Why does Socket.io server override node http middleware? - node.js

Question

I have made a React application which relies fully on WebSockets after the initial HTTP Upgrade. For security reasons i use a cookie AND a JWT token in my WebSockets connection.
It all works fine, but when opening a new tab, socket.io cookies get reissued and I want users to stay logged in over multiple tabs. So i want to set a cookie if the client doesn't already have one. If it already has one, then use that cookie.
So I want to handle the first HTTP polling requests and created middleware for that in Node's http server:
// HTTP SERVER
const server = require('http').createServer(function (request, response) {
console.log('test');
console.log(request);
if(!request.headers.cookie) { // cookie pseudo-logic
response.writeHead(200, {
'Set-Cookie': 'mycookie=test',
'Content-Type': 'text/plain'
});
}
// Socket.IO server instance
const io = require('socket.io')(server, {
origins: config.allowedOrigins,
cookie: false, // disable default io cookie
});
server.listen(port, () => console.log(`Listening on port ${port}`));
I use Socket.io as WebSockets framework. The problem however is that this middleware get's ignored, when registering the Socket.io server. When i comment out the Socket.io server, the middleware is active and the request get's logged.
It looks like Socket.io's server is overriding the handler for node http's server. In the Socket.io docs however they provide this example:
var app = require('http').createServer(handler)
var io = require('socket.io')(app);
var fs = require('fs');
app.listen(80);
function handler (req, res) {
fs.readFile(__dirname + '/index.html',
function (err, data) {
if (err) {
res.writeHead(500);
return res.end('Error loading index.html');
}
res.writeHead(200);
res.end(data);
});
}
io.on('connection', function (socket) {
socket.emit('news', { hello: 'world' });
socket.on('my other event', function (data) {
console.log(data);
});
});
Thus indicating that it should be possible to handle thw first http polling requests and also the socket requests. I managed to get it work with Express, but I don't understand why node's http server can't.
Anybody who knows what's happening?
Thanks in advance,
Mike

Answer 1

Because normal usage of socket.io does not want regular http middleware to see socket.io connection requests (they would normally trigger 404 responses), socket.io places its own request handler first in line before any others, even ones that existed before it was installed.
You can see how it does that here: https://github.com/socketio/engine.io/blob/master/lib/server.js#L437 in the engine.io source.
I can think of the following ways for you to pre-process a request before socket.io sees it:
Use a proxy and do your cookie stuff in a proxy before socket.io even sees the request.
Patch socket.io/engine.io code to add a callback hook for what you want to do.
Copy the technique used by socket.io/engine.io to put your own request handler first in line after socket.io is configured.
Find a way to override the socket.io server object's handleRequest() method which is what gets called when there's an incoming connection request. You can see its code here.