Chat room | Revel - A Web Application Framework for Go!

The Chat app demonstrates:

Using channels to implement a chat room with a publish-subscribe model.
Using Comet and Websockets

Here are the contents of the app:

chat/app/
	chatroom	       # Chat room routines
		chatroom.go

	controllers
		app.go         # The welcome screen, allowing user to pick a technology
		refresh.go     # Handlers for the "Active Refresh" chat demo
		longpolling.go # Handlers for the "Long polling" ("Comet") chat demo
		websocket.go   # Handlers for the "Websocket" chat demo

	views
		...            # HTML and Javascript

The Chat Room

First, let’s look at how the chat room is implemented, in app/chatroom/chatroom.go.

The chat room runs as an independent go-routine, started on initialization:

func init() {
	go chatroom()
}

The chatroom() function simply selects on three channels and executes the requested action.

var (
	// Send a channel here to get room events back.  It will send the entire
	// archive initially, and then new messages as they come in.
	subscribe = make(chan (chan<- Subscription), 10)
	// Send a channel here to unsubscribe.
	unsubscribe = make(chan (<-chan Event), 10)
	// Send events here to publish them.
	publish = make(chan Event, 10)
)

func chatroom() {
	archive := list.New()
	subscribers := list.New()

	for {
		select {
		case ch := <-subscribe:
			// Add subscriber to list and send back subscriber channel + chat log.
		case event := <-publish:
			// Send event to all subscribers and add to chat log.
		case unsub := <-unsubscribe:
			// Remove subscriber from subscriber list.
		}
	}
}

Let’s see how each of those is implemented.

case ch := <-subscribe:
    var events []Event
    for e := archive.Front(); e != nil; e = e.Next() {
        events = append(events, e.Value.(Event))
    }
    subscriber := make(chan Event, 10)
    subscribers.PushBack(subscriber)
    ch <- Subscription{events, subscriber}

A Subscription is created with two properties:

The chat log (archive)
A channel that the subscriber can listen on to get new messages.

The Subscription is then sent back over the channel that the subscriber supplied.

Publish

case event := <-publish:
    for ch := subscribers.Front(); ch != nil; ch = ch.Next() {
        ch.Value.(chan Event) <- event
    }
    if archive.Len() >= archiveSize {
        archive.Remove(archive.Front())
    }
    archive.PushBack(event)

The published event is sent to the subscribers’ channels one by one. Then the event is added to the archive, which is trimmed if necessary.

Unsubscribe

case unsub := <-unsubscribe:
    for ch := subscribers.Front(); ch != nil; ch = ch.Next() {
        if ch.Value.(chan Event) == unsub {
            subscribers.Remove(ch)
        }
    }

The subscriber channel is removed from the list.

Handlers

Now that you know how the chat room works, we can look at how the handlers expose that functionality using different techniques.

Active Refresh

The Active Refresh chat room javascript refreshes the page every five seconds to get any new messages (see Refresh/Room.html):

// Scroll the messages panel to the end
var scrollDown = function() {
    $('#thread').scrollTo('max');
}

// Reload the whole messages panel
var refresh = function() {
    $('#thread').load('/refresh/room?user={{.user}} #thread .message', function() {
        scrollDown();
    })
}

// Call refresh every 5 seconds
setInterval(refresh, 5000);

This is the handler to serve the above in app/controllers/refresh.go:

func (c Refresh) Room(user string) revel.Result {
	subscription := chatroom.Subscribe()
	defer subscription.Cancel()
	events := subscription.Archive
	for i, _ := range events {
		if events[i].User == user {
			events[i].User = "you"
		}
	}
	return c.Render(user, events)
}

It subscribes to the chatroom and passes the archive to the template to be rendered (after changing the user name to “you” as necessary).

Long Polling (Comet)

The Long Polling chat room (see LongPolling/Room.html) makes an ajax request that the server keeps open until a new message comes in. The javascript uses a lastReceived timestamp to tell the server the last message it knows about.

var lastReceived = 0;
var waitMessages = '/longpolling/room/messages?lastReceived=';
var say = '/longpolling/room/messages?user={{.user}}';

$('#send').click(function(e) {
    var message = $('#message').val();
    $('#message').val('');
    $.post(say, {message: message});
});

// Retrieve new messages
var getMessages = function() {
    $.ajax({
        url: waitMessages + lastReceived,
        success: function(events) {
            $(events).each(function() {
                display(this);
                lastReceived = this.Timestamp;
            });
            getMessages();
        },
        dataType: 'json'
    });
}
getMessages();

The handler for the above in app/controllers/longpolling.go

func (c LongPolling) WaitMessages(lastReceived int) revel.Result {
	subscription := chatroom.Subscribe()
	defer subscription.Cancel()

	// See if anything is new in the archive.
	var events []chatroom.Event
	for _, event := range subscription.Archive {
		if event.Timestamp > lastReceived {
			events = append(events, event)
		}
	}

	// If we found one, grand.
	if len(events) > 0 {
		return c.RenderJSON(events)
	}

	// Else, wait for something new.
	event := <-subscription.New
	return c.RenderJSON([]chatroom.Event{event})
}

In this implementation, it can simply block on the subscription channel, assuming it has already sent back everything in the archive.

Websocket

The Websocket chat room (see WebSocket/Room.html) opens a websocket connection as soon as the user has loaded the page.

// Create a socket
var socket = new WebSocket('ws://127.0.0.1:9000/websocket/room/socket?user={{.user}}');

// Message received on the socket
socket.onmessage = function(event) {
    display(JSON.parse(event.data));
}

$('#send').click(function(e) {
    var message = $('#message').val();
    $('#message').val('');
    socket.send(message);
});

The first thing to do is to subscribe to new events, join the room, and send down the archive. Here is what websocket.go looks like:

func (c WebSocket) RoomSocket(user string, ws revel.ServerWebSocket) revel.Result {
	// Join the room.
	subscription := chatroom.Subscribe()
	defer subscription.Cancel()

	chatroom.Join(user)
	defer chatroom.Leave(user)

	// Send down the archive.
	for _, event := range subscription.Archive {
		if ws.MessageSendJSON(&event) != nil {
			// They disconnected
			return nil
		}
	}
	....

Next, we have to listen for new events from the subscription. However, the websocket library only provides a blocking call to get a new frame. To select between them, we have to wrap it (websocket.go):

// In order to select between websocket messages and subscription events, we
// need to stuff websocket events into a channel.
newMessages := make(chan string)
go func() {
    var msg string
    for {
        err := ws.MessageReceiveJSON(&msg)
        if err != nil {
            close(newMessages)
            return
        }
        newMessages <- msg
    }
}()

Now we can select for new websocket messages on the newMessages channel.

The last bit does exactly that – it waits for a new message from the websocket (if the user has said something) or from the subscription (someone else in the chat room has said something) and propagates the message to the other.

// Now listen for new events from either the websocket or the chatroom.
for {
    select {
    case event := <-subscription.New:
        if ws.MessageSendJSON(&event) != nil {
            // They disconnected.
            return nil
        }
    case msg, ok := <-newMessages:
        // If the channel is closed, they disconnected.
        if !ok {
            return nil
        }

        // Otherwise, say something.
        chatroom.Say(user, msg)
    }
}
return nil

websocket.go

If we detect the websocket channel has closed, then we just return nil.