I've been a ruby/php web application developer for quite some time and I'm used to the idea of horizontal scaling of server instances to handle more requests. Horizontal scaling - meaning separate instances of an application sitting behind a load-balancer that share nothing and are unaware of each other.
The main question I have is, since Node.js and it's emphasis on evented-io allows for a single box running a node.js server to handle 'thousands' of simultaneous requests - is load-balancing/horizontal scaling used to scale nodejs applications? Is scaling a node app limited to vertical scaling (throwing more RAM/Processing power at the problem)?
My second question has to do with node.js horizontal scaling and websockets. I've seen quite a few Node.js 'chat' tutorials out there that make use of websockets. (favorite: http://martinsikora.com/nodejs-and-websocket-simple-chat-tutorial)
Since websockets effectively keep an open line of communication open between a browser and a server, would a horizontally scaled architecture typical of the PHP/Ruby world cause a chat application like the one explained in the link to break - as new websocket connection requests would be assigned to different processes/servers and there would be no one central resource tracking all connected clients?
Node.js supports horizontal scaling in much the way you describe via the built-in cluster module.
Regarding your second question about the use of websockets/socket.io in this environment, you have to use something like Redis to store shared state across multiple instances of your application as described here.
Node.js's cluster functionality is limited to single server with multiple processor. Mainly it leverages number of processors in server. I think the question if more about the scenario when we want to scale horizontally with multiple servers with a Load balancer facade.