Connectivity is unreliable. The best mobile apps treat the network as an enhancement, not a requirement. An offline-first architecture stores data locally and syncs in the background.
The Core Principle: Local Is the Source of Truth
Read and write to a local database first (SQLite/Drift, Realm, WatermelonDB). The UI never waits for the network. A sync layer reconciles local changes with the server when a connection is available.
Optimistic Writes
- Apply the change locally and update the UI immediately.
- Queue the mutation for sync with a 'pending' flag.
- On reconnect, push queued mutations; on success, mark them synced.
- On conflict, resolve with a strategy (last-write-wins, or merge).
Conflict Resolution
The hard part of sync. Simple apps use last-write-wins with timestamps. Collaborative apps need CRDTs or server-authoritative merges. Decide early — retrofitting conflict handling is painful.
Don't Forget the Queue
Persist the mutation queue to disk. If the app is killed before sync, an in-memory queue loses the user's offline edits — the exact failure offline-first is meant to prevent.
