How to make multiple Spring Webclient calls in parallel and wait for the result?

Question

I am new to Reactive programming and I would like to make two API calls in parallel and process the results and return a simple array or list of items.

I have two functions, one returns a Flux and the other returns a Mono and I make a very simple filtering logic on the Flux emitted items depending on the result of that Mono.

I tried to use zipWith but only one item made it to the end no matter what filtering logic. Also I tried with block but that is not allowed inside the controller :/

@GetMapping("/{id}/offers")
fun viewTaskOffers(
        @PathVariable("id") id: String,
        @AuthenticationPrincipal user: UserPrincipal
) : Flux<ViewOfferDTO> {
    data class TaskOfferPair(
        val task: TaskDTO,
        val offer: ViewOfferDTO
    )

    return client.getTaskOffers(id).map {
            it.toViewOfferDTO()
        }.zipWith(client.getTask(id), BiFunction {
            offer: ViewOfferDTO, task: TaskDTO -> TaskOfferPair(task, offer)
        }).filter {
            it.offer.workerUser.id == user.id || it.task.creatorUser == user.id
        }.map {
            it.offer
        }
}

• getTaskOffers returns a Flux of OfferDTO
• getTask returns a Mono of TaskDTO

If you cannot answer my question please tell me atleast how to do multiple API calls in parallel and wait for the results in WebClient

Answer 1

Here is a use case for a parallel call.

public Mono<UserInfo> fetchCarrierUserInfo(User user) {
        Mono<UserInfo> userInfoMono = fetchUserInfo(user.getGuid());
        Mono<CarrierInfo> carrierInfoMono = fetchCarrierInfo(user.getCarrierGuid());

        return Mono.zip(userInfoMono, carrierInfoMono).map(tuple -> {
            UserInfo userInfo = tuple.getT1();
            userInfo.setCarrier(tuple.getT2());
            return userInfo;
        });
    }

Here:

• fetchUserInfo makes http call to get user info from another service and returns Mono
• fetchCarrierInfo method makes HTTP call to get carrierInfo from another service and returns Mono
• Mono.zip() merges given monos into a new Mono that will be fulfilled when all of the given Monos have produced an item, aggregating their values into a Tuple2.

Then, call fetchCarrierUserInfo().block() it to get the final result.

Answer 2

As you already figured out, zipWith won't help you there, since it will produce min(a.size, b.size), which will always be 1, in case one of them is Mono.

But since those two are independent, you can simply split them:

val task: Mono<TaskDTO> = client.getTask(id)
val result: Flux<ViewOfferDTO> = 
task.flatMapMany {t ->
        client.getTaskOffers(id).map {offer ->
            t to offer
        }
    }.filter {
        it.second.workerUser.id == user.id || it.first.creatorUser == user.id
    }.map {
        it.second
}

Note that if you want to have pair of elements, you can use built-in Pair.

Also, this check doesn't make much sense, since you have only Mono: it.first.creatorUser

Answer 3

Convert your Mono to a Flux using repeat():

client.getTask(id).cache().repeat();

So your code would become

    return client.getTaskOffers(id).map {
        it.toViewOfferDTO()
    }.zipWith(client.getTask(id).cache().repeat(), BiFunction {
        offer: ViewOfferDTO, task: TaskDTO -> TaskOfferPair(task, offer)
    }).filter {
        it.offer.workerUser.id == user.id || it.task.creatorUser == user.id
    }.map {
        it.offer
    }