Play Framework - Is it possible to combine use of Promise and WebSocket in Play?

Question

I'm using Play 2.0.1 with Java. Up to this point, I have loaded a page displaying data from a database using Promise. Here's the original Controller code:

public static Result index() {
    // Generate the page
    final MainPage page = new MainPage();

    Promise<MainPage> promiseMainPage = Akka.future(
            new Callable<MainPage>() {
                public MainPage call() throws Exception {
                    page.generate();
                    return page;
                }
            });

    return async(promiseMainPage.map(new Function<MainPage, Result>() {
            @Override
            public Result apply(MainPage mainPage) throws Throwable {
                return ok(views.html.index.render(mainPage));
            }
     }));
}

This all works fine; the promised page does sent to the browser while the server does not block for the database query (executed in page.generate()) to complete. However, now I would like to use WebSocket to update the page with new/modified information retrieved from the DB. So I've used the Chat example to do just that (even simplified, since I would like to use just the outgoing channel: server to client). I've added the following to the end of index.scala.html:

<script type="text/javascript" charset="utf-8">

    $(function() {
    var WS = window['MozWebSocket'] ? MozWebSocket : WebSocket;
    var socket = new WS("@(routes.Application.webSocket().webSocketURL(request))");

    var receiveEvent = function(event) {
        var data = JSON.parse(event.data);

        var connectionStatus = data["connectionStatus"];
        var connectionStatusHtml = '<font color="red">* Not connected</font>';
        if (connectionStatus != undefined) {
            connectionStatusHtml = '<font color="blue">' + connectionStatus + '</font>';
        }

        $('#connectionStatus').html(connectionStatusHtml);
    }

    socket.onmessage = receiveEvent;
    })

</script>

I've updated routes file and created a handler for webSocket() request.

At this point, when I try to browse the page, I get the following error from play:

[error] play - Waiting for a promise, but got an error: null
java.lang.RuntimeException: null
    at play.libs.F$Promise$2.apply(F.java:113) ~[play_2.9.1.jar:2.0.1]
    at akka.dispatch.Future$$anonfun$map$1.liftedTree3$1(Future.scala:625) ~[akka-actor.jar:2.0.1]
    at akka.dispatch.Future$$anonfun$map$1.apply(Future.scala:624) ~[akka-actor.jar:2.0.1]
    at akka.dispatch.Future$$anonfun$map$1.apply(Future.scala:621) ~[akka-actor.jar:2.0.1]
    at akka.dispatch.DefaultPromise.akka$dispatch$DefaultPromise$$notifyCompleted(Future.scala:943) [akka-actor.jar:2.0.1]
    at akka.dispatch.DefaultPromise$$anonfun$tryComplete$1$$anonfun$apply$mcV$sp$4.apply(Future.scala:920) [akka-actor.jar:2.0.1]

This happens at return ok(views.html.index.render(mainPage));. Commenting out the script from HTML file solves this, but of course no WebSocket will open.

Is it possible to combine use of Promise and WebSocket in Play? Perhaps I miss used it?

Answer 1

I do not believe you can use a promise with a web socket. However, you can use Akka to execute a task in the background. For example, the following code should let Akka handling the longer operation (database query) in the background:

public static WebSocket<String> webSocket() {
    return new WebSocket<String>() {
        public void onReady(final WebSocket.In<String> in, final WebSocket.Out<String> out) {
            in.onMessage(new F.Callback<String>() {
                public void invoke(String event) {
                    Akka.system().scheduler().scheduleOnce(
                            Duration.Zero(),
                            new Runnable() {
                                public void run() {
                                    String result = foo(event);
                                    out.write(result);
                                }
                            },
                            Akka.system().dispatcher()
                    );
                }
            });
            //Handle in.onClose();
        }
    };
}