Simple server-push broadcast flow with Akka - scala

Question

I am struggling to implement a - rather simple - Akka flow.
Here is what I think I need:
I have a single server and n clients and want to be able to react to external events by broadcasting messages (JSON) to the clients. The clients can register/deregister at any time.
So for example:
1 client is registered
server throws an event ("Hello World!")
server broadcasts a "Hello World!" to all clients (one client)
a new client opens a websocket connection
server throws another event ("Hello Akka!")
server broadcasts a "Hello Akka!" to all clients (two clients)
Here is what I have so far:
def route: Route = {
val register = path("register") {
// registration point for the clients
handleWebSocketMessages(serverPushFlow)
}
}
// ...
def broadcast(msg: String): Unit = {
// use the previously created flow to send messages to all clients
// ???
}
// my broadcast sink to send messages to the clients
val broadcastSink: Sink[String, Source[String, NotUsed]] = BroadcastHub.sink[String]
// a source that emmits simple strings
val simpleMsgSource = Source(Nil: List[String])
def serverPushFlow = {
Flow[Message].mapAsync(1) {
case TextMessage.Strict(text) => Future.successful(text)
case streamed: TextMessage.Streamed => streamed.textStream.runFold("")(_ ++ _)
}
.via(Flow.fromSinkAndSource(broadcastSink, simpleMsgSource))
.map[Message](string => TextMessage(string))
}

Answer 1

To be able to use a broadcastHub, you have to define two flows. One that runs your websocket TextMessage to the broadcastHub. You have to run it, it produces a Source that you connect to each client.
Here it is this concept described in simple runnable app.
import akka.NotUsed
import akka.actor.ActorSystem
import akka.stream.ActorMaterializer
import akka.stream.scaladsl.{BroadcastHub, Sink, Source}
import org.slf4j.LoggerFactory
import scala.concurrent.duration._
object BroadcastSink extends App {
private val logger = LoggerFactory.getLogger("logger")
implicit val actorSystem = ActorSystem()
implicit val actorMaterializer = ActorMaterializer()
val broadcastSink: Sink[String, Source[String, NotUsed]] =
BroadcastHub.sink[String]
val simpleMsgSource = Source.tick(500.milli, 500.milli, "Single Message")
val sourceForClients: Source[String, NotUsed] = simpleMsgSource.runWith(broadcastSink)
sourceForClients.to(Sink.foreach(t => logger.info(s"Client 1: $t"))).run()
Thread.sleep(1000)
sourceForClients.to(Sink.foreach(t => logger.info(s"Client 2: $t"))).run()
Thread.sleep(1000)
sourceForClients.to(Sink.foreach(t => logger.info(s"Client 3: $t"))).run()
Thread.sleep(1000)
sourceForClients.to(Sink.foreach(t => logger.info(s"Client 4: $t"))).run()
Thread.sleep(1000)
actorSystem.terminate()
}
Prints
10:52:01.774 Client 1: Single Message
10:52:02.273 Client 1: Single Message
10:52:02.273 Client 2: Single Message
10:52:02.773 Client 2: Single Message
10:52:02.773 Client 1: Single Message
10:52:03.272 Client 3: Single Message
10:52:03.272 Client 2: Single Message
10:52:03.272 Client 1: Single Message
10:52:03.772 Client 1: Single Message
10:52:03.772 Client 3: Single Message
10:52:03.773 Client 2: Single Message
10:52:04.272 Client 2: Single Message
10:52:04.272 Client 4: Single Message
10:52:04.272 Client 1: Single Message
10:52:04.273 Client 3: Single Message
10:52:04.772 Client 1: Single Message
10:52:04.772 Client 2: Single Message
10:52:04.772 Client 3: Single Message
10:52:04.772 Client 4: Single Message
10:52:05.271 Client 4: Single Message
10:52:05.271 Client 1: Single Message
10:52:05.271 Client 3: Single Message
10:52:05.272 Client 2: Single Message
If clients are known in advance, you don't need BrodacastHub and you can use alsoTo method:
def webSocketHandler(clients: List[Sink[Message, NotUsed]]): Flow[Message, Message, Any] = {
val flow = Flow[Message]
clients.foldLeft(flow) {case (fl, client) =>
fl.alsoTo(client)
}
}