Google Doc

01Two people type into the same sentence at once — what must be true afterward?

Both edits survive and every replica converges to the identical document — no lost keystrokes, no divergent copies.

02How live is "live" — polling or push?

Push over a persistent connection: a collaborator’s keystroke shows up in well under a second, character by character.

03Do collaborators see each other, not just the text?

Yes: cursors, selections, and presence — ephemeral state that flows over the same connection but is never persisted.

04Undo - my edits, or everyone's?

Your own only: undo reverses your operations transformed against everything since - undoing a collaborator is a permission violation, not a feature.

05Comments and suggestions - in scope?

Out of scope: they are metadata anchored to ranges, not text operations - a different data model bolted beside the op log.

06How many simultaneous editors before the experience degrades?

Tens work fine; beyond that, sequencing stays correct but cursor noise dominates - cap active editors and admit the overflow read-only.

01What is the correctness bar for concurrent edits?

Convergence: replicas may transiently differ, but all reach the identical document — the transform function must handle every pair of concurrent operations.

02How fast must my own keystroke feel vs a collaborator’s?

Mine: instant (applied locally before any server round trip). Theirs: under ~500 ms door to door.

03What stops the operation log from growing forever?

Periodic snapshots: the log compacts to a snapshot + recent ops; loading a document is snapshot + replay, not replay-from-birth.

04What is the concurrency shape — many docs or many editors?

Both, differently: tens of editors per document (ordering is per-doc), millions of documents (documents shard cleanly — each is independent).

05What happens on reconnect?

The client replays from its last acknowledged sequence number: server sends missed ops, client transforms its unacked local ops against them, and the doc heals.