play-scala-chatroom-example

This is a simple chatroom using Play and Websockets with the Scala API.

This project makes use of dynamic streams from Pekko Streams, notably BroadcastHub and MergeHub. By combining MergeHub and BroadcastHub, you can get publish/subscribe functionality.

The good bit

The flow is defined once in the controller, and used everywhere from the chat action:

import javax.inject._
import play.api.mvc._

import org.apache.pekko.stream.scaladsl._
import scala.concurrent._

class HomeController @Inject()(val controllerComponents: ControllerComponents) extends BaseController {

    private type WSMessage = String

  // chat room many clients -> merge hub -> broadcasthub -> many clients
  private val (chatSink, chatSource) = {

    // Don't log MergeHub$ProducerFailed as error if the client disconnects.
    // recoverWithRetries -1 is essentially "recoverWith"
    val source = MergeHub.source[WSMessage]
      .log("source")
      .recoverWithRetries(-1, { case _: Exception => Source.empty })

    val sink = BroadcastHub.sink[WSMessage]
    source.toMat(sink)(Keep.both).run()
  }

  private val userFlow: Flow[WSMessage, WSMessage, _] = {
    Flow[WSMessage].via(Flow.fromSinkAndSource(chatSink, chatSource)).log("userFlow")
  }

  def chat: WebSocket = {
    WebSocket.acceptOrResult[WSMessage, WSMessage] {
      case rh if sameOriginCheck(rh) =>
        Future.successful(userFlow).map { flow =>
          Right(flow)
        }.recover {
          case e: Exception =>
            Left(InternalServerError("Cannot create websocket"))
        }

      case rejected =>
        Future.successful {
          Left(Forbidden("forbidden"))
        }
    }
  }
}

Prerequisites

You will need JDK 11 and sbt installed.

Running

sbt run

Go to http://localhost:9000 and open it in two different browsers. Typing into one browser will cause it to show up in another browser.

Server backend

By default, the project uses the Pekko HTTP Server backend. To switch to the Netty Server backend, enable the PlayNettyServer sbt plugin in the build.sbt file. In the build.sbt of this project, you'll find a commented line for this setting; simply uncomment it to make the switch. For more detailed information, refer to the Play Framework documentation.

Tributes

This project is originally taken from Johan Andrén's Akka-HTTP version:

Johan also has a blog post explaining dynamic streams in more detail:

• http://markatta.com/codemonkey/blog/2016/10/02/chat-with-akka-http-websockets/