Using side effects in Akka Streams to implement commands received from a websocket - websocket

Question

I want to be able to click a button on a website, have it represent a command, issue that command to my program via a websocket, have my program process that command (which will produce a side effect), and then return the results of that command to the website to be rendered.
The websocket would be responsible for updating state changes applied by different actors that are within the users view.
Example: Changing AI instructions via the website. This modifies some values, which would get reported back to the website. Other users might change other AI instructions, or the AI would react to current conditions changing position, requiring the client to update the screen.
I was thinking I could have an actor responsible for updating the client with changed information, and just have the receiving stream update the state with the changes?
Is this the right library to use? Is there a better method to achieve what I want?

Answer 1

You can use akka-streams and akka-http for this just fine. An example when using an actor as a handler:
package test
import akka.actor.{Actor, ActorRef, ActorSystem, Props, Stash, Status}
import akka.http.scaladsl.Http
import akka.http.scaladsl.model.ws.{Message, TextMessage}
import akka.http.scaladsl.server.Directives._
import akka.stream.scaladsl.{Flow, Sink, Source, SourceQueueWithComplete}
import akka.stream.{ActorMaterializer, OverflowStrategy, QueueOfferResult}
import akka.pattern.pipe
import scala.concurrent.{ExecutionContext, Future}
import scala.io.StdIn
object Test extends App {
implicit val actorSystem = ActorSystem()
implicit val materializer = ActorMaterializer()
implicit def executionContext: ExecutionContext = actorSystem.dispatcher
val routes =
path("talk") {
get {
val handler = actorSystem.actorOf(Props[Handler])
val flow = Flow.fromSinkAndSource(
Flow[Message]
.filter(_.isText)
.mapAsync(4) {
case TextMessage.Strict(text) => Future.successful(text)
case TextMessage.Streamed(textStream) => textStream.runReduce(_ + _)
}
.to(Sink.actorRefWithAck[String](handler, Handler.Started, Handler.Ack, Handler.Completed)),
Source.queue[String](16, OverflowStrategy.backpressure)
.map(TextMessage.Strict)
.mapMaterializedValue { queue =>
handler ! Handler.OutputQueue(queue)
queue
}
)
handleWebSocketMessages(flow)
}
}
val bindingFuture = Http().bindAndHandle(routes, "localhost", 8080)
println("Started the server, press enter to shutdown")
StdIn.readLine()
bindingFuture
.flatMap(_.unbind())
.onComplete(_ => actorSystem.terminate())
}
object Handler {
case object Started
case object Completed
case object Ack
case class OutputQueue(queue: SourceQueueWithComplete[String])
}
class Handler extends Actor with Stash {
import context.dispatcher
override def receive: Receive = initialReceive
def initialReceive: Receive = {
case Handler.Started =>
println("Client has connected, waiting for queue")
context.become(waitQueue)
sender() ! Handler.Ack
case Handler.OutputQueue(queue) =>
println("Queue received, waiting for client")
context.become(waitClient(queue))
}
def waitQueue: Receive = {
case Handler.OutputQueue(queue) =>
println("Queue received, starting")
context.become(running(queue))
unstashAll()
case _ =>
stash()
}
def waitClient(queue: SourceQueueWithComplete[String]): Receive = {
case Handler.Started =>
println("Client has connected, starting")
context.become(running(queue))
sender() ! Handler.Ack
unstashAll()
case _ =>
stash()
}
case class ResultWithSender(originalSender: ActorRef, result: QueueOfferResult)
def running(queue: SourceQueueWithComplete[String]): Receive = {
case s: String =>
// do whatever you want here with the received message
println(s"Received text: $s")
val originalSender = sender()
queue
.offer("some response to the client")
.map(ResultWithSender(originalSender, _))
.pipeTo(self)
case ResultWithSender(originalSender, result) =>
result match {
case QueueOfferResult.Enqueued => // okay
originalSender ! Handler.Ack
case QueueOfferResult.Dropped => // due to the OverflowStrategy.backpressure this should not happen
println("Could not send the response to the client")
originalSender ! Handler.Ack
case QueueOfferResult.Failure(e) =>
println(s"Could not send the response to the client: $e")
context.stop(self)
case QueueOfferResult.QueueClosed =>
println("Outgoing connection to the client has closed")
context.stop(self)
}
case Handler.Completed =>
println("Client has disconnected")
queue.complete()
context.stop(self)
case Status.Failure(e) =>
println(s"Client connection has failed: $e")
e.printStackTrace()
queue.fail(new RuntimeException("Upstream has failed", e))
context.stop(self)
}
}
There are lots of places here which could be tweaked, but the basic idea remains the same. Alternatively, you could implement the Flow[Message, Message, _] required by the handleWebSocketMessages() method by using GraphStage. Everything used above is also described in detail in akka-streams documentation.