Node.js, multi-threading and Socket.io

Question

I'm looking to get Socket.io to work multi-threaded with native load balancing ("cluster") in Node.js v.0.6.0 and later.

From what I understand, Socket.io uses Redis to store its internal data. My understanding is this: instead of spawning a new Redis instance for every worker, we want to force the workers to use the same Redis instance as the master. Thus, connection data would be shared across all workers.

Something like this in the master:

RedisInstance = new io.RedisStore;

The we must somehow pass RedisInstance to the workers and do the following:

io.set('store', RedisInstance);

Inspired by this implementation using the old, 3rd party cluster module, I have the following non-working implementation:

var cluster = require('cluster');
var http = require('http');
var numCPUs = require('os').cpus().length;

if (cluster.isMaster) {
  // Fork workers.
  for (var i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  var sio = require('socket.io')
  , RedisStore = sio.RedisStore
  , io = sio.listen(8080, options);

  // Somehow pass this information to the workers
  io.set('store', new RedisStore);

} else {
  // Do the work here
  io.sockets.on('connection', function (socket) {
    socket.on('chat', function (data) {
      socket.broadcast.emit('chat', data);
    })
  });
}

Thoughts? I might be going completely in the wrong direction, anybody can point to some ideas?

Answer 1

Actually your code should look like this:

var cluster = require('cluster');
var http = require('http');
var numCPUs = require('os').cpus().length;

if (cluster.isMaster) {
  // Fork workers.
  for (var i = 0; i < numCPUs; i++) {
    cluster.fork();
  }
} else {
  var sio = require('socket.io')
  , RedisStore = sio.RedisStore
  , io = sio.listen(8080, options);

  // Somehow pass this information to the workers
  io.set('store', new RedisStore);

  // Do the work here
  io.sockets.on('connection', function (socket) {
    socket.on('chat', function (data) {
      socket.broadcast.emit('chat', data);
    })
  });
}

Another option is to open Socket.IO to listen on multiple ports and have something like HAProxy load-balance stuff. Anyway you know the most important thing: using RedisStore to scale outside a process!

Resources:

http://nodejs.org/docs/latest/api/cluster.html
How can I scale socket.io?
How to reuse redis connection in socket.io?
Node: Scale socket.io / nowjs - scale across different instances
http://delicious.com/alessioaw/socket.io