How to use FallbackFuture to handle TimeoutException?

Question

I have my task and fallback for it:

ListenableFuture<T> listenableAsyncTask = executorService.submit(asyncTaskCallable);
ListenableFuture<T> listenableFallbackTask = executorService.submit(fallBackTaskCallable);

From them, I form a fail tolerant ListenableFuture:

ListenableFuture<T> failTolerantListenableFuture = Futures.withFallback(listenableAsyncTask, new FutureFallback<T>() {
                @Override
                public ListenableFuture<T> create(Throwable t) throws Exception {
                    return listenableFallbackTask;
                }
            });

And I have a list of fail tolerant futures:

List<ListenableFuture<T>> listenableFutures = ...;

It's time to get the result, in a certain amount of time:

result = Futures.allAsList(listenableFutures).get(50,TimeUnit.MILLISECONDS);

At this point, I expect that if a task failed to finish within 50ms, the return output will be handled by the fallBackTask, which is a lightweight one.

But not as I planed, I got the following exception:

java.util.concurrent.TimeoutException: Timeout waiting for task.

Which causes me losing all results from other succeed tasks. It seems that the fallback didn't work in this case for me. Or I misunderstood the concept?

ColinD · Accepted Answer · 2016-04-12 17:22:15Z

We need to distinguish between "the Future fails" and "the call to Future.get fails."

"The Future fails" if the task you submitted throws an exception. (For purposes of withFallback, we also consider cancellation to be a failure. That's not relevant here, though, and the behavior may change someday.)
"The call to Future.get fails" if any of the following happen:
- the Future fails
- the call times out
- the call is interrupted

withFallback handles only the case in which the Future fails, without handling cases of timeout or interruption.

If your goal is to retrieve all the primary results that are done within 50 milliseconds, with all other cases falling back to the secondary results, you can try something like this, which uses withTimeout to automatically fail the Future after a given timeout:

List<ListenableFuture<T>> originalFutures = ...;
List<ListenableFuture<T>> defaultAfterTimeoutFutures = new ArrayList<>();
for (ListenableFuture<T> f : originalFutures) {
  f = Futures.withTimeout(f, 50, MILLISECONDS, executor);
  f = Futures.withFallback(f, ...);
  defaultAfterTimeoutFutures.add(f);
}
result = Futures.allAsList(defaultAfterTimeoutFutures).get();

But note that that last get call may wait longer than 50 milliseconds: If a primary Future fails, then the get call must wait until its fallback is done. If you don't want to wait for the fallbacks, then you will need to wrap them with withTimeout, as well. And if you do wrap them, then they will fail after the timeout, at which point allAsList will also fail. If you don't want that, then you'll need to either use successfulAsList (instead of allAsList) or wrap the wrappers with withFallback again, this time with a value that is always available immediately.

Thank you a lot for the clear explanation on WithFallback wrapping :)

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