Closing an Akka stream from inside a GraphStage (Akka 2.4.2)

Question

In Akka Stream 2.4.2, PushStage has been deprecated. For Streams 2.0.3 I was using the solution from this answer:

which was:

import akka.stream.stage._

    val closeStage = new PushStage[Tpe, Tpe] {
      override def onPush(elem: Tpe, ctx: Context[Tpe]) = elem match {
        case elem if shouldCloseStream ⇒
          // println("stream closed")
          ctx.finish()
        case elem ⇒
          ctx.push(elem)
      }
    }

How would I close a stream in 2.4.2 immediately, from inside a GraphStage / onPush() ?

kiritsuku kiritsuku · Accepted Answer · 2016-02-22T11:11:30

Use something like this:

val closeStage = new GraphStage[FlowShape[Tpe, Tpe]] {
  val in = Inlet[Tpe]("closeStage.in")
  val out = Outlet[Tpe]("closeStage.out")

  override val shape = FlowShape.of(in, out)

  override def createLogic(inheritedAttributes: Attributes) = new GraphStageLogic(shape) {
    setHandler(in, new InHandler {
      override def onPush() = grab(in) match {
        case elem if shouldCloseStream ⇒
          // println("stream closed")
          completeStage()
        case msg ⇒
          push(out, msg)
      }
    })
    setHandler(out, new OutHandler {
      override def onPull() = pull(in)
    })
  }
}

It is more verbose but one the one side one can define this logic in a reusable way and on the other side one no longer has to worry about differences between the stream elements because the GraphStage can be handled in the same way as a flow would be handled:

val flow: Flow[Tpe] = ???
val newFlow = flow.via(closeStage)

Closing an Akka stream from inside a GraphStage (Akka 2.4.2)

2 Answers