Keep calling a rest API until server completes the task

Question

I am calling a REST API synchronously from a Spring boot app (in JAVA 11) to download a file.

This is how the REST API looks like:

@GetMapping("/download/{userId}/{fileName}")
public String download(final int userId, final String fileName) {
        if(requested file is available) {
             return {file location URL}
        } else {
             Begin generating a new file (takes 5 seconds to more than 5 minutes)
             return "file generation in progress" 
        } 
    }

The API returns two things, either it returns the status of the file if it is generating the file (which takes anywhere from 5 seconds to 5 minutes or more), or the file location URL itself if the file is generated.

The problem I am trying to solve is, I need to call this "/download" API every 5 seconds and check if the file is ready or not.

Something like this:

First call the "/download" -> API returns "file generation in progress" status
so, call it again after 5 seconds -> API returns the "file generation in progress" status again
so, call it again after 5 seconds -> API returns the "file generation in progress" status again (But by this time assume that the file is generated)
so, call it another time after 5 seconds -> API returns the file location URL
Stop calling the API and do the next things

Is there a way I can achieve this? I tried looking CompletableFuture & @Scheduled but I am so new to both of these options and couldn't find any way to implement them in my code. Any help would be appreciated!

Could you just call the API and check the how much is the download going, like percentage? check this : https://stackoverflow.com/questions/22273045/java-getting-download-progress — PatrickChen, May 10 '20 at 05:43

score 1 · Answer 1 · answered May 10 '20 at 06:22

1

You can make use of the Java Failsafe Library. Refer the answer from [Retry a method based on result (instead of exception). I have modified the code from the answer to match your scenario.

private String downloadFileWithRetry() {
    final RetryPolicy<String> retryPolicy = new RetryPolicy<String>()
        .withMaxAttempts(-1)
        .handleResultIf("file generation in progress"::equalsIgnoreCase);

    return Failsafe
        .with(retryPolicy)
        .onSuccess(response -> System.out.println("Generated file Url is ".concat(response.getResult())))
        .get(this::downloadFile);
}

private String downloadFile() {
    return "file generation in progress";
}

answered May 10 '20 at 06:22

Ramachandran Murugaian

591
3
11

Thanks, Ramachandran. I am trying your solution out and it does call the API on repeat, but if the API throws an exception, Failsafe doesn't break out of the loop. I am trying methods like Failsafe.abortIf() and Failsafe.abortWhen() but they aren't working for me. – AMagic May 12 '20 at 03:26
@AMagic make use of handleIf method like `.handleIf(it -> false)` – Ramachandran Murugaian May 12 '20 at 04:55

Keep calling a rest API until server completes the task

1 Answers1