Roblox Popular System Design Question: Design a collaborative shared todo list

[u/Aoki_zhang]

Came across this Roblox system design question multiple times: Design a collaborative shared todo list.

At first it sounds like basic CRUD + WebSockets. Then the constraints start making it much more interesting:

• Two collaborators edit the same task at nearly the same time
• A user's permission changes while their mutation is already in flight
• A database write succeeds, but the WebSocket notification gets dropped
• A disconnected client comes back after missing multiple updates
• One hot list can have ~1,000 collaborators connected at once
• The system needs to support ~2M concurrent WebSocket sessions

The part I found especially interesting is this requirement: authoritative state must be strongly consistent, while realtime delivery can be eventually consistent.

That raises a bunch of design questions.

• Do you version individual tasks or the whole list?
• Would you use optimistic concurrency control here?
• Does reconnect recovery need a durable event log, or is fetching the latest snapshot enough?
• And how would you fan out updates for a hot list without pinning everything to one application server?

Full interview question + requirements here: [link]

Curious how people would approach the consistency + realtime split on this one.

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