Forward redis pubsub message to specific socket.io client in express - node.js

Question

In my express server, I am using socket.io and Redis pubsub.
The server subscribes a Redis message channel, and forwards Redis message to a specific websocket client when someone publishes new message via Redis.
From what i read from socket.io and Redis documentation, I can send message to a specific client by using the varible socket and call socket.broadcast.to(mySocketID).emit.
But in the following code, what should I do if I wanna emit message to mySocketID inside the redis subscriber, which is out of the scope of io.on('connection')?
var redis = require('redis');
var server = require('http').createServer(app);
var io = require("socket.io")(server);
io.on('connection', function (socket) {
socket.on('whatever_event', function() {
socket.broadcast.to(mySocketID).emit('TEST', 'hello ' + mySocketID);
}
}
var subscriber = redis.createClient();
subscriber.on('message', function (channel, redisMessage) {
// I want to send redisMessage to the websocket client
// Can I access to "socket" in io.on('connection' ...) ?
});

Answer 1

I did something similar with WebSockets a while back (If I understand what you're trying to do). Effectively, I had to create a in-process event proxy layer between the Redis Pub/Sub (plain ol' EventEmitter) and the websocket. Without this layer you're going to have a lot of unneeded connection overhead, by using it you can limit the number of open connections to Redis.
So, the subscriber.on section sends a EventEmitter event (yourEventsObject.emit(/*...*/)). Then inside your socket.io connection callback you respond to these events with listeners (yourEventsObject.on(/*...*/)). When your socket.io connection closes, you need to make sure and clean up these EventEmitter listeners so you don't get errors with closed sockets.
Here is the full file on full source on and the article describing the problem/solution with websockets.