What options do you have for storing data on the device, and how do you choose between them?

What the interviewer wants to hear

• Key-value stores (DataStore, UserDefaults) for small settings
• A database (Room, SwiftData, Core Data, SQLite) for structured, queryable data
• Files for large blobs like images or downloads
• Keychain or Keystore for secrets such as tokens
• Consider size, querying needs, security and migrations

Model answer

I pick storage by the shape and sensitivity of the data. Small settings like theme or onboarding done go in a key-value store: DataStore on Android, UserDefaults on iOS, shared_preferences in Flutter. Structured data that I need to query, sort or observe, like a list of orders, goes in a database such as Room, SwiftData, Core Data or SQLite through a library like Drift. Large blobs like images, PDFs or downloaded media go into files in the app sandbox, often in the cache directory if they can be re-downloaded. Secrets such as refresh tokens go in the Keychain or in storage encrypted with the Android Keystore, never in plain preferences. I also plan for schema migrations from day one and avoid storing more personal data than the feature needs.

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