Real-time Doc Collaboration (Google Docs)
Difficulty: Hard
What It Tests
Conflict-free collaborative editing, cursor sync, version history, convergence guarantees.
Topics Covered
- Real-time Collaboration (OT / CRDT)
- Real-time Updates
- Database Design: SQL vs NoSQL
- Blob / Object Storage
Hello Interview Breakdown
Read the full Hello Interview breakdown →
Video Walkthroughs
- Design Google Docs — System Design Interview →
- Google Docs System Design: Collaborative Editor & Operational Transformation →
Approach Hints
- Use Operational Transformation (OT) — each client sends operations; the server transforms concurrent ops before applying and broadcasting
- A central server serializes all operations to guarantee convergence: all clients reach the same document state regardless of arrival order
- Persist the full operation log for version history and point-in-time restore; take periodic snapshots to avoid replaying the full log
- Separate cursor/selection sync (100ms update rate, ephemeral) from document content sync (apply + broadcast on every op)