Replacing a traditional java for loop logic into reactive programming using spring webflux and webclient

Question

I'm converting Spring Boot (MVC) application to Reactive Programming (Spring Webflux)

For making external rest calls, I'm using WebClient.

List<String> targetUrls = Arrays.asList("one","two","three");

        int iteration = 1;
        for(String target:targetUrls){

            // make a rest call here 
            // if 200 break the loop
            // else  loop continues
        }

The rest call here returns a Mono.

Is there a way to iterate over the list of urls(here in this case) and break the loop based on some response

Tried flatMap on the Mono and based on response can make a rest call to another url --> This works in case we know before hand the number of urls

thinkgruen · Answer 1 · 2022-06-21T14:37:13.993

0

I'm not sure how you call the urls from the WebClient, but just assuming the URLs are passed to a method WebClient.get(String url), I will continue with the example.

The steps would be somewhat like you will first get a Flux of the URLs, then use flatMap (because your response is a Mono), then use map on these.

Flux.fromIterable(targetUrls)
  .flatMap(webClient::get)
  .map(response -> doSomethingWith(response))

Now you should have a Flux of responses. If the error handling from the WebClient isn't enough, you can also insert something inbetween.

Flux.fromIterable(targetUrls)
  .flatMap(webClient::get)
  .doOnError(errorHandler::handle)
  .map(response -> doSomethingWith(response))

edited Jun 21 '22 at 14:37

answered Jun 21 '22 at 14:25

thinkgruen

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Thanks for your response. I'm a beginner to reactive programming but if I understand correctly the above two examples will execute all the rest calls ( let say 3 rest calls here ) and we have a flux of responses. but my requirement here is to have the second rest call only if the first response is not 200OK. If the first response is 200 OK , break the loop or cancel the flux or do not make further api calls – Sunny Pukkalli Jun 21 '22 at 14:58
I haven't validated for your use case, but I think according to [this explanation and diagram](https://projectreactor.io/docs/core/release/api/reactor/core/publisher/Flux.html#doOnError-java.util.function.Consumer-), any failure will stop processing. You could also use `doOnNext` and then handle the response code on your own. I am not 100% sure if the WebClient treats a 4xx and 5xx as an error already. I think it does. – thinkgruen Jun 21 '22 at 17:19
In case it doesn't [here](https://stackoverflow.com/a/60688375/3393379) are a few more pointers. – thinkgruen Jun 21 '22 at 17:24

score 0 · Answer 2 · answered Jun 21 '22 at 15:09

There are several patterns you could apply to implement this logic

You can use flatMap to execute multiple queries concurrently. By default, flatMap will process Queues.SMALL_BUFFER_SIZE = 256 number of in-flight inner sequences.

Flux.fromIterable(targetUrls)
  .flatMap(url -> webClient.get(url)) 
  .map(response -> process(response))

You could control concurrency flatMap(url -> webClient.get(url), concurrency) or use concatMap operator if you want to execute queries sequentially.

Another useful pattern in case you need response to make decision about next request - use expand operator. For example, in case of paginated requests.

webclient.get(url)
        .expand(response -> {
            if (<should stop>) {
                // stop the loop
                return Mono.empty();
            }

            return webclient.get(url);
        });

Replacing a traditional java for loop logic into reactive programming using spring webflux and webclient

2 Answers2