Who is logged in on the node, by whatever kind of client, told to whoever asks.

Cauldron2D.Sessions.track("alice", :terminal)
Cauldron2D.Sessions.list()
#=> [%{id: "alice", kind: :terminal, since: 1789718400, pid: #PID<0.312.0>}]
Cauldron2D.Sessions.subscribe()
receive do
  {:cauldron_sessions, :opened, session} -> ...
  {:cauldron_sessions, :closed, session} -> ...
end

A session is the calling process — a client on an ssh connection, a page in a browser — listed from track/2 until the process exits or tracks again. kind is whatever the client calls itself. A subscribed process hears every opening and closing until it unsubscribes or exits.