Designing a Live Collaborative Editor

The plain-language idea

The prompt: "Design the frontend architecture for a Google-Docs-style collaborative text editor where multiple users edit the same document simultaneously." This is a genuinely hard, advanced prompt, and it's worth being explicit about the expectation up front: naming the right concepts and reasoning about why they're needed is the real signal here, not implementing a conflict-resolution algorithm from scratch in the room.

See it in a real thing: designing a collaborative editor

The core problem. When two users edit the same document at nearly the same moment, their edits need to merge into one consistent final state without simply overwriting each other. If user A inserts a word at position 10 and user B deletes a word at position 5 within the same second, naively applying both edits in the order they arrive can corrupt the document — the positions each edit refers to shift under each other.

Two real, established families of algorithms solve this class of problem, and naming both is the actual interview signal:

• Operational Transformation (OT) — transforms each incoming operation against the operations that happened concurrently, so it can still be applied correctly to the current document state.
• CRDTs (Conflict-free Replicated Data Types) — structure the document's data itself so that concurrent edits merge deterministically no matter what order they're applied in, without needing a central authority to sequence them.

Both exist to let concurrent edits merge deterministically without a central lock forcing users to take turns. Knowing that they exist, what problem they solve, and roughly how they differ is the expected depth — implementing either one is not.

Presence. Showing other users' live cursor positions and selections is a real, separate piece of this design. Cursor movement is high-frequency and low-stakes — losing a stray cursor update costs nothing — so it typically travels over a lightweight, frequent real-time channel kept separate from the actual document-edit stream, which needs stronger delivery guarantees.

Local optimistic edits vs. reconciled shared state. A user's own keystrokes have to feel instant — applied to their local view immediately, with zero round-trip latency, regardless of what's happening on the network. The eventual, merged state — once other users' concurrent edits are reconciled in via OT or a CRDT — updates the document around that local edit rather than blocking it.

What people usually miss

Watch out

• "Just lock the document while someone's editing" is the wrong instinct, and naming why matters. A pessimistic lock kills the actual product requirement (simultaneous editing) — the whole point of OT/CRDTs is enabling concurrency, not preventing it.
• Presence and the document-edit stream are two different real-time channels with two different guarantee levels, not one undifferentiated "real-time sync" blob — conflating them misses a real architectural distinction.
• The expectation is naming and reasoning, not implementing. Candidates who freeze because they can't derive a transform function from scratch are solving the wrong problem — no interviewer expects that in 30-45 minutes.
• Local edits must never wait on the network to feel correct. A design where a keystroke visibly waits for a server round-trip before appearing has already failed the product requirement, independent of which merge algorithm sits underneath.

Why this matters in interviews

This prompt shows up specifically to see whether a candidate has the vocabulary and reasoning for genuinely hard, unsolved-by-simple-means problems — the bar is informed reasoning about the right concepts, not whiteboard-implementing a CRDT.