"How would you design an offline-first mobile application?"
Mobile networks are unreliable. This tests your ability to design resilient apps that work seamlessly regardless of connectivity.
Explain the offline-first philosophy and why it matters.
Describe local storage and sync strategies.
Address conflict resolution for concurrent edits.
Discuss UX patterns for offline states.
Offline-first means the app works fully offline and syncs when connectivity returns. My approach: use a local database (SQLite/Room on Android, Core Data on iOS) as the single source of truth. All reads come from local storage; writes queue locally and sync in background. For sync, I use a timestamp-based approach with server-side conflict resolution: each record has a lastModified timestamp, and the server applies last-writer-wins or merges based on business rules. I implement a sync queue with exponential backoff retry. For UX, I show sync status indicators and let users manually trigger sync. In my messaging app, we used CRDTs for conflict-free merging and achieved seamless offline editing with automatic sync when back online.
Conflict resolution is the hardest part — show you have thought about it.
Discuss UX implications: how does the user know they are offline?
Mention specific local databases you have used.
SQLite/Room for Android, Core Data or SQLite for iOS, and Realm for cross-platform. For simple key-value storage, SharedPreferences or UserDefaults. Choose based on data complexity.
Options: last-writer-wins (simplest), field-level merging (more complex but preserves more data), or CRDTs (conflict-free but limited data types). Choose based on your data model and user expectations.
Store what users need for their typical offline session. Use smart caching: frequently accessed data first, with background prefetching. Set storage limits and clean up old data.
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