---

name: mobile-storage description: > Use this skill when the user asks about mobile local storage, SQLite, Room, Core Data, Hive, Isar, SharedPreferences, UserDefaults, file storage, or database migration. version: "1.0.0" author: "j4flmao" license: "MIT" compatibility: claude-code: true cursor: true codex: true windsurf: true tags: [mobile, storage, phase-4, universal]

Mobile Storage

Purpose

Design mobile local storage architectures selecting the right persistence strategy — SQLite, Room, Core Data, Hive, Isar, key-value, or file storage — with migration planning.

Agent Protocol

Trigger

User request includes: mobile storage, local database, sqlite mobile, room database, core data, hive, isar, sharedpreferences, userdefaults, file storage mobile, migration mobile.

Input Context

• Platform (iOS, Android, Flutter, React Native)
• Data complexity (simple K/V vs relational vs document)
• Storage library (Room, Core Data, Hive, Isar, WatermelonDB)
• Migration requirements

Output Artifact

A markdown document containing:

• Storage layer architecture
• Schema design
• CRUD operations
• Migration plan

Response Format

No preamble. No postamble. No explanations.

Max Response Length

4096 tokens

Decision Trees

Storage Type Selection

What kind of data?
├── Simple key-value (settings, preferences, feature flags)
│   ├── Secure (tokens, auth) → Keychain / EncryptedSharedPrefs / flutter_secure_storage
│   └── Non-sensitive → UserDefaults / SharedPreferences / AsyncStorage
├── Complex queryable (user records, products, orders)
│   ├── Relational with JOINs/aggregations → SQLite (Room, CoreData, sqflite, WatermelonDB)
│   └── Document/NoSQL (nested, simple queries) → Isar, Hive, Realm
├── Large files (images, video, documents)
│   ├── Cache → Temporary directory, LRU eviction
│   └── Permanent → Application documents directory
└── Real-time sync across devices → CloudKit, Firebase, custom API + local cache

Migration Strategy

Is schema likely to change?
├── Yes, planned evolution (most apps)
│   ├── Version schema from v1
│   ├── Write incremental migrations (1→2, 2→3, never skip)
│   ├── Test migration from every previous version
│   └── Have rollback plan (destructive vs non-destructive)
├── Schema rarely changes (utilities, tools)
│   └── Single version, handle via JSON blobs for flexibility
└── Frequent changes (rapid prototyping)
    └── Consider destructive migration (delete and recreate) for non-production

Sync Strategy

Does data sync across devices?
├── No sync needed → Local-first, local DB is source of truth
├── Online-first (network authoritative, local is cache)
│   └── Cache API responses, stale-while-revalidate
└── Offline-first (local authoritative, background sync)
    └── Local-first writes, sync queue, conflict resolution

Workflow

Step 1: Select Storage Type

Step 2: Design Schema

Define entities/tables with proper column types, indexes for query paths, and relationships.

Step 3: Implement CRUD Operations

Set up DAOs/repositories with insert, read, update, delete operations and query methods.

Step 4: Plan Migrations

Version the schema from day one, write migration scripts for each version change, and test rollback paths.

Storage Selection

Need	iOS	Android	Flutter	RN
Simple K/V	UserDefaults	SharedPreferences	SharedPreferences	AsyncStorage
Secure K/V	Keychain	EncryptedSharedPrefs	flutter_secure_storage	react-native-keychain
Relational	Core Data	Room	sqflite / drift	WatermelonDB
Document	Realm	Realm	Isar / Hive	Realm
File	FileManager	Internal Storage	path_provider	react-native-fs

Implementation — Relational Databases

Android — Room

// Entity
@Entity(
  tableName = "orders",
  indices = [Index("customer_id"), Index("status", "created_at")]
)
data class OrderEntity(
  @PrimaryKey val id: String,
  @ColumnInfo(name = "customer_id") val customerId: String,
  val status: String,
  val total: Double,
  @ColumnInfo(name = "created_at") val createdAt: Long
)

// DAO
@Dao
interface OrderDao {
  @Query("SELECT * FROM orders WHERE customer_id = :customerId ORDER BY created_at DESC")
  fun getOrders(customerId: String): Flow<List<OrderEntity>>

  @Query("SELECT COUNT(*) FROM orders WHERE status = :status")
  fun countByStatus(status: String): Flow<Int>

  @Insert(onConflict = OnConflictStrategy.REPLACE)
  suspend fun upsertAll(orders: List<OrderEntity>)

  @Query("DELETE FROM orders WHERE id = :id")
  suspend fun deleteById(id: String)

  @Transaction
  @Query("SELECT * FROM orders o INNER JOIN customers c ON o.customer_id = c.id WHERE o.status = :status")
  fun getOrdersWithCustomer(status: String): Flow<List<OrderWithCustomer>>
}

