Integration with Reactive Streams, materializes into a org.reactivestreams.Publisher.

Sink operators




This method gives you the capability to publish the data from the Sink through a Reactive Streams Publisher. Generally, in Akka Streams a Sink is considered a subscriber, which consumes the data from source. To integrate with other Reactive Stream implementations Sink.asPublisher provides a Publisher materialized value when run. Now, the data from this publisher can be consumed by subscribing to it. We can control if we allow more than one downstream subscriber from the single running Akka stream through the fanout parameter. In Java 9, the Reactive Stream API was included in the JDK, and Publisher is available through Flow.Publisher. Since those APIs are identical but exist at different package namespaces and does not depend on the Reactive Streams package a separate publisher sink for those is available through


In the example we are using a source and then creating a Publisher. After that, we see that when fanout is true multiple subscribers can subscribe to it, but when it is false only the first subscriber will be able to subscribe and others will be rejected.

sourceval source = Source(1 to 5)

val publisher = source.runWith(Sink.asPublisher(false))
Source.fromPublisher(publisher).runWith(Sink.foreach(println)) // 1 2 3 4 5
  .runWith(Sink.foreach(println)) //No output, because the source was not able to subscribe to the publisher.
sourceSource<Integer, NotUsed> source = Source.range(1, 5);

Publisher<Integer> publisherFalse =
    source.runWith(Sink.asPublisher(AsPublisher.WITHOUT_FANOUT), system);
CompletionStage<Integer> resultFromFirstSubscriberFalse =
        .runWith(Sink.fold(0, (acc, element) -> acc + element), system);
CompletionStage<Integer> resultFromSecondSubscriberFalse =
        .runWith(Sink.fold(1, (acc, element) -> acc * element), system);

resultFromFirstSubscriberFalse.thenAccept(System.out::println); // 15
        ::println); // No output, because the source was not able to subscribe to the publisher.

Reactive Streams semantics

emits the materialized publisher

completes after the source is consumed and materialized publisher is created

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