I defined an interface to download a file

@GetMapping("/video.mp4")
public Mono<Void> video(ServerHttpResponse response) {
    ZeroCopyHttpOutputMessage zeroCopyResponse = (ZeroCopyHttpOutputMessage) response;
    response.getHeaders().set(HttpHeaders.CONTENT_DISPOSITION, "attachment; filename=video.mp4");
    response.getHeaders().setContentType(MediaType.parseMediaType("video/mpeg4"));
    File file = new File("/Users/Shared/video.mp4");
    return zeroCopyResponse.writeWith(file, 0, file.length());
}

And I defined a test interface to run http requests

@GetMapping("test")
public Mono<Void> a() throws InterruptedException {
    CountDownLatch latch = new CountDownLatch(20);
    ExecutorService executorService = Executors.newFixedThreadPool(5);
    for (int i = 0; i < 20; i++) {
        int index = i;
        executorService.execute(() -> {
            try {
                Request request = new Request.Builder()
                .url("http://127.0.0.1:8080/video.mp4")
                .build();
                Response response = new OkHttpClient().newCall(request).execute();
                if (response.code() == 200) {
                    ResponseBody body = response.body();
                    if (body == null) {
                        throw new DownloadException("Body is null");
                    }
                    System.out.println("s: " + index);
                } else {
                    System.out.println("f: " + index);
                }
            } catch (Throwable e) {
                System.out.println("e: " + index);
                e.printStackTrace();
            } finally {
                latch.countDown();
            }
        });
    }
    latch.await();
    return Mono.empty();
}

But I got timeout exception on several of them

I did some tests

Test Result Conclusion
Use HttpClient instead of OkHttp Timeout exception Http client has no problem
Do not use CountDownLatch Work well I do not know why
New Thread instead of ExecutorService Timeout exception ExecutorService has no problem
Use MVC instead of WebFlux Work well Spring mvc is work well
MVC(run http interface) request WebFlux(download file interface) Timeout exception The interface to download file has problem
WebFlux(run http interface) request MVC(download file interface) Work well The interface to download file has problem

Under the same conditions，use CountDownLatch makes timeout？

So I doubt

  1. WebFlux bug
  2. I write the wrong code to download a file

spring boot version: 2.6.0

Comment From: bclozel

Given your description here, it's hard to understand what's the actual setup (how many applications are there and what's the relationship between them, libraries versions, etc). I must say that the CountDownLatch + blocking client seems odd, as WebClient with flatMap + then operators will do a much better job there.

In my opinion, looking at the conclusions of your experiments, so far this points to a setup issue on the client side more than a WebFlux bug: each service can work well independently in several scenarios. If the goal is to somehow stress test an existing service, tools like Gatling, JMeter or any other similar tool will do a much better job. You can also independently test your client on a different testing service that serves static files.

I'm closing this issue as there's no evidence of a WebFlux bug. If you need more assistance, feel free to ask on StackOverflow - don't forget to provide more details there (and a link to a sample people can take a look at), as the current description makes it hard to reproduce the behavior you're seeing.