The probe
This is the hardest question in the question bank. The problem: two users type simultaneously into the same document. Both changes must be preserved, in a consistent order, on both clients, in real time. This requires Operational Transformation (OT) or CRDTs — the most complex distributed consistency problem in a product context.
Step 1 — Clarify
- How many simultaneous editors? (Google Docs: up to ~100 simultaneous, typical: 2–10) - Conflict resolution: last-writer-wins (loses data), OT (complex, correct), CRDT (simpler, eventually consistent)
- Rich text (bold, links, images) or plain text? Rich text requires operation types beyond insert/delete
- Real-time (< 200ms) or near-real-time (< 1s)?
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The Core Problem — Why naive approaches fail
Naive approach 1: Each keystroke triggers a POST to the server; server updates the document; all clients re-fetch.
Fails because: At 100 keystrokes/sec × 10 users = 1000 req/sec just for one document. And the user sees their own edits delayed by a round-trip.
Naive approach 2: Last-writer-wins — each client sends the full document state periodically.
Fails because: If A types “Hello” and B types “World” simultaneously, one overwrites the other.
