Does composite flow make loop? - scala

Question

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

Answer 1

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)