Replacing a traditional java for loop logic into reactive programming using spring webflux and webcl-CodePudding

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

CodePudding user response：

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))

CodePudding user response：

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);
        });