iOS Storage Guide

Purpose: Navigation hub for ALL storage decisions — database vs files, local vs cloud, specific locations
iOS Version: iOS 17+ (iOS 26+ for latest features)
Context: Complete storage decision framework integrating SwiftData (WWDC 2023), CKSyncEngine (WWDC 2023), and file management best practices

When to Use This Skill

✅ Use this skill when:

• Starting a new project and choosing storage approach
• Asking "where should I store this data?"
• Deciding between SwiftData, Core Data, SQLite, or files
• Choosing between CloudKit and iCloud Drive for sync
• Determining Documents vs Caches vs Application Support
• Planning data architecture for offline/online scenarios
• Migrating from one storage solution to another
• Debugging "files disappeared" or "data not syncing"

❌ Do NOT use this skill for:

• SwiftData implementation details (use axiom-swiftdata skill)
• SQLite/GRDB specifics (use axiom-sqlitedata or axiom-grdb skills)
• CloudKit sync implementation (use axiom-cloudkit-ref skill)
• File protection APIs (use axiom-file-protection-ref skill)

Related Skills:

• Existing database skills: axiom-swiftdata, axiom-sqlitedata, axiom-grdb
• New file skills: axiom-file-protection-ref, axiom-storage-management-ref, axiom-storage-diag
• New cloud skills: axiom-cloudkit-ref, axiom-icloud-drive-ref, axiom-cloud-sync-diag

Core Philosophy

"Choose the right tool for your data shape. Then choose the right location."

Storage decisions have two dimensions:

1. Format: How is data structured? (Queryable records vs files)
2. Location: Where is it stored? (Local vs cloud, which directory)

Getting the format wrong forces workarounds. Getting the location wrong causes data loss or backup bloat.

---

The Complete Decision Tree

Level 1: Format — What Are You Storing?

What is the shape of your data?

├─ STRUCTURED DATA (queryable records, relationships, search)
│   Examples: User profiles, task lists, notes, contacts, transactions
│   → Continue to "Structured Data Path" below
│
└─ FILES (documents, images, videos, downloads, caches)
    Examples: Photos, PDFs, downloaded content, thumbnails, temp files
    → Continue to "File Storage Path" below

---

Structured Data Path

Modern Apps (iOS 17+)

// ✅ CORRECT: SwiftData for modern structured persistence
import SwiftData

@Model
class Task {
    var title: String
    var isCompleted: Bool
    var dueDate: Date

    init(title: String, isCompleted: Bool = false, dueDate: Date) {
        self.title = title
        self.isCompleted = isCompleted
        self.dueDate = dueDate
    }
}

// Query with type safety
@Query(sort: \Task.dueDate) var tasks: [Task]

Why SwiftData:

• Modern Swift-native API (no Objective-C)
• Type-safe queries
• Built-in CloudKit sync support
• Observable models integrate with SwiftUI
• Use skill: axiom-swiftdata for implementation details

When NOT to use SwiftData:

• Need advanced SQLite features (FTS5, complex joins)
• Existing Core Data app (migration overhead)
• Ultra-performance-critical (direct SQLite is faster)

Advanced Control Needed

// ✅ CORRECT: SQLiteData or GRDB for advanced features
import SQLiteData

// Full-text search, custom indices, raw SQL when needed
let results = try db.prepare("SELECT * FROM users WHERE name MATCH ?", "John")

Use SQLiteData when:

• Need full-text search (FTS5)
• Custom SQL queries and indices
• Maximum performance (direct SQLite)
• Migration from existing SQLite database
• Use skill: axiom-sqlitedata for modern SQLite patterns

Use GRDB when:

• Need reactive queries (ValueObservation)
• Complex database operations
• Type-safe query builders
• Use skill: axiom-grdb for advanced patterns

Legacy Apps (iOS 16 and earlier)

// ❌ LEGACY: Core Data (avoid for new projects)
import CoreData

// NSManagedObject, NSFetchRequest, NSPredicate...

Only use Core Data if:

• Maintaining existing Core Data app
• Can't upgrade to iOS 17 minimum deployment

---

File Storage Path

Decision Tree for Files

What kind of file is it?

