How to disable the buffering of messages on an Akka WebSocket server? - scala

Question

I have a very simple Akka WebSocket server that pushes lines from a file to a connected client with an interval of 400ms per line. Everything works fine, except for the fact that the web server seems to buffer messages for about a minute before broadcasting them.
So when a client connects, I see at the server end that every 400ms a line is read and pushed to the Sink, but on the client side I get nothing for a minute and then a burst of about 150 messages (corresponding to a minute of messages).
Is there a setting that I'm overlooking?
object WebsocketServer extends App {
implicit val actorSystem = ActorSystem("WebsocketServer")
implicit val materializer = ActorMaterializer()
implicit val executionContext = actorSystem.dispatcher
val file = Paths.get("websocket-server/src/main/resources/EURUSD.txt")
val fileSource =
FileIO.fromPath(file)
.via(Framing.delimiter(ByteString("\n"), Int.MaxValue))
val delayedSource: Source[Strict, Future[IOResult]] =
fileSource
.map { line =>
Thread.sleep(400)
println(line.utf8String)
TextMessage(line.utf8String)
}
def route = path("") {
extractUpgradeToWebSocket { upgrade =>
complete(upgrade.handleMessagesWithSinkSource(
Sink.ignore,
delayedSource)
)
}
}
val bindingFuture = Http().bindAndHandle(route, "localhost", 8080)
bindingFuture.onComplete {
case Success(binding) ⇒
println(s"Server is listening on ws://localhost:8080")
case Failure(e) ⇒
println(s"Binding failed with ${e.getMessage}")
actorSystem.terminate()
}
}

Answer 1

So the approach with Thread.sleep(400) was wrong. I should've used the .throttle mechanic on sources:
val delayedSource: Source[Strict, Future[IOResult]] =
fileSource
.throttle(elements = 1, per = 400.millis)
.map { line =>
println(line.utf8String)
TextMessage(line.utf8String)
}
This fixed the issue.