Reactive Streams Interop

Dependency

To use Pekko Streams, add the module to your project:

sbt

val PekkoVersion = "1.1.5"
libraryDependencies += "org.apache.pekko" %% "pekko-stream" % PekkoVersion

Maven

<properties>
  <scala.binary.version>2.13</scala.binary.version>
</properties>
<dependencyManagement>
  <dependencies>
    <dependency>
      <groupId>org.apache.pekko</groupId>
      <artifactId>pekko-bom_${scala.binary.version}</artifactId>
      <version>1.1.5</version>
      <type>pom</type>
      <scope>import</scope>
    </dependency>
  </dependencies>
</dependencyManagement>
<dependencies>
  <dependency>
    <groupId>org.apache.pekko</groupId>
    <artifactId>pekko-stream_${scala.binary.version}</artifactId>
  </dependency>
</dependencies>

Gradle

def versions = [
  ScalaBinary: "2.13"
]
dependencies {
  implementation platform("org.apache.pekko:pekko-bom_${versions.ScalaBinary}:1.1.5")

  implementation "org.apache.pekko:pekko-stream_${versions.ScalaBinary}"
}

Overview

Pekko Streams implements the Reactive Streams standard for asynchronous stream processing with non-blocking back pressure.

Since Java 9 the APIs of Reactive Streams has been included in the Java Standard library, under the java.util.concurrent.Flow namespace. For Java 8 there is instead a separate Reactive Streams artifact with the same APIs in the package org.reactivestreams.

Pekko streams provides interoperability for both these two API versions, the Reactive Streams interfaces directly through factories on the regular Source and Sink APIs. For the Java 9 and later built in interfaces there is a separate set of factories in org.apache.pekko.stream.scaladsl.JavaFlowSupportorg.apache.pekko.stream.javadsl.JavaFlowSupport.

In the following samples the standalone Reactive Stream API factories has been used but each such call can be replaced with the corresponding method from JavaFlowSupport and the JDK java.util.concurrent.Flow._java.util.concurrent.Flow.* interfaces.

Note that it is not possible to use JavaFlowSupport on Java 8 since the needed interfaces simply is not available in the Java standard library.

The two most important interfaces in Reactive Streams are the Publisher and Subscriber.

Scala

copysourceimport org.reactivestreams.Publisher
import org.reactivestreams.Subscriber
import org.reactivestreams.Processor

Java

copysourceimport org.reactivestreams.Publisher;
import org.reactivestreams.Subscriber;
import org.reactivestreams.Processor;

Let us assume that a library provides a publisher of tweets:

Scala

copysourcedef tweets: Publisher[Tweet]

Java

copysourcePublisher<Tweet> tweets();

and another library knows how to store author handles in a database:

Scala

copysourcedef storage: Subscriber[Author]

Java

copysourceSubscriber<Author> storage();

Using a Pekko Streams Flow we can transform the stream and connect those:

Scala

copysourceval authors = Flow[Tweet].filter(_.hashtags.contains(pekkoTag)).map(_.author)

Source.fromPublisher(tweets).via(authors).to(Sink.fromSubscriber(storage)).run()

Java

copysourcefinal Flow<Tweet, Author, NotUsed> authors =
    Flow.of(Tweet.class).filter(t -> t.hashtags().contains(PEKKO)).map(t -> t.author);

Source.fromPublisher(rs.tweets()).via(authors).to(Sink.fromSubscriber(rs.storage()));

The Publisher is used as an input Source to the flow and the Subscriber is used as an output Sink.

A Flow can also be converted to a RunnableGraph[Processor[In, Out]] which materializes to a Processor when run() is called. run() itself can be called multiple times, resulting in a new Processor instance each time.

Scala

copysourceval processor: Processor[Tweet, Author] = authors.toProcessor.run()

tweets.subscribe(processor)
processor.subscribe(storage)

Java

copysourcefinal Processor<Tweet, Author> processor = authors.toProcessor().run(system);

rs.tweets().subscribe(processor);
processor.subscribe(rs.storage());

A publisher can be connected to a subscriber with the subscribe method.

It is also possible to expose a Source as a Publisher by using the Publisher-Sink:

Scala

copysourceval authorPublisher: Publisher[Author] =
  Source.fromPublisher(tweets).via(authors).runWith(Sink.asPublisher(fanout = false))

authorPublisher.subscribe(storage)

Java

copysourcefinal Publisher<Author> authorPublisher =
    Source.fromPublisher(rs.tweets())
        .via(authors)
        .runWith(Sink.asPublisher(AsPublisher.WITHOUT_FANOUT), system);

authorPublisher.subscribe(rs.storage());

A publisher that is created with Sink.asPublisher(fanout = false)Sink.asPublisher(AsPublisher.WITHOUT_FANOUT) supports only a single subscription. Additional subscription attempts will be rejected with an IllegalStateException.

A publisher that supports multiple subscribers using fan-out/broadcasting is created as follows:

Scala

copysourcedef alert: Subscriber[Author]
def storage: Subscriber[Author]

Java

copysourceSubscriber<Author> alert();
Subscriber<Author> storage();

Scala

copysourceval authorPublisher: Publisher[Author] =
  Source.fromPublisher(tweets).via(authors).runWith(Sink.asPublisher(fanout = true))

authorPublisher.subscribe(storage)
authorPublisher.subscribe(alert)

Java

copysourcefinal Publisher<Author> authorPublisher =
    Source.fromPublisher(rs.tweets())
        .via(authors)
        .runWith(Sink.asPublisher(AsPublisher.WITH_FANOUT), system);

authorPublisher.subscribe(rs.storage());
authorPublisher.subscribe(rs.alert());

The input buffer size of the operator controls how far apart the slowest subscriber can be from the fastest subscriber before slowing down the stream.

To make the picture complete, it is also possible to expose a Sink as a Subscriber by using the Subscriber-Source:

Scala

copysourceval tweetSubscriber: Subscriber[Tweet] =
  authors.to(Sink.fromSubscriber(storage)).runWith(Source.asSubscriber[Tweet])

tweets.subscribe(tweetSubscriber)

Java

copysourcefinal Subscriber<Author> storage = rs.storage();

final Subscriber<Tweet> tweetSubscriber =
    authors.to(Sink.fromSubscriber(storage)).runWith(Source.asSubscriber(), system);

rs.tweets().subscribe(tweetSubscriber);

It is also possible to use re-wrap Processor instances as a Flow by passing a factory function that will create the Processor instances:

Scala

copysource// An example Processor factory
def createProcessor: Processor[Int, Int] = Flow[Int].toProcessor.run()

val flow: Flow[Int, Int, NotUsed] = Flow.fromProcessor(() => createProcessor)

Java

copysource// An example Processor factory
final Creator<Processor<Integer, Integer>> factory =
    new Creator<Processor<Integer, Integer>>() {
      public Processor<Integer, Integer> create() {
        return Flow.of(Integer.class).toProcessor().run(system);
      }
    };

final Flow<Integer, Integer, NotUsed> flow = Flow.fromProcessor(factory);

Please note that a factory is necessary to achieve reusability of the resulting Flow.

Other implementations

Implementing Reactive Streams makes it possible to plug Pekko Streams together with other stream libraries that adhere to the standard. An incomplete list of other implementations:

• Reactor (1.1+)
• RxJava
• Ratpack
• Slick