Cannot get properly chunked response using Akka streams

Question

Using the example provided in the Play! Framework documentation, I've created a modified version that issues some delayed events, so I could observe them occurring at the same rate in the client side:

public Result playExampleDelayed() {
    Source<ByteString, ?> source = Source.<ByteString> actorRef(5, OverflowStrategy.dropNew())
            .mapMaterializedValue(sourceActor -> {
                for (int i = 0; i < 10; ++i) {
                    Thread.sleep(1000);
                    sourceActor.tell(ByteString.fromString("tick " + i), null);
                }
                sourceActor.tell(new Status.Success(NotUsed.getInstance()), null);
                return null;
            });
    return ok().chunked(source);
}

However, using curl, we get all the events in a single step, when the source is completed.

Using a different source type I can get the intended behavior:

public Result tick() {
    Source<ByteString, ?> source = Source.<ByteString> tick(Duration.create(0, TimeUnit.SECONDS),
            Duration.create(1, TimeUnit.SECONDS), ByteString.fromString("tick"));
    return ok().chunked(source);
}

In this case, I'll get in the console one chunk per second.

According to the Akka documentation, I'd expect the first example to work. What am I doing wrong?

Answer 1

You're using Thread.sleep in your mapMaterializedValue call. This is a synchronous call that happens right after you run() your stream. Blocking in here (e.g. with a Thread.sleep) will block the whole materialization. Therefore, all the messages will be picked up by the actor at the very end of the loop execution.

Bottom line: always steer away from using Thread.sleep when using Akka.

On the contrary, Source.tick uses a scheduler (async, non-blocking), and is therefore a far more performant, robust and elegant solution.