Design a Document Presence Service

Design the presence layer for a collaborative document suite: the little colored avatars, live cursors, and "typing" dots that show who else is in a document right now and where they are working. This is the awareness signal that sits alongside the editor, not the editor itself.

Operating context. Clients open long-lived connections when they view a document and emit lightweight signals — joined, moved cursor, selected a range, actively typing, went idle, left. Every other viewer of that same document should see those signals almost immediately. Presence is soft state: it is fine to lose it on a crash as long as a stale ghost cleans itself up quickly. The document's own content sync (the OT/CRDT stream) is handled by a separate team and is out of your scope.

Out of scope. The operational-transform / CRDT document-editing pipeline, authentication and authorization, persistent storage of document content, and offline conflict resolution. Assume an auth layer hands you a trusted user identity per connection.

What to produce. A high-level architecture covering: the connection layer and wire protocol; how a presence signal fans out to every subscriber of a document; the ephemeral data model and how heartbeats plus TTLs evict ghosts; how you shard connections and presence state so a single hot document does not melt one node; and the failure story when a presence node dies. Sketch the components and the path a cursor move takes from one client to all the others; we will probe specifics in checkpoints.