Axiom Data

Data & Persistence

You MUST use this skill for ANY data persistence, database, storage, CloudKit, or serialization work.

When to Use

Use this skill when working with:

• Databases (SwiftData, Core Data, GRDB, SQLiteData)
• Schema migrations
• CloudKit sync
• File storage (iCloud Drive, local storage)
• Data serialization (Codable, JSON)
• Storage strategy decisions
• Keychain / secure credential storage
• Encryption, signing, key management (CryptoKit)

Quick Reference

Automated Scanning

Core Data audit → Launch core-data-auditor agent or /axiom:audit core-data (safety violations, architectural gaps — migration options, thread-confinement, N+1 queries, merge policies, context isolation) Codable audit → Launch codable-auditor agent or /axiom:audit codable (safety violations, semantic gaps — try? swallowing errors, JSONSerialization, date handling, silent field drops, wrapper-hidden fallbacks, cross-file strategy drift, enum future-case crashes) iCloud audit → Launch icloud-auditor agent or /axiom:audit icloud (entitlement checks, file coordination, CloudKit anti-patterns) Storage audit → Launch storage-auditor agent or /axiom:audit storage (wrong file locations, missing backup exclusions, data loss risks) Database schema audit → Launch database-schema-auditor agent or /axiom:audit database-schema (unsafe ALTER TABLE, DROP operations, missing idempotency, foreign key misuse) SwiftData audit → Launch swiftdata-auditor agent or /axiom:audit swiftdata (struct models, missing VersionedSchema, relationship defaults, background context misuse, N+1 patterns)

Decision Tree

1. SwiftData? → skills/swiftdata.md, skills/swiftdata-migration.md
2. Core Data? → skills/core-data.md, skills/core-data-diag.md
3. GRDB? → skills/grdb.md
4. SQLiteData? → skills/sqlitedata.md, skills/sqlitedata-ref.md
5. ANY schema migration? → skills/database-migration.md (ALWAYS — prevents data loss)
6. Realm migration? → skills/realm-migration-ref.md
7. SwiftData vs SQLiteData? → skills/sqlitedata-migration.md
8. Cloud sync architecture? → skills/cloud-sync.md
9. CloudKit? → skills/cloudkit-ref.md
10. iCloud Drive? → skills/icloud-drive-ref.md
11. Cloud sync errors? → skills/cloud-sync-diag.md
12. Codable/JSON serialization? → skills/codable.md
13. File storage strategy? → skills/storage.md, skills/storage-diag.md, skills/storage-management-ref.md
14. File protection? → See axiom-security (skills/file-protection-ref.md)
15. Keychain / storing tokens, passwords, secrets securely? → See axiom-security (skills/keychain.md), See axiom-security (skills/keychain-diag.md), See axiom-security (skills/keychain-ref.md)
16. SecItem errors (errSecDuplicateItem, errSecItemNotFound, errSecInteractionNotAllowed)? → See axiom-security (skills/keychain-diag.md)
17. Encryption, signing, Secure Enclave, CryptoKit? → See axiom-security (skills/cryptokit.md), See axiom-security (skills/cryptokit-ref.md)
18. Quantum-secure cryptography, HPKE, ML-KEM? → See axiom-security (skills/cryptokit.md)
19. Want Core Data safety scan? → core-data-auditor (Agent)
20. Want Codable anti-pattern scan? → codable-auditor (Agent)
21. Want iCloud sync audit? → icloud-auditor (Agent)
22. Want storage location audit? → storage-auditor (Agent)
23. Want database schema/migration safety scan? → database-schema-auditor (Agent)
24. Want SwiftData code audit? → swiftdata-auditor (Agent)
25. tvOS data persistence? → See axiom-swift (skills/tvos.md) (CRITICAL: no persistent local storage) + skills/sqlitedata.md (CloudKit SyncEngine)
26. SwiftData @MainActor / background context threading? → /skill axiom-concurrency
27. Structured data generation with Foundation Models? → /skill axiom-ai

Sync patterns

• HealthKit anchored/observer queries as a generalizable change-tracking pattern → See axiom-health (skills/sync-and-background.md)

Anti-Rationalization

Critical Pattern: Migrations

ALWAYS read skills/database-migration.md when adding/modifying database columns.

This prevents:

• "FOREIGN KEY constraint failed" errors
• "no such column" crashes
• Data loss from unsafe migrations

Example Invocations

User: "I need to add a column to my SwiftData model" → Read: skills/database-migration.md (critical - prevents data loss)

User: "How do I query SwiftData with complex filters?" → Read: skills/swiftdata.md

User: "CloudKit sync isn't working" → Read: skills/cloud-sync-diag.md

User: "Should I use SwiftData or SQLiteData?" → Read: skills/sqlitedata-migration.md

User: "Check my Core Data code for safety issues" → Launch: core-data-auditor agent

User: "Scan for Codable anti-patterns before release" → Launch: codable-auditor agent

User: "Audit my iCloud sync implementation" → Launch: icloud-auditor agent

User: "Check if my files are stored in the right locations" → Launch: storage-auditor agent

User: "Audit my database migrations for safety" → Launch: database-schema-auditor agent

User: "Check my SwiftData models for issues" → Launch: swiftdata-auditor agent

User: "How do I persist data on tvOS?" → Invoke: See axiom-swift (skills/tvos.md) + Read: skills/sqlitedata.md

User: "My tvOS app loses data between launches" → Invoke: See axiom-swift (skills/tvos.md)

User: "How do I store an auth token securely?" → Invoke: See axiom-security (skills/keychain.md)

User: "errSecDuplicateItem but I checked and the item doesn't exist" → Invoke: See axiom-security (skills/keychain-diag.md)

User: "How do I encrypt data with AES in Swift?" → Invoke: See axiom-security (skills/cryptokit.md)

User: "I need to sign data with the Secure Enclave" → Invoke: See axiom-security (skills/cryptokit.md)

User: "What's ML-KEM and should I use it?" → Invoke: See axiom-security (skills/cryptokit.md)