├─ USER-CREATED CONTENT (documents, photos created by user)
│   Where: Documents/ directory
│   Backed up: ✅ Yes (iCloud/iTunes)
│   Purged: ❌ Never
│   Visible in Files app: ✅ Yes
│   Example: User's edited photos, documents, exported data
│   → See "Documents Directory" section below
│
├─ APP-GENERATED DATA (not user-visible, must persist)
│   Where: Library/Application Support/
│   Backed up: ✅ Yes
│   Purged: ❌ Never
│   Visible in Files app: ❌ No
│   Example: Database files, user settings, downloaded assets
│   → See "Application Support Directory" section below
│
├─ RE-DOWNLOADABLE / REGENERABLE CONTENT
│   Where: Library/Caches/
│   Backed up: ❌ No (set isExcludedFromBackup)
│   Purged: ✅ Yes (under storage pressure)
│   Example: Thumbnails, API responses, downloaded images
│   → See "Caches Directory" section below
│
└─ TEMPORARY FILES (can be deleted anytime)
    Where: tmp/
    Backed up: ❌ No
    Purged: ✅ Yes (aggressive, even while app running)
    Example: Image processing intermediates, export staging
    → See "Temporary Directory" section below

Documents Directory

// ✅ CORRECT: User-created content in Documents
func saveUserDocument(_ data: Data, filename: String) throws {
    let documentsURL = FileManager.default.urls(
        for: .documentDirectory,
        in: .userDomainMask
    )[0]

    let fileURL = documentsURL.appendingPathComponent(filename)

    // Enable file protection
    try data.write(to: fileURL, options: .completeFileProtection)
}

Key rules:

• ✅ DO store: User-created documents, exported files, user-visible content
• ❌ DON'T store: Downloaded data that can be re-fetched, caches, temp files
• ⚠️ WARNING: Everything here is backed up to iCloud. Large re-downloadable files will bloat backups and may get your app rejected.

Use skill: axiom-file-protection-ref for encryption options

Application Support Directory

// ✅ CORRECT: App data in Application Support
func getAppDataURL() -> URL {
    let appSupportURL = FileManager.default.urls(
        for: .applicationSupportDirectory,
        in: .userDomainMask
    )[0]

    // Create app-specific subdirectory
    let appDataURL = appSupportURL.appendingPathComponent(
        Bundle.main.bundleIdentifier ?? "AppData"
    )

    try? FileManager.default.createDirectory(
        at: appDataURL,
        withIntermediateDirectories: true
    )

    return appDataURL
}

Use for:

• SwiftData/SQLite database files
• User preferences
• Downloaded assets that must persist
• Configuration files

Caches Directory

// ✅ CORRECT: Re-downloadable content in Caches
func cacheDownloadedImage(data: Data, for url: URL) throws {
    let cacheURL = FileManager.default.urls(
        for: .cachesDirectory,
        in: .userDomainMask
    )[0]

    let filename = url.lastPathComponent
    let fileURL = cacheURL.appendingPathComponent(filename)

    try data.write(to: fileURL)

    // Mark as excluded from backup (explicit, though Caches is auto-excluded)
    var resourceValues = URLResourceValues()
    resourceValues.isExcludedFromBackup = true
    try fileURL.setResourceValues(resourceValues)
}

Key rules:

• ✅ The system CAN and WILL delete files here under storage pressure
• ✅ Always have a way to re-download or regenerate
• ❌ Don't store anything that can't be recreated

Use skill: axiom-storage-management-ref for purge policies and disk space management

Temporary Directory

// ✅ CORRECT: Truly temporary files in tmp
func processImageWithTempFile(image: UIImage) throws {
    let tmpURL = FileManager.default.temporaryDirectory
    let tempFileURL = tmpURL.appendingPathComponent(UUID().uuidString + ".jpg")

    // Write temp file
    try image.jpegData(compressionQuality: 0.8)?.write(to: tempFileURL)

    // Process...
    processImage(at: tempFileURL)

    // Clean up (though system will auto-clean eventually)
    try? FileManager.default.removeItem(at: tempFileURL)
}

Key rules:

• System can delete files here AT ANY TIME (even while app is running)
• Always clean up after yourself
• Don't rely on files persisting between app launches

---

Cloud Storage Decisions

Should Data Sync to Cloud?

