SocketIO on a Node.js cluster

I have a standalone Node.js app which has SocketIO server that listens on a certain port, e.g. 8888. Now I am trying to run this app in a cluster and because cluster randomly assigns workers to requests, SocketIO clients in XHR polling mode once handshaken and authorized with one worker get routed to another worker where they're not handshaken and the mess begins.

And because workers don't share anything, I can't find a workaround. Is there a known solution to this issue?

There is no "simple" solution. What you have to do is the following:

  • If a client connects to a worker, save the connection-id together with the worker-id and a potential additional identification-id in a global (=for all workers accessible) store (i.e. redis).
  • If a client gets routed to another worker, use the store to look up which worker is reponsible for this client (either with the connection-id or with the additional identification-id and then hand it over to that worker (either with the nodejs-worker-master-worker-communication or via redis-pub-sub)

I habe implemented such thing with sock.js and an additional degree of complexity: I have two node.js servers with four workers each, so I had to use redis-pub-sub for worker/worker communication, because it is not guaranteed that they are on the same machine.

Actually there is a simple solution: using Redis to store sockets states.
Everything is explained in Socket.IO documentation:

The default 'session' storage in Socket.IO is in memory (MemoryStore).

The MemoryStore only allows you to deploy socket.io on a single process. If you want to scale to multiple process and / or multiple servers you can use our RedisStore which uses the Redis NoSQL database as man in the middle.

So in order to change the store instance to RedisStore we add this:

var RedisStore = require('socket.io/lib/stores/redis')
  , redis  = require('socket.io/node_modules/redis')
  , pub    = redis.createClient()
  , sub    = redis.createClient()
  , client = redis.createClient();

// Needs to be done after 'listen()'
io.set('store', new RedisStore({
  redisPub : pub
, redisSub : sub
, redisClient : client
}));

Of course you will need to have a redis server running.