Connection between js and akka-http websockets fails 95% of the time - websocket

Question

I'm trying to setup a basic connection between an akka-http websocket server and simple javascript.
1 out of roughly 20 connections succeeds, the rest fails. I have no idea why the setup of the connection is so unreliable.
Application.scala:
import akka.actor.ActorSystem
import akka.http.scaladsl.Http
import akka.stream.ActorMaterializer
import services.WebService
import scala.concurrent.Await
import scala.concurrent.duration._
import java.util.concurrent.TimeoutException
object Application extends App {
implicit val system = ActorSystem("api")
implicit val materializer = ActorMaterializer()
import system.dispatcher
val config = system.settings.config
val interface = config.getString("app.interface")
val port = config.getInt("app.port")
val service = new WebService
val binding = Http().bindAndHandle(service.route, interface, port)
try {
Await.result(binding, 1 second)
println(s"server online at http://$interface:$port/")
} catch {
case exc: TimeoutException =>
println("Server took to long to startup, shutting down")
system.shutdown()
}
}
WebService.scala:
import actors.{PublisherActor, SubscriberActor}
import akka.actor.{Props, ActorSystem}
import akka.http.scaladsl.model.ws.{Message, TextMessage}
import akka.http.scaladsl.server.Directives
import akka.stream.Materializer
import akka.stream.scaladsl.{Source, Flow}
import scala.concurrent.duration._
class WebService(implicit fm: Materializer, system: ActorSystem) extends Directives {
import system.dispatcher
system.scheduler.schedule(15 second, 15 second) {
println("Timer message!")
}
def route =
get {
pathSingleSlash {
getFromResource("web/index.html")
} ~
path("helloworld") {
handleWebsocketMessages(websocketActorFlow)
}
}
def websocketActorFlow: Flow[Message, Message, Unit] =
Flow[Message].collect({
case TextMessage.Strict(msg) =>
println(msg)
TextMessage.Strict(msg.reverse)
})
}
client side:
<input type="text" id="inputMessage"/><br/>
<input type="button" value="Send!" onClick="sendMessage()"/><br/>
<span id="response"></span>
<script type="application/javascript">
var connection;
function sendMessage() {
connection.send(document.getElementById("inputMessage").value);
}
document.addEventListener("DOMContentLoaded", function (event) {
connection = new WebSocket("ws://localhost:8080/helloworld");
connection.onopen = function (event) {
connection.send("connection established");
};
connection.onmessage = function (event) {
console.log(event.data);
document.getElementById("response").innerHTML = event.data;
}
});
</script>
if the connection to the server fails I get a timeout message after 5 seconds which says the following:
[DEBUG] [07/23/2015 07:59:54.517] [api-akka.actor.default-dispatcher-27] [akka://api/user/$a/flow-76-3-publisherSource-prefixAndTail] Cancelling akka.stream.impl.MultiStreamOutputProcessor$SubstreamOutput#a54778 (after: 5000 ms)
No matter if the connection fails or succeeds, I always get the following log message:
[DEBUG] [07/23/2015 07:59:23.849] [api-akka.actor.default-dispatcher-4] [akka://api/system/IO-TCP/selectors/$a/0] New connection accepted

Answer 1

Look at that error message carefully... it is coming from a source I would not have expected, some "MultiStreamOutputProcessor" when I only expect to handle a single stream.
That tells me - along with the webSocketActorFlow - that maybe you are getting messages and they aren't being caught by the flow, and so you're ending up with substreams you never expected.
So instead of it "only working some of the time," maybe it is "working most of the time but unable to handle all of the input as you have demanded in the flow, and you are left with un-selectable streams that have to die first.
See if you can either a) make sure you get a grip on the streams so you don't end up with stragglers, b) bandaid adjust timeouts, and c) detect such occurences and cancel processing the downstream
https://groups.google.com/forum/#!topic/akka-user/x-tARRaJ0LQ
akka {
stream {
materializer {
subscription-timeout {
timeout=30s
}
}
}
}
http://grokbase.com/t/gg/akka-user/1561gr0jgt/debug-message-cancelling-akka-stream-impl-multistreamoutputprocessor-after-5000-ms