Design a Collaborative Notes Service

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.