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!

Answer 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";
}