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scalaakkaakka-stream

Does composite flow make loop?

I am trying to understand, how the following code snippet works: 

val flow: Flow[Message, Message, Future[Done]] =
      Flow.fromSinkAndSourceMat(printSink, helloSource)(Keep.left)

Two guys gave a very wonderful explanation on this thread. I understand the concept of the Composite flow, but how does it work on the websocket client.

Consider the following code:

import akka.actor.ActorSystem
import akka.{ Done, NotUsed }
import akka.http.scaladsl.Http
import akka.stream.ActorMaterializer
import akka.stream.scaladsl._
import akka.http.scaladsl.model._
import akka.http.scaladsl.model.ws._

import scala.concurrent.Future

object SingleWebSocketRequest {
  def main(args: Array[String]) = {
    implicit val system = ActorSystem()
    implicit val materializer = ActorMaterializer()
    import system.dispatcher

    // print each incoming strict text message
    val printSink: Sink[Message, Future[Done]] =
      Sink.foreach {
        case message: TextMessage.Strict =>
          println(message.text)
      }

    val helloSource: Source[Message, NotUsed] =
      Source.single(TextMessage("hello world!"))

    // the Future[Done] is the materialized value of Sink.foreach
    // and it is completed when the stream completes
    val flow: Flow[Message, Message, Future[Done]] =
      Flow.fromSinkAndSourceMat(printSink, helloSource)(Keep.left)

    // upgradeResponse is a Future[WebSocketUpgradeResponse] that
    // completes or fails when the connection succeeds or fails
    // and closed is a Future[Done] representing the stream completion from above
    val (upgradeResponse, closed) =
      Http().singleWebSocketRequest(WebSocketRequest("ws://echo.websocket.org"), flow)

    val connected = upgradeResponse.map { upgrade =>
      // just like a regular http request we can access response status which is available via upgrade.response.status
      // status code 101 (Switching Protocols) indicates that server support WebSockets
      if (upgrade.response.status == StatusCodes.SwitchingProtocols) {
        Done
      } else {
        throw new RuntimeException(s"Connection failed: ${upgrade.response.status}")
      }
    }

    // in a real application you would not side effect here
    // and handle errors more carefully
    connected.onComplete(println)
    closed.foreach(_ => println("closed"))
  }
}

It is a websocket client, that send a message to the websocket server and the printSink receives it and print it out.

How can it be, that printSink receives messages, there is no a connection between the Sink and Source.

Is it like a loop?

enter image description here

Stream flow is from left to right, how it comes that the Sink can consume messages from websocket server?

Solution

  • Flow.fromSinkAndSourceMat puts an independent Sink and a Source to a shape of the Flow. Elements going into that Sink do not end up at the Source.

    From the Websocket client API perspective, it needs a Source from which requests will be sent to the server and a Sink that it will send the responses to. The singleWebSocketRequest could take a Source and a Sink separately, but that would be a bit more verbose API.

    Here is a shorter example that demonstrates the same as in your code snippet but is runnable, so you can play around with it:

    import akka._
import akka.actor._
import akka.stream._
import akka.stream.scaladsl._

implicit val sys = ActorSystem()
implicit val mat = ActorMaterializer()

def openConnection(userFlow: Flow[String, String, NotUsed])(implicit mat: Materializer) = {
  val processor = Flow[String].map(_.toUpperCase)
  processor.join(userFlow).run()
}

val requests = Source(List("one", "two", "three"))
val responses = Sink.foreach(println)
val userFlow = Flow.fromSinkAndSource(responses, requests)

openConnection(userFlow)