Creating a Source out of an EventStream

Question

I am using PlayFramework 2.5.3 and would like to create an akka.stream.scaladsl.Source from an akka.event.EventStream (the eventstream is part of an actor system). The event stream would produce events from a certain type, so I would need to subscribe to that certain type of events and push them by using play.api.mvc.Results.chunked. Is there any easy way to create such a Source using Akka Streams 2.4.5?

Answer 1

You can use Source.actorRef together with the subscription. Source.actorRef is a source which materializes into an ActorRef, so you can do this:

// choose the buffer size of the actor source and how the actor
// will react to its overflow
val eventListenerSource = Source.actorRef[YourEventType](32, OverflowStrategy.dropHead)

// run the stream and obtain all materialized values
val (eventListener, ...) = eventListenerSource
    .viaMat(...)(Keep.left)
    <...>
    .run()

// subscribe the source actor to the stream
actorSystem.eventStream.subscribe(eventListener, classOf[YourEventType])

// now events emitted by the source will go to the actor
// and through it to the stream

Note that actorRef source is somewhat limited, for example, it naturally does not support backpressure overflow strategy for its internal buffer. You may want to use Source.actorPublisher with an actor which extends ActorPublisher[YourEventType] trait, it will give you a bit more control. However, since EventStream is a pure push-based source, you won't be able to do much more with ActorPublisher than with Source.actorRef, so you may just as well use the simpler approach.