How to use ExecutorService to poll until a result arrives

Question

I have a scenario where I have to poll a remote server checking if a task has completed. Once it has, I make a different call to retrieve the result.

I originally figured I should use a SingleThreadScheduledExecutor with scheduleWithFixedDelay for polling:

ScheduledExecutorService executor = Executors.newSingleThreadScheduledExecutor();
ScheduledFuture future = executor.scheduleWithFixedDelay(() -> poll(jobId), 0, 10, TimeUnit.SECONDS);

public void poll(String jobId) {
   boolean jobDone = remoteServer.isJobDone(jobId);
   if (jobDone) {
       retrieveJobResult(jobId);
   }
}

But since I can only provide a Runnable to scheduleWithFixedDelay which can't return anything, I don't understand when the future will be complete, if ever. What does calling future.get() even mean? What result am I waiting for?

The first time I detect the remote task has completed, I want to execute a different remote call and set its result as the value of the future. I figured I could use CompletableFuture for this, that I would forward to my poll method, which would in turn forward it to my retrieveTask method that would eventually complete it:

CompletableFuture<Object> result = new CompletableFuture<Object>();
ScheduledFuture future = executor.scheduleWithFixedDelay(() -> poll(jobId, result), 0, 10, TimeUnit.SECONDS);

public void poll(String jobId, CompletableFuture<Object> result) {
   boolean jobDone = remoteServer.isJobDone(jobId);
   if (jobDone) {
       retrieveJobResult(jobId, result);
   }
}

public void retrieveJobResult(String jobId, CompletableFuture<Object> result) {
    Object remoteResult = remoteServer.getJobResult(jobId);
    result.complete(remoteResult);
}

But this has a ton of issues. For one, CompletableFuture doesn't even seem to be intended for this kind of use. Instead I should be doing CompletableFuture.supplyAsync(() -> poll(jobId)) I think, but how would I then properly shutdown the executor and cancel the future it returned when my CompletableFuture is canceled/complete? It feels like polling should be implemented in some completely different way.

Answer 1

I think CompletableFutures are a fine way to do this:

ScheduledExecutorService executor = Executors.newSingleThreadScheduledExecutor();

private void run() {
    final Object jobResult = pollForCompletion("jobId1")
            .thenApply(jobId -> remoteServer.getJobResult(jobId))
            .get();

}

private CompletableFuture<String> pollForCompletion(final String jobId) {
    CompletableFuture<String> completionFuture = new CompletableFuture<>();
    final ScheduledFuture<Void> checkFuture = executor.scheduleAtFixedRate(() -> {
        if (remoteServer.isJobDone(jobId)) {
            completionFuture.complete(jobId);
        }
    }, 0, 10, TimeUnit.SECONDS);
    completionFuture.whenComplete((result, thrown) -> {
        checkFuture.cancel(true);
    });
    return completionFuture;
}