Lightweight way of waiting for a group of asynchro

Question

We're writing some code, in a single blocking method, which calls out to multiple, slow third party services asynchronously. These async calls are wrapped in code that implement the same interface method. We wish to fire off the async calls and wait until they've all returned before returning our blocking method call.

I hope that's clear!

Is there a suitable design pattern / library for implementing this... it must be a fairly common pattern. Thanks in advance.

Answer 1

You could use a CountDownLatch initialized with the number of async calls and have each async handler decrement the latch. The "outer" blocking method would simply "await" for the full countdown, e.g.:

// Untested, Java pseudocode...
public void awaitAllRemoteCalls() {
    final CountDownLatch allDoneSignal = new CountDownLatch(N);
    // For each remote N calls...
    thirdPartyAsyncCall.call(new AsyncHandler(Object remoteData) {
        // Handle the remote data...
        allDoneSignal.countDown();
    });
    allDoneSignal.await();
}

Answer 2

I'm not sure how you're doing things, but I'd have whatever starts the async tasks (preferably by using an Executor) return a Future<?> for each task you start. Then you'd simply need to put all the Future<?>s in a Collection and iterate through it calling get():

List<Future<?>> futures = startAsyncTasks();
for (Future<?> future : futures) {
  future.get();
}
// all async tasks are finished

I've left out exception handling for get() here, but that's the general idea.