How to use an Akka Streams SourceQueue with PlayFramework

Question

I would like to use a SourceQueue to push elements dynamically into an Akka Stream source. Play controller needs a Source to be able to stream a result using the chuncked method.
As Play uses its own Akka Stream Sink under the hood, I can't materialize the source queue myself using a Sink because the source would be consumed before it's used by the chunked method (except if I use the following hack).

I'm able to make it work if I pre-materialize the source queue using a reactive-streams publisher, but it's a kind of 'dirty hack' :

def sourceQueueAction = Action{

    val (queue, pub) = Source.queue[String](10, OverflowStrategy.fail).toMat(Sink.asPublisher(false))(Keep.both).run()

    //stupid example to push elements dynamically
    val tick = Source.tick(0 second, 1 second, "tick")
    tick.runForeach(t => queue.offer(t))

    Ok.chunked(Source.fromPublisher(pub))
  }

Is there a simpler way to use an Akka Streams SourceQueue with PlayFramework?

Thanks

Answer 1

The solution is to use mapMaterializedValue on the source to get a future of its queue materialization :

def sourceQueueAction = Action {
    val (queueSource, futureQueue) = peekMatValue(Source.queue[String](10, OverflowStrategy.fail))

    futureQueue.map { queue =>
      Source.tick(0.second, 1.second, "tick")
        .runForeach (t => queue.offer(t))
    }
    Ok.chunked(queueSource)

  }

  //T is the source type, here String
  //M is the materialization type, here a SourceQueue[String]
  def peekMatValue[T, M](src: Source[T, M]): (Source[T, M], Future[M]) = {
    val p = Promise[M]
    val s = src.mapMaterializedValue { m =>
      p.trySuccess(m)
      m
    }
    (s, p.future)
  }

Answer 2

Would like to share an insight I got today, though it may not be appropriate to your case with Play.

Instead of thinking of a Source to trigger, one can often turn the problem upside down and provide a Sink to the function that does the sourcing.

In such a case, the Sink would be the "recipe" (non-materialized) stage and we can now use Source.queue and materialize it right away. Got queue. Got the flow that it runs.