ServeWS handles websocket requests from the peer.

Client is a middleman between the websocket connection and the hub.

A ConnMessage is a message that just gets sent to a single socket connection.

Hub maintains the set of active clients and broadcasts messages to the clients.

PubSub encapsulates the various subscriptions to the different channels.

A RealmMessage is a message that should be sent to a socket Realm.

A UserMessage is a message that should be sent to a user (across all of the sockets that they are connected to, unless the channel says otherwise).

A Realm is basically a set of clients.