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Apps & Interfaces · 07.04

App Presence: Who's Here Right Now

Live co-presence over the realtime bus, ephemeral by design, so two operators can tell they're looking at the same App at once.

Presence

Ephemeral on purpose — presence is never a database write

Presence answers one narrow question — who else is looking at this App, or this specific thread, right now — and it answers it entirely in memory and over the realtime bus, never in the database. A viewer's console heartbeats a join while it's open; the service keeps one roster entry per viewer and rebroadcasts the full roster on every change, published to both the App's own room and the shared workspace room (the same dual-publish pattern DATA_CHANGED uses), so the dashboard — which subscribes to the workspace room — receives it without any special-casing. A background sweep expires viewers whose heartbeat has gone stale and rebroadcasts when the roster shrinks.

Presence is explicitly best-effort and non-throwing: if the realtime bus is unavailable, presence just degrades to an empty roster — it can never be the thing that breaks message delivery or page rendering.

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