Challenges when developing real-time (websockets) web-applications

When we do classic web development we are used to clients requesting resources from our servers. However, when we are using websockets there will be no request for information. Instead we have clients that subscribe to events. So how do we target the clients subscribing? And how to we send to only a subset of the subscribers? And also... How do we handle the fact that some subscribers can be between pages (connections) when we send a message?

I will try to give my view on the questions above in this live-coding session