public void demonstrateADetachedGraphStage() throws Exception { // tests: CompletionStage<Integer> result1 = Source.from(Arrays.asList(1, 2, 3)) .via(new TwoBuffer<>()) .runFold(0, (acc, n) -> acc + n, mat); assertEquals(new Integer(6), result1.toCompletableFuture().get(3, TimeUnit.SECONDS)); TestSubscriber.ManualProbe<Integer> subscriber = TestSubscriber.manualProbe(system); TestPublisher.Probe<Integer> publisher = TestPublisher.probe(0, system); RunnableGraph<NotUsed> flow2 = Source.fromPublisher(publisher).via(new TwoBuffer<>()).to(Sink.fromSubscriber(subscriber)); flow2.run(mat); Subscription sub = subscriber.expectSubscription(); // this happens even though the subscriber has not signalled any demand publisher.sendNext(1); publisher.sendNext(2); sub.cancel(); }