axiom-data

What this skill does

# 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` |
| GRDB performance, indexes, EXPLAIN QUERY PLAN, cursors | See `skills/grdb-performance.md` |
| Full-text search (FTS5) in GRDB or SQLiteData | See `skills/sqlite-fts-ref.md` |
| Storing or querying JSON inside a SQLite column (JSON1, JSONB) | See `skills/sql-json-ref.md` |
| GRDB shared across app + widget/extension (App Group) | See `skills/grdb-app-groups.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)
**GRDB performance audit** → Launch `grdb-performance-auditor` agent or `/axiom:audit grdb-performance` (raw SQL string interpolation, missing FK indexes in raw SQL, missing PRAGMA optimize for raw-GRDB apps, journal mode mismatch for app-group DBs, missing observesSuspensionNotifications for shared DBs, prefix-redundant indexes in raw SQL, legacy Record subclass)
**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

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`
3a. GRDB perf, slow query, schema design for perf? → `skills/grdb-performance.md`
3b. FTS5 (full-text search, any layer)? → `skills/sqlite-fts-ref.md`
3c. DB shared across app + extension/widget/Live Activity? → `skills/grdb-app-groups.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)
23a. Want GRDB performance/app-group scan? → grdb-performance-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

| 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