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
| Symptom / Task | Reference |
|---|---|
| SwiftData @Model, @Query, ModelContext | See skills/swiftdata.md |
| SwiftData schema migration, VersionedSchema | See skills/swiftdata-migration.md |
| SwiftData migration crashes, data loss | See skills/swiftdata-migration-diag.md |
| Migrating from Realm to SwiftData | See skills/realm-migration-ref.md |
| SwiftData vs SQLiteData decision | See skills/sqlitedata-migration.md |
| GRDB queries, ValueObservation, DatabaseMigrator | See skills/grdb.md |
| SQLiteData @Table, CRUD, SyncEngine | See skills/sqlitedata.md |
| SQLiteData advanced patterns, CTEs, views | See skills/sqlitedata-ref.md |
| Core Data stack, relationships, concurrency | See skills/core-data.md |
| Core Data migration crashes, thread errors | See skills/core-data-diag.md |
| ANY schema migration safety | See skills/database-migration.md |
| Codable, JSON encoding/decoding | See skills/codable.md |
| Cloud sync architecture, offline-first | See skills/cloud-sync.md |
| CloudKit, CKSyncEngine, CKRecord | See skills/cloudkit-ref.md |
| iCloud Drive, ubiquitous containers | See skills/icloud-drive-ref.md |
| Cloud sync errors, conflict resolution | See skills/cloud-sync-diag.md |
| Storage strategy, where to store data | See skills/storage.md |
| Storage issues, files disappeared | See skills/storage-diag.md |
| Storage management, disk pressure | See skills/storage-management-ref.md |
| Keychain / secure credential storage | See axiom-security (skills/keychain.md) |
| Keychain errors (errSecDuplicateItem) | See axiom-security (skills/keychain-diag.md) |
| Keychain API reference | See axiom-security (skills/keychain-ref.md) |
| Encryption / signing / key management | See axiom-security (skills/cryptokit.md) |
| CryptoKit API reference | See axiom-security (skills/cryptokit-ref.md) |
| File protection, NSFileProtection | See axiom-security (skills/file-protection-ref.md) |
| tvOS data persistence (no local storage) | See axiom-swift (skills/tvos.md) |
| tvOS + CloudKit SyncEngine | See skills/sqlitedata.md |
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, incomplete CKError matrix coverage, missing account-change observation, polling vs CKSubscriptions, SwiftData + CloudKit unsupported features, compound risks like uncoordinated I/O across extensions) Storage audit → Launch storage-auditor agent or /axiom:audit storage (wrong file locations, missing backup exclusions, sensitive data on disk vs Keychain, missing App Group containers, unbounded cache growth, orphan files, compound risks like user data in tmp/ + critical content) Database schema audit → Launch database-schema-auditor agent or /axiom:audit database-schema (unsafe ALTER TABLE, DROP operations, missing idempotency, FK constraints declared but not enforced, incomplete upgrade paths, compound risks like INSERT OR REPLACE on FK-referenced tables) SwiftData audit → Launch swiftdata-auditor agent or /axiom:audit swiftdata (struct models, missing schema registration, array relationships without defaults, background context misuse, N+1 patterns, stale predicates, CloudKit conformance gaps, compound risks like struct model + array relationship)
Decision Tree
- SwiftData? →
skills/swiftdata.md,skills/swiftdata-migration.md - Core Data? →
skills/core-data.md,skills/core-data-diag.md - GRDB? →
skills/grdb.md - SQLiteData? →
skills/sqlitedata.md,skills/sqlitedata-ref.md - ANY schema migration? →
skills/database-migration.md(ALWAYS — prevents data loss) - Realm migration? →
skills/realm-migration-ref.md - SwiftData vs SQLiteData? →
skills/sqlitedata-migration.md - Cloud sync architecture? →
skills/cloud-sync.md - CloudKit? →
skills/cloudkit-ref.md - iCloud Drive? →
skills/icloud-drive-ref.md - Cloud sync errors? →
skills/cloud-sync-diag.md - Codable/JSON serialization? →
skills/codable.md - File storage strategy? →
skills/storage.md,skills/storage-diag.md,skills/storage-management-ref.md - File protection? → See axiom-security (skills/file-protection-ref.md)
- 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)
- SecItem errors (errSecDuplicateItem, errSecItemNotFound, errSecInteractionNotAllowed)? → See axiom-security (skills/keychain-diag.md)
- Encryption, signing, Secure Enclave, CryptoKit? → See axiom-security (skills/cryptokit.md), See axiom-security (skills/cryptokit-ref.md)
- Quantum-secure cryptography, HPKE, ML-KEM? → See axiom-security (skills/cryptokit.md)
- Want Core Data safety scan? → core-data-auditor (Agent)
- Want Codable anti-pattern scan? → codable-auditor (Agent)
- Want iCloud sync audit? → icloud-auditor (Agent)
- Want storage location audit? → storage-auditor (Agent)
- Want database schema/migration safety scan? → database-schema-auditor (Agent)
- Want SwiftData code audit? → swiftdata-auditor (Agent)
- tvOS data persistence? → See axiom-swift (skills/tvos.md) (CRITICAL: no persistent local storage) +
skills/sqlitedata.md(CloudKit SyncEngine) - SwiftData @MainActor / background context threading? →
/skill axiom-concurrency - 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
| Thought | Reality |
|---|---|
| "Just adding a column, no migration needed" | Schema changes without migration crash users. database-migration prevents data loss. |
| "I'll handle the migration manually" | Manual migrations miss edge cases. database-migration covers rollback and testing. |
| "Simple query, I don't need the skill" | Query patterns prevent N+1 and thread-safety issues. The skill has copy-paste solutions. |
| "CloudKit sync is straightforward" | CloudKit has 15+ failure modes. cloud-sync-diag diagnoses them systematically. |
| "I know Codable well enough" | Codable has silent data loss traps (try? swallows errors). codable skill prevents production bugs. |
| "I'll use local storage on tvOS" | tvOS has NO persistent local storage. System deletes Caches at any time. See axiom-swift (skills/tvos.md) for the iCloud-first pattern. |
| "UserDefaults is fine for this token" | UserDefaults is unencrypted, backed up to iCloud, and visible to MDM profiles. One audit catches it. keychain stores tokens securely. |
| "I'll encrypt it myself with CommonCrypto" | CryptoKit replaced CommonCrypto's buffer-management nightmares with one-line APIs. cryptokit prevents misuse. |
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)