Does this data need to sync across user's devices?

├─ NO → Use local storage (paths above)
│
└─ YES → What kind of data?
    │
    ├─ STRUCTURED DATA (queryable, relationships)
    │   → Use CloudKit
    │   → See "CloudKit Path" below
    │
    ├─ FILES (documents, images)
    │   → Use iCloud Drive (ubiquitous containers)
    │   → See "iCloud Drive Path" below
    │
    └─ SMALL PREFERENCES (<1 MB, key-value pairs)
        → Use NSUbiquitousKeyValueStore
        → See "Key-Value Store" below

CloudKit Path (Structured Data Sync)

// ✅ CORRECT: SwiftData with CloudKit sync (iOS 17+)
import SwiftData

let container = try ModelContainer(
    for: Task.self,
    configurations: ModelConfiguration(
        cloudKitDatabase: .private("iCloud.com.example.app")
    )
)

Three approaches to CloudKit:

1. SwiftData + CloudKit (Recommended, iOS 17+):

   • Automatic sync for SwiftData models
   • Private database only
   • Easiest approach
   • Use skill: axiom-swiftdata for details

2. CKSyncEngine (Custom persistence, iOS 17+):

   • For SQLite, GRDB, or custom stores
   • Manages sync automatically
   • Modern replacement for manual CloudKit
   • Use skill: axiom-cloudkit-ref for CKSyncEngine patterns

3. Raw CloudKit APIs (Legacy):

   • CKContainer, CKDatabase, CKRecord
   • Manual sync management
   • Only if CKSyncEngine doesn't fit
   • Use skill: axiom-cloudkit-ref for raw API reference

iCloud Drive Path (File Sync)

// ✅ CORRECT: iCloud Drive for file-based sync
func saveToICloud(_ data: Data, filename: String) throws {
    // Get ubiquitous container
    guard let iCloudURL = FileManager.default.url(
        forUbiquityContainerIdentifier: nil
    ) else {
        throw StorageError.iCloudUnavailable
    }

    let documentsURL = iCloudURL.appendingPathComponent("Documents")
    try FileManager.default.createDirectory(
        at: documentsURL,
        withIntermediateDirectories: true
    )

    let fileURL = documentsURL.appendingPathComponent(filename)
    try data.write(to: fileURL)
}

When to use iCloud Drive:

• User-created documents that sync
• File-based collaboration
• Simple file sync (like Dropbox)

Use skill: axiom-icloud-drive-ref for implementation details

Key-Value Store (Small Preferences)

// ✅ CORRECT: Small synced preferences
let store = NSUbiquitousKeyValueStore.default

store.set(true, forKey: "darkModeEnabled")
store.set(2.0, forKey: "textSize")
store.synchronize()

Limitations:

• Max 1 MB total storage
• Max 1024 keys
• Max 1 MB per value
• For preferences ONLY, not data storage

---

Common Patterns and Anti-Patterns

✅ DO: Choose Based on Data Shape

// ✅ CORRECT: Structured data → SwiftData
@Model
class Note {
    var title: String
    var content: String
    var tags: [Tag]  // Relationships
}

// ✅ CORRECT: Files → FileManager + proper directory
let imageData = capturedPhoto.jpegData(compressionQuality: 0.9)
try imageData?.write(to: documentsURL.appendingPathComponent("photo.jpg"))

❌ DON'T: Use Files for Structured Data

// ❌ WRONG: Storing queryable data as JSON files
let tasks = [Task(...), Task(...), Task(...)]
let jsonData = try JSONEncoder().encode(tasks)
try jsonData.write(to: appSupportURL.appendingPathComponent("tasks.json"))

// Why it's wrong:
// - Can't query individual tasks
// - Can't filter or sort efficiently
// - No relationships
// - Entire file loaded into memory
// - Concurrent access issues

