Foundation Set 10: Storage & Persistence (25 Q&A)

💾 Storage & Persistence - 25 Quick Q&A

Q1: UserDefaults use case?

A: Small preferences/settings, not complex relational data.

Q2: Keychain use case?

A: Secrets (tokens/passwords/keys).

Q3: Core Data vs SQLite directly?

A: Core Data offers object graph + change tracking; SQLite offers raw control.

Q4: File storage best for?

A: Large blobs (images, media, exports).

Q5: Why not store images in Core Data by default?

A: DB bloat and slower fetch patterns.

Q6: What is lightweight migration?

A: Auto migration for compatible Core Data model changes.

Q7: When migration fails?

A: Breaking schema changes needing custom mapping.

Q8: What is data corruption mitigation?

A: Backups, checksums, atomic writes, recovery path.

Q9: Atomic file write means?

A: Write temp file then replace to avoid partial writes.

Q10: Why background context in Core Data?

A: Keep heavy writes off main thread.

Q11: Merge conflicts in persistence?

A: Concurrent updates from multiple contexts/devices.

Q12: Conflict policy examples?

A: Store-trump, object-trump, custom merge.

Q13: Why indexing matters?

A: Speeds query/filter/sort on large datasets.

Q14: What is write amplification?

A: One logical update causing many disk operations.

Q15: Cache + persistence relationship?

A: Cache for speed, persistence for durability.

Q16: How to design app launch data load?

A: Load critical subset first, lazy-load rest.

Q17: Why version your stored models?

A: Supports safe schema evolution and rollback.

Q18: Offline-first persistence principle?

A: Local commit first, sync later.

Q19: What to log for storage debugging?

A: Migration version, DB size, query latency, error codes.

Q20: How to avoid giant table scans?

A: Proper predicates/indexes + pagination.

Q21: GDPR/CCPA storage implication?

A: Data deletion/export workflows and retention control.

Q22: Why partition data domains?

A: Better ownership, safer migration, reduced contention.

Q23: Interview tradeoff example?

A: Core Data productivity vs raw SQLite flexibility.

Q24: One anti-pattern?

A: Treating all data with same storage strategy.

Q25: Staff-level summary?

A: Persistence design should optimize durability, performance, evolvability, and compliance together.