Every process has "my-actor" defined in its global environment. It provides methods related to basic actor functionality. In this case I'm calling the "announce" method to say something to all of the process' subscribers.
user-interface and discovery-system are two hypothetical processes whose pids (process identifiers) are passed in to this process as arguments. user-interface would be responsible for creating a window with a text display area and a text entry box. discovery-system would use our remote service discovery model to find others chatting in the same "room."
