Game + Web Server using ExpressJS

Question

Game + Web Server using ExpressJS

I'm currently trying to develop a simple Flash game which talks to a node.js server.

My question is this: How might I go about making a server which differentiates web requests from game requests?

Here are the details of what I've done:

Previously, I used the net and static modules to handle requests from the game client and the browser, respectively.

TwoServers.js

// Web server
var file = new staticModule.Server('./public');
http.createServer(function(req, res){
    req.addListener('end', function(){
        file.serve(req, res, function(err, result){
                     // do something
        });
    });
}).listen(port1, "127.0.0.1");

// Game Server
var server = net.createServer(function(socket)
{
    // handle messages to/from Flash client
    socket.setEncoding('utf8');
    socket.write('foo');
    socket.on('data', onMessageReceived);
});
server.listen(port2, "127.0.0.1");

I'd like to do the above with just an Express server listening in on a single port, but I'm not sure how to go about doing that.

Here's what I'm thinking it might look like (doesn't actually work):

OneServer.js

var app = express();
app.configure(function()
{
    // ...
    app.use('/',express.static(path.join(__dirname, 'public')));  // The static server
});

app.get('/', function(req, res)    // This is incorrect (expects http requests)
{
    // Handle messages to/from Flash client
    var socket = req.connection;
    socket.setEncoding('utf8');
    socket.write('foo');
    socket.on('data', onMessageReceived);
});
app.listen(app.get('port'));   // Listen in on a single port

But I'd like to be able to differentiate from web page requests and requests from the game.

Note: Actionscript's XMLSocket makes TCP requests, so using app.get('/') is incorrect for two reasons:

When Flash writes to the socket, it isn't using the http protocol, so app.get('/') will not be fired when the game tries to connect.
Since I don't have access to correct the net.Socket object, I cannot expect to be reading or writing from/to the correct socket. Instead, I'll be reading/writing from/to the socket associated with the web page requests.

Any help on this would be much appreciated (especially if I'm reasoning about this the wrong way).

actionscript-3
node.js
sockets
express

Answer 1

When a TCP connection is opened to a given port, the server (Node + Express) has no way of telling who made that connection (whether it's a browser or your custom client).

Therefore, your custom client must speak HTTP if it wishes to communicate with the Express server sitting on port 80. Otherwise, the data you send over a freshly opened socket (in your custom protocol) will just look like garbage to Express, and it will close the connection.

However, this doesn't mean you can't get a TCP stream to speak a custom protocol over – you just have to speak HTTP first and ask to switch protocols. HTTP provides a mechanism exactly to accomplish this (the Upgrade header), and in fact it is how WebSockets are implemented.

When your Flash client first opens a TCP connection to your server, it should send: (note line breaks MUST be sent as CRLF characters, aka \r\n)

GET /gamesocket HTTP/1.1
Upgrade: x-my-custom-protocol/1.0
Host: example.com
Cache-Control: no-cache

The value of Upgrade is your choice, Host MUST be sent for all HTTP requests, and the Cache-Control header ensures no intermediate proxies service this request. Notice the blank line, which indicates the request is complete.

The server responds:

HTTP/1.1 101 Switching Protocols
Upgrade: x-my-custom-protocol/1.0
Connection: Upgrade

Again, a blank line indicates the headers are complete, and after that final CRLF, you are now free to send any data you like in any format over the TCP connection.

To implement the server side of this:

app.get('/gamesocket', function(req, res) {
    if (req.get('Upgrade') == 'x-my-custom-protocol/1.0') {
         res.writeHead(101, { Upgrade: req.get('Upgrade'), Connection: 'Upgrade' });

         // `req.connection` is the raw net.Socket object
         req.connection.removeAllListeners(); // make sure Express doesn't listen to the data anymore... we've got it from here!

         // now you can do whatever with the socket
         req.connection.setEncoding('utf8');             
         req.connection.write('foo');
         req.connection.on('data', onMessageReceived);
    } else res.send(400); // bad request
});

Of course, remember that TCP is not a message-based protocol, it only provides a stream, and thus the data events of a Socket can either fragment a single logical message into multiple events or even include several logical messages in a single event. Be prepared to manually buffer data.

Your other option here is to use socket.io, which implements a WebSockets server plus its own protocol on top of the WebSockets protocol. The WebSockets protocol is message-based. It mostly works just like I've outlined here, and then after HTTP negotiation adds a message framing layer on top of the TCP connection so that the application doesn't have to worry about the data stream. (Using WebSockets also opens the possibility of connecting to your server from a HTML page if necessary.)

There is a Flash socket.io client available.