// ✅ CORRECT: Use SwiftData instead
@Model class Task { ... }

❌ DON'T: Store Re-downloadable Content in Documents

// ❌ WRONG: Downloaded images in Documents (bloats backup!)
func downloadProfileImage(url: URL) throws {
    let data = try Data(contentsOf: url)
    let documentsURL = FileManager.default.urls(
        for: .documentDirectory,
        in: .userDomainMask
    )[0]
    try data.write(to: documentsURL.appendingPathComponent("profile.jpg"))
}

// ✅ CORRECT: Use Caches instead
func downloadProfileImage(url: URL) throws {
    let data = try Data(contentsOf: url)
    let cacheURL = FileManager.default.urls(
        for: .cachesDirectory,
        in: .userDomainMask
    )[0]
    let fileURL = cacheURL.appendingPathComponent("profile.jpg")
    try data.write(to: fileURL)

    // Mark excluded from backup
    var resourceValues = URLResourceValues()
    resourceValues.isExcludedFromBackup = true
    try fileURL.setResourceValues(resourceValues)
}

❌ DON'T: Use CloudKit for Simple File Sync

// ❌ WRONG: Storing files as CKAssets with manual sync
let asset = CKAsset(fileURL: documentURL)
let record = CKRecord(recordType: "Document")
record["file"] = asset
// ... manual upload, conflict handling, etc.

// ✅ CORRECT: Use iCloud Drive for files
// Files automatically sync via ubiquitous container
try data.write(to: iCloudDocumentsURL.appendingPathComponent("doc.pdf"))

---

Quick Reference Table

Data Type	Format	Local Location	Cloud Sync	Use Skill
User tasks, notes	Structured	Application Support	SwiftData + CloudKit	axiom-swiftdata → axiom-cloudkit-ref
User photos (created)	File	Documents	iCloud Drive	axiom-file-protection-ref → axiom-icloud-drive-ref
Downloaded images	File	Caches	None (re-download)	axiom-storage-management-ref
Thumbnails	File	Caches	None (regenerate)	axiom-storage-management-ref
Database file	File	Application Support	CKSyncEngine (if custom)	axiom-sqlitedata → axiom-cloudkit-ref
Temp processing	File	tmp	None	N/A
User settings	Key-Value	UserDefaults	NSUbiquitousKeyValueStore	N/A

---

Debugging: Data Missing or Not Syncing?

Files disappeared:

• Check if stored in Caches or tmp (system purged them)
• Check file protection level (may be inaccessible when locked)
• Use skill: axiom-storage-diag

Backup too large:

• Check if re-downloadable content is in Documents (should be in Caches)
• Check if isExcludedFromBackup is set on large files
• Use skill: axiom-storage-management-ref

Data not syncing:

• CloudKit: Check CKSyncEngine status, account availability
  • Use skill: axiom-cloud-sync-diag
• iCloud Drive: Check ubiquitous container entitlements, file coordinator
  • Use skill: axiom-icloud-drive-ref, axiom-cloud-sync-diag

---

Migration Checklist

When changing storage approach:

Database to Database (e.g., Core Data → SwiftData):

• [ ] Create SwiftData models matching Core Data entities
• [ ] Write migration code to copy data
• [ ] Test with production-size datasets
• [ ] Keep old database for rollback

Files to Database:

• [ ] Identify all JSON/plist files storing structured data
• [ ] Create SwiftData models
• [ ] Write one-time migration on first launch
• [ ] Verify all data migrated, then delete old files

Local to Cloud:

• [ ] Ensure proper entitlements (CloudKit/iCloud)
• [ ] Handle initial upload carefully (bandwidth)
• [ ] Test conflict resolution
• [ ] Provide user control (opt-in)

---

Last Updated: 2025-12-12
Skill Type: Discipline
Related WWDC Sessions:

• WWDC 2023-10187: Meet SwiftData
• WWDC 2023-10188: Sync to iCloud with CKSyncEngine
• WWDC 2024-10137: What's new in SwiftData