Richard Searle

This post demonstrates a solution for the following use case:

1. Connect a server and print all the data received as a UTF8 String.
2. Retry connect every 4 seconds
3. Terminate on user command

The underlying requirement is gathering data from a Terminal Server, where the serial port is exposed as TCP Server.

src is created from BroadcastHub, so all instances of flow are referencing the same instance. It provides the not-yet-completed Source so flow reads data from the server. It terminates flow when its underlying Promise is completed.

The Future exposed by flow._2 completes when the remote server closes the connection, the server is not accessible or the src is completed. A special SENTINEL exception is used to detect the last case and stop the retry loop.

  val (flag, src) = Source.maybe[Nothing].toMat(BroadcastHub.sink(2))(Keep.both).run()

  val SENTINEL = new IOException("stop")

  def make: Unit = {
    val tcpFlow = Tcp().outgoingConnection(new InetSocketAddress("127.0.0.1", 2558))
    val flow = tcpFlow.runWith(src, Sink.foreach(bs => println(bs.utf8String)))
    flow._2.onComplete(_ match {
      case Failure(SENTINEL) => //stop
      case x@_ => system.scheduler.scheduleOnce(4 seconds)(make)
    }
    )
  }

  make //start

  StdIn.readLine("Press enter to stop")
  flag.failure(SENTINEL)