> {
                    // some error handling here if not passing validation.
                })
            })
        })
}

private Mono<Void> verify(Parent parent) {

    if(parent == null)
        return Mono.error(// some error here);
    else
        Mono.empty();
}

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