Project: multi-tab offline case review

Build contract

• Send scoped cross-tab invalidation hints, revalidate on focus after missed messages, and keep unsaved fields separate from fetched server state.
• Persist a bounded account-scoped outbox with stable operation IDs, version preconditions, retry limits, and distinct accepted, conflicted, and rejected outcomes.
• Bridge snapshot to change stream through a server cursor; detect duplicate, gap, expiry, filter change, and account change.
• Test expiring presence separately from server write versions and expose a same-field conflict repair view.

Implementation checkpoint

function outboxState(responseStatus) {
  if (responseStatus === 412) return "conflicted";
  if (responseStatus === 403) return "rejected";
  return responseStatus >= 200 && responseStatus < 300 ? "accepted" : "retry-later";
}
console.log(outboxState(412));
// Output: conflicted

Cost and boundaries

The outbox and retained case snapshots use O(n) device storage for n pending or cached records; cap both item count and total bytes, and plan for browser eviction. Snapshot refresh costs O(m) transfer for m records, while a valid delta stream costs O(d) for d changes since the cursor. Presence heartbeats create traffic proportional to active editors and can expand sharply if every update is broadcast to every participant. Cross-tab hints are cheap but can trigger duplicate fetches across many tabs; coalesce repeated versions. Measure stale-window duration, replay age, duplicate suppression, gap recovery time, and the number of unsaved edits retained through a conflict.

Failure drill

Save in Tab A while Tab B has unsaved text. A direct broadcast of the whole case must not replace Tab B’s note. Close Tab B before the next change; reopening it must revalidate without receiving the missed message. Lose the server response after a committed write and verify replay makes one audit entry. Cause a 412 and ensure the edit does not claim Saved. Omit a delta sequence and require a fresh snapshot rather than a silently stale queue. Put one editor to sleep; presence must expire without blocking another editor. Move the device clock and confirm it cannot win a server version conflict.

Acceptance checks

• Unsaved local text survives cross-tab invalidation and a version conflict.
• A replayed offline operation creates at most one server mutation.
• An expired or mismatched cursor triggers a fresh scoped snapshot.
• Presence expires without acting as an edit lock or authorization grant.

Common Mistakes

• Assuming a browser channel is a durable message queue.
• Creating a new idempotency key after a lost response.
• Keeping a delta cursor across a filter or account change.
• Treating presence or client timestamps as the write authority.

Related lessons

Cross-Tab Invalidation and Version Checks; Offline Outbox, Idempotency, and Replay; Snapshot, Delta Cursor, and Gap Recovery; Collaborative Conflicts and Presence Expiry; Conditional Writes and Lost-Update Prevention.