Let clients work disconnected and reconcile changes on reconnect without losing edits or resurrecting deletions. Use when building an application that must work on an unreliable network.
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---
name: offline-sync
description: Let clients work disconnected and reconcile changes on reconnect without losing edits or resurrecting deletions. Use when building an application that must work on an unreliable network.
---
# Offline sync
Offline capability turns every client into a replica that will diverge.
The work is not caching but reconciliation: deciding what happens when
two versions of the truth meet, and making sure nothing the user did is
silently discarded.
## Method
1. **Queue intents, not final states.** Recording that the user
completed a task merges better than recording that the document now
looks like this, which overwrites concurrent work.
2. **Make every queued operation idempotent and identified.** Reconnect
retries duplicate requests routinely, and without an id the server
cannot tell a retry from a repeat (see idempotency).
3. **Handle deletes explicitly with tombstones.** Without them, a client
that was offline during a delete will happily recreate the record on
sync.
4. **Decide the conflict policy per data type.** Last write wins is
acceptable for a preference and unacceptable for a document body or a
balance; choose per field rather than globally.
5. **Bound the offline window.** State how long a client may be offline
and still sync cleanly, since unbounded divergence eventually cannot
be reconciled and needs a full refresh path.
6. **Show sync state honestly.** Pending, syncing, synced, and failed,
visible to the user, because silent failure is how offline apps lose
trust and data.
7. **Test the ugly paths.** Reconnect mid-sync, conflicting edits from
two devices, clock skew, and a server that rejects a queued
operation (see clock-skew).
## Boundaries
- Offline sync guarantees convergence at best; it cannot preserve the
intent of two people who edited the same thing differently.
- Local storage has quota limits and can be evicted by the platform, so
it is not durable storage (see browser-storage-safety).
- Sensitive data cached on a device needs its own protection and a
wipe path (see data-classification).
