Presence tracking for collaborative entity editing.

Uses Phoenix.Presence to track who is currently editing an entity or data record. The first person to join a topic becomes the "owner" (can edit), and everyone else becomes "spectators" (read-only mode).

How It Works

1. When a user opens an edit form, they join a Presence topic (e.g., "entity_edit:5")
2. Presence tracks all connected users with metadata (user info, joined_at timestamp)
3. Users are sorted by joined_at to determine order (FIFO)
4. First user in the sorted list = owner (readonly?: false)
5. All other users = spectators (readonly?: true)
6. When owner leaves, Presence removes them automatically
7. All connected users receive presence_diff event
8. Each user re-evaluates: "Am I first now?"
9. New first user auto-promotes to owner

Automatic Cleanup

Phoenix.Presence automatically detects when LiveView processes die and removes them immediately via process monitoring. When a user closes a tab or navigates away:

1. WebSocket disconnects
2. LiveView process terminates
3. Presence automatically cleaned up (via process monitoring)
4. All other users receive presence_diff event instantly

No manual cleanup or timeout configuration needed!

Topics

• Entity editing: "entity_edit:<entity_id>"
• Data editing: "data_edit:<data_id>"