Scaling Socket.IO across multiple servers

Question

I've been searching around looking for help on setting up a multi-server cluster for a Node.js Socket.IO install. This is what I am trying to do:

Have 1 VIP in an F5 loadbalancer, pointing to n number of Node servers running Express, and Socket.IO
Have client connect to that 1 VIP via io.connect and then have it filter to one the of the servers behind the loadbalancer.
When a message is emitted on any one of those servers, it is is sent to all users who are listening for that event and connect via the other servers.

For example - if we have Server A, Server B and Server C behind LB1 (F5), and User A is connected to Server A, User B is connected to Server B and User C is connected to Server C.

In a "chat" scenario - basically if a message is emitted from Server A to message event - Server B and C should also send the message to their connected client. I read that this is possible with using socket-io.redis, but it needs a Redis box - which server should that be install on? If all the servers are connected to the same Redis box - does this work automatically?

var io = require('socket.io')(server);
var redis = require('socket.io-redis');
io.adapter(redis({ host: 'localhost', port: 6379 }));

Any help would be greatly appreciated thanks!

It seems like a good solve for this would be to enable peer-to-peer communication between different socket.io instances on the express servers, this way they can all stay in sync with one another. You can do this using something like scuttlebutt and with its event emitter — Travis Kaufman
Using the socket.io-redis adapter will indeed automatically (with some configuration) achieve what you're looking for. It does not matter which node your Redis server will listen on, as long as all of the Socket.IO instances are subscribed to it. — hexacyanide
Have you read this: socket.io/docs/using-multiple-nodes? If so, what did you not understand about it? It appears to be designed for your particular situation. — jfriend00
Yes - I should have tried this before, but @hexacyanide was correct - it works exactly out of the box as long as everyone is attached to the the same redis box. For anyone coming to this post later - it just works. sorry for wasting everyone's time :-( — gregavola
If you're done with your question, you can either answer it yourself or delete it rather than just leave it sit here. — jfriend00

gregavola gregavola · Accepted Answer · 2015-06-14T15:35:05

The answer to this question is that you must set up a single Redis server that is either outside your SocketIO cluster - and have all nodes connect to it.

Then you simply add this at the top of your code and it just works magically without any issues.

var io = require('socket.io')(server);
var redis = require('socket.io-redis');
io.adapter(redis({ host: 'localhost', port: 6379 }));

Scaling Socket.IO across multiple servers

1 Answers