Design a real-time collaborative document editor

Setting

You're building the real-time layer behind a collaborative document editor (Google Docs-style): multiple people type into the same document at once, and every participant's screen needs to converge to the same content, with each person's cursor visible to the others.

The ask

Design the system that lets N clients edit the same document concurrently, merges their concurrent edits correctly, and keeps everyone's view in sync in real time.

Constraints & scale

• Documents up to a few hundred pages; up to ~50 concurrent editors on a single popular document
• Two people typing in the same paragraph at the same moment must both have their changes preserved and end up with the same resulting text on every client — no silent overwrite
• A client that briefly disconnects (flaky wifi) must reconnect and catch up to the current state without re-syncing the whole document from scratch
• Keystroke-to-screen latency for the local typist must feel instant, even before the edit round-trips to the server or other clients
• The document must persist durably — a server restart must not lose in-flight edits

What a strong answer covers

Requirements → merge strategy (OT / CRDT, and why) → high-level diagram (client, real-time transport, merge/sequencing service, persistence) → 1–2 deep dives → trade-offs. Likely probes: how concurrent edits to the same region are merged deterministically, how a reconnecting client catches up cheaply, and where persistence happens without adding to typing latency.

Graded on

Every session — this problem included — is scored against the same 10-dimension senior/staff rubric: