Google Docs (Real-Time Collaboration)

Phase 1 — Clarify & Scope

Interviewer: Design Google Docs — a real-time collaborative document editor. What do you want to clarify?

Candidate: Key questions:

1. What document types — rich text only, or also spreadsheets and presentations?
2. How many concurrent editors per document?
3. What's the scale — documents and users?
4. What's the core hard problem you want to focus on — conflict resolution, storage, or real-time sync?
5. Offline editing support required?

Interviewer: Rich text documents only. Up to 100 concurrent editors per document. 1 billion documents total, 10 million active documents at any time, 50 million daily active users. Focus on conflict resolution and real-time sync. Offline editing required — changes must merge correctly when a user comes back online.

Candidate: The fundamental challenge is maintaining document consistency when multiple users edit simultaneously. If Alice inserts "Hello" at position 0 and Bob inserts "World" at position 0 concurrently, the final document depends on the order operations are applied — and different clients will see different results without a coordination protocol.

Key sizing: $\text{Operations/sec} = 50\text{M users} \times 10 \text{ keystrokes/min} / 60 \approx 8.3\text{M ops/sec}$

But operations are bursty per-document. Active documents: $\text{Ops/sec per document (average)} = \frac{8.3\text{M}}{10\text{M active docs}} \approx 0.83 \text{ ops/sec}$

For a hot document with 100 concurrent editors, maybe 100 ops/sec — very manageable.

Interviewer: What are the core requirements?

Candidate:

• Real-time collaborative editing — changes appear on all clients within 500 ms