What's the use and meaning of second type para

Question

What's the use and meaning of second type parameter in akka.Source?

Sample Code:-

  def stream: ServiceCall[Source[String, NotUsed], Source[String, NotUsed]]

According to as much code I have seen up to now by default second type parameter is set to akka.NotUsed. But I don't know what's the significance of it.

Answer 1

The second parameter is the materialized value, that is, when you run the source, it's the value that gets returned by the run method to you. All stream shapes in Akka streams have them, that is, sources, sinks, flows, bidiflows, etc. With sinks, it's really obvious, if you're folding the stream, you're going to end up with a single value, that value is given to you through the materialized value, which is a future of the result:

def fold[U, T](zero: U)(f: (U, T) ⇒ U): Sink[T, Future[U]]

For sources though, it's less obvious, but here's one example:

def actorRef[T](bufferSize: Int, overflowStrategy: OverflowStrategy): Source[T, ActorRef]

This is a Source that, when you run it, materializes to an ActorRef. Every message that you send to the actor will be emitted from the source. If you wanted to use this in Lagom, you would do something like this:

def stream = ServiceCall { _ =>
  Source.actorRef[String](16, OverflowStrategy.dropHead)
    .mapMaterializedValue { actor =>
      // send messages here, or maybe pass the actor to somewhere else
      actor ! "Hello world"
      // And return NotUsed so that it now materializes to `NotUsed`, as required by Lagom
      NotUsed
    }
}