Presence

Presence is what each person’s device says about itself right now: online, a cursor, a selection. It changes many times a second and is worthless a few seconds later, so it never goes into a log (crates/node/src/presence.rs).

How it moves

• POST /apps/{app}/presence { seq, state } — state is hex (at most 4 KiB of bytes), null to retract. Anyone who reaches the app may publish, and the author is the caller the node authenticated: their account, and as the device, the session’s passkey (or the API key).
• The node that takes the update applies it to its own in-memory table and passes it to every other node (internal/presence), without waiting for them. A client on any node therefore sees the whole cluster.
• GET /apps/{app}/presence returns who is present now. GET /apps/{app}/presence/stream is a server-sent event stream: a snapshot event, then a presence event per change (state: null when someone left). It counts against the node’s stream limit.

Rules

Rule	Why
One entry per (account, device)	A person on two devices is two cursors
The highest seq wins; an older update is ignored	Updates may cross between nodes in any order
A retract is an entry with no state	An older “set” arriving late cannot bring someone back
The same seq and state again is a heartbeat: it refreshes the entry and is passed on too	Staying present costs one small message
An entry not refreshed within PRESENCE_TTL_MS (7 s) is swept, checked every second	A closed tab leaves without saying goodbye
At most MAX_AUTHORS_PER_APP (512) per app; a new author evicts the stalest	Bounded memory per app

What it is not

Presence is best effort. It is not replicated by Raft, not sealed to disk, and not part of any receipt; a node that restarts starts with an empty table and fills up within one heartbeat. Anything that must survive belongs in the app’s state, through a call.