Using (socket.io + RedisStore) to communicate across multiple servers - node.js

Question

I am working on a multiplayer online game, using Node.js and Socket.io. I expect a lot of players to join the game, so I am hosting it on Amazon Opworks.
The problem is that the servers aren't able to send socket events to clients connected to a different server. I am using RedisStore to manage socket.io sessions. I believed RedisStore and socket.io took care of this inter-server communication under the hood in a seamless manner. Here is a reference to another question: How does socket.io send messages across multiple servers?
But that's not the case it seems. Messages do not go through to other clients if they are on different servers; the app works if there is only one server, but fails if I use multiple servers loadbalanced using ELB on Opsworks.
This is just an extract from the whole code. Please ignore syntax errors etc if any.
app.js
//Redis Client Initialization
var redis = require("redis");
redis_client = require('redis-url').connect('redis://xxxxx');
//setting RedisStore for socket.io
var RedisStore = require('socket.io/lib/stores/redis')
, redis = require('socket.io/node_modules/redis')
, pub = redis.createClient(11042,'-----')
, sub = redis.createClient(11042,'-----')
, client = redis.createClient(11042,'-----');
// using RedisStore in combo with Socket.io to allow communications across multiple servers
io.set('store', new RedisStore({
redis : redis,
redisPub : pub,
redisSub : sub,
redisClient : client
}));
//socket communication specific code
io.of('/game')
.on('connection', function (socket) {
socket.on('init' , function(data){
var user_id = data.user_id; // collecting user_id that was sent by the client
var socket_id = socket.id;
redis_client.set("user_socket:"+user_id, socket_id, function(err, reply){
//stored a referece to the socket id for that user in the redis database
});
});
socket.on('send_message', function(data){
var sender = data.sender_id;
var reciepient = data.reciepient_id ; // id of the user to whom message is to be sent to
redis_client.get("user_socket:"+reciepient, function(err,socket_id){
if(socket_id){
var socket = io.of('/game').sockets[socket_id];
socket.emit("message", {sender : sender}); // This fails. Messages to others servers dont go through.
}
})
})
});

Answer 1

You can't broadcast directly to socket objects on other servers. What Redis does is allows you to broadcast to 'rooms' on other servers. Thankfully with socket.io 1.x, new connections automatically join a room with the name of their socket id. To solve your problem, change:
if(socket_id){
var socket = io.of('/game').sockets[socket_id];
socket.emit("message", {sender : sender}); // This fails. Messages to others servers dont go through.
}
to emit to the room instead of calling emit on a socket object:
if(socket_id){
io.to(socket_id).emit("message", {sender : sender}); // This fails. Messages to others servers dont go through.
}
And you might have more luck.