Offline and Connectivity

For apps that must work offline, design for local-first with background sync.

Patterns (in order of complexity):

1. Optimistic updates — apply changes to local UI immediately, sync in background. Roll back on server failure. Sufficient for most apps.
2. Queue-based sync — mutations go into an outbox queue, drained when connectivity returns. Failed items stay in queue for retry.
3. Conflict resolution — use ETags or version numbers to detect conflicts. Return 409 with both versions. Simple apps use server-wins; collaborative apps need merge UI or CRDTs.

Practical defaults:

• Track last_synced_at per entity for delta sync
• Show clear connectivity status to the user
• Mutations MUST be queued locally; user work MUST NOT be silently discarded
• Offline scenarios SHOULD be tested — airplane mode, flaky connections, long offline periods

References:

• web.dev: Offline Cookbook
• CRDTs

Change History