// Database
@Database(entities = [OrderEntity::class, CustomerEntity::class], version = 2)
abstract class AppDatabase : RoomDatabase() {
  abstract fun orderDao(): OrderDao
  abstract fun customerDao(): CustomerDao

  companion object {
    @Volatile private var INSTANCE: AppDatabase? = null

    fun getInstance(context: Context): AppDatabase {
      return INSTANCE ?: synchronized(this) {
        Room.databaseBuilder(context, AppDatabase::class.java, "app.db")
          .addMigrations(MIGRATION_1_2)
          .build()
          .also { INSTANCE = it }
      }
    }

    val MIGRATION_1_2 = object : Migration(1, 2) {
      override fun migrate(db: SupportSQLiteDatabase) {
        db.execSQL("ALTER TABLE orders ADD COLUMN notes TEXT DEFAULT ''")
        db.execSQL("CREATE INDEX idx_orders_notes ON orders(notes)")
      }
    }

    val MIGRATION_2_3 = object : Migration(2, 3) {
      override fun migrate(db: SupportSQLiteDatabase) {
        db.execSQL("CREATE TABLE customers (id TEXT PRIMARY KEY, name TEXT NOT NULL)")
        db.execSQL("CREATE INDEX idx_orders_customer ON orders(customer_id)")
      }
    }
  }
}

iOS — Core Data

import CoreData

class CoreDataStack {
  static let shared = CoreDataStack()
  let container: NSPersistentContainer

  private init() {
    container = NSPersistentContainer(name: "AppModel")
    container.loadPersistentStores { _, error in
      if let error = error { fatalError("Core Data failed: \(error)") }
    }
    container.viewContext.automaticallyMergesChangesFromParent = true
  }

  var context: NSManagedObjectContext { container.viewContext }

  func save() {
    if context.hasChanges {
      try? context.save()
    }
  }

  // Migration
  // Configure in AppModel.xcdatamodeld:
  // - Add model version (Editor > Add Model Version)
  // - Set current version in File Inspector
  // - Core Data handles lightweight migration automatically for:
  //   - Adding/removing optional attributes
  //   - Renaming with renamingID
  //   - Adding/removing relationships
  // For heavy migration: NSMigrationManager with custom mapping model
}

// Fetch request with predicate and sort
extension OrderEntity {
  static func fetchByStatus(_ status: String, in context: NSManagedObjectContext) -> [OrderEntity] {
    let request = OrderEntity.fetchRequest()
    request.predicate = NSPredicate(format: "status == %@", status)
    request.sortDescriptors = [NSSortDescriptor(key: "createdAt", ascending: false)]
    return (try? context.fetch(request)) ?? []
  }

  // NSFetchedResultsController for reactive updates
  static func fetchedResultsController(status: String) -> NSFetchedResultsController<OrderEntity> {
    let request = OrderEntity.fetchRequest()
    request.predicate = NSPredicate(format: "status == %@", status)
    request.sortDescriptors = [NSSortDescriptor(key: "createdAt", ascending: false)]

    return NSFetchedResultsController(
      fetchRequest: request,
      managedObjectContext: CoreDataStack.shared.context,
      sectionNameKeyPath: nil,
      cacheName: nil
    )
  }
}

Flutter — Drift (SQLite)

import 'package:drift/drift.dart';
import 'package:drift/native.dart';

// Table definition
class Orders extends Table {
  TextColumn get id => text()();
  TextColumn get customerId => text()();
  TextColumn get status => text()();
  RealColumn get total => real()();
  DateTimeColumn get createdAt => dateTime()();

  @override
  Set<Column> get primaryKey => {id};

  @override
  List<Set<Column>> get uniqueKeys => [{customerId, status}];  // Composite index
}

// Database
@DriftDatabase(tables: [Orders, Customers])
class AppDatabase extends _$AppDatabase {
  AppDatabase() : super(_openConnection());

  @override
  int get schemaVersion => 2;

  @override
  MigrationStrategy get migration {
    return MigrationStrategy(
      onCreate: (m) async {
        await m.createAll();
      },
      onUpgrade: (m, from, to) async {
        if (from == 1) {
          await m.addColumn(orders, orders.notes);
        }
      },
    );
  }

  // Reactive queries — return Stream
  Stream<List<Order>> watchOrders(String customerId) {
    return (select(orders)
        ..where((o) => o.customerId.equals(customerId))
        ..orderBy([(o) => OrderingTerm(expression: o.createdAt, mode: OrderingMode.desc)])
    ).watch();
  }
}

LazyDatabase _openConnection() {
  return LazyDatabase(() async {
    final file = File(await getDatabasesPath() + '/app.db');
    return NativeDatabase(file);
  });
}

React Native — WatermelonDB

// Schema
import { appSchema, tableSchema } from '@nozbe/watermelondb';

export const schema = appSchema({
  version: 2,
  tables: [
    tableSchema({
      name: 'orders',
      columns: [
        { name: 'customer_id', type: 'string', isIndexed