What's the sync layer implementation for this project?

what's the sync layer for this? websockets on DO storing the presence state in memory?

Yes—roughly:

• The client uses tldraw’s useSync, connecting over a WebSocket to /api/connect/:roomId (client/pages/Room.tsx:12-16).
• The Worker routes each room ID to a uniquely named Cloudflare Durable Object, so one DO coordinates each room (worker/worker.ts:16-20).
• Inside the DO, @tldraw/sync-core’s TLSocketRoom handles synchronization, applying changes and broadcasting them to connected clients (worker/TldrawDurableObject.ts:49-65).
• Document records are persisted in the DO’s built-in SQLite through SQLiteSyncStorage; they are not only in memory.
• Presence/session state is ephemeral, primarily managed by TLSocketRoom. However, it’s not purely ordinary process memory: session snapshots are serialized into each WebSocket’s attachment so sessions can survive DO hibernation and be resumed (worker/TldrawDurableObject.ts:58-79).
• Cloudflare’s WebSocket Hibernation API keeps sockets alive while allowing the DO’s memory to be freed. Disconnects trigger presence removal broadcasts (worker/TldrawDurableObject.ts:131-158).

So: WebSockets + one Durable Object per room; durable document state in SQLite; ephemeral presence/session state with WebSocket-attached snapshots for hibernation recovery. Static assets go separately to R2.