How would you make a mobile app work offline and sync reliably?

• Reads and writes go to the local database (Room, SwiftData). The UI observes it, so it works offline and updates instantly.
• Writes are also recorded in an outbox in the same local transaction. A sync worker pushes it when there’s a network, in order, with client-generated UUIDs and idempotent requests (PUT /items/{id} or an Idempotency-Key), so retries after lost responses don’t duplicate.
• Pulls use a server-issued, commit-ordered cursor: GET /changes?cursor=… returns created, updated and deleted items (tombstones), and the new cursor is saved in the same transaction as the applied changes.
• Conflicts are detected with versions (each write carries the version it was based on) and resolved per field: last write wins by server order for low-stakes fields, merges for independent fields, domain rules where they exist, CRDTs for collaborative text. Make outcomes visible to the user.
• Platform: WorkManager with network constraints and backoff on Android; sync on foreground plus BGAppRefreshTask on iOS; real-time channels (WebSocket, silent push) only trigger the normal pull.

Senior follow-ups: tombstone retention and full resyncs, migrating unsynced outbox entries across app versions, partial sync for large datasets, and monitoring outbox size and sync failures per app version.