Design a collaborative notes service, similar to a lightweight online document editor. Users should be able to create, read, update, delete, and share notes; edit from multiple devices; synchronize changes; collaborate near real time; and recover from accidental edits. ### Constraints & Assumptions - Focus on backend architecture and synchronization, not rich UI details. - Multiple users may edit the same note concurrently. - Clients may briefly go offline and later reconnect. - The system should preserve history or support basic recovery. - Avoid full-document last-write-wins for collaborative text edits because it can lose data. ### Clarifying Questions to Ask - Is the document plain text, rich text, or structured blocks? - Do we need offline editing in v1? - What collaboration latency is expected? - How long should history be retained? - Are notes shared by individual users, teams, or public links? ### Part 1 - Architecture and Data Model Describe services, storage, APIs, and major data entities. #### What This Part Should Cover - Metadata service, collaboration service, sync service, auth, WebSockets, pub-sub, operation log, snapshots, and caches. - User, note, permission, snapshot, operation, and audit data. - CRUD APIs and live-editing protocol. ### Part 2 - Synchronization and Conflict Handling Explain how real-time editing, offline sync, and concurrent edits are handled. #### What This Part Should Cover - Operation logs and monotonic note versions. - Operational Transformation or CRDTs. - Idempotency keys and server acknowledgements. - Snapshot plus replay for catch-up. - Why full-note overwrites are unsafe. ### Part 3 - Scalability, Reliability, and Security Describe how to scale the collaboration service and keep data safe. #### What This Part Should Cover - Partitioning or sticky routing by note ID. - Durable append before acknowledgement. - Pub-sub across collaboration servers. - History retention and compaction. - Permissions, encryption, rate limiting, audit logs, and observability. ### What a Strong Answer Covers - Clear separation of metadata, document state, operations, and live sessions. - Correct collaborative-editing strategy. - Durable operation logging and snapshots. - Handling of offline clients and retries. - Scaling and hot-document tradeoffs. ### Follow-up Questions - How would you handle a note with thousands of simultaneous viewers? - Why choose CRDTs over Operational Transformation, or vice versa? - How would you recover a note to an earlier version? - How would you prevent unauthorized users from receiving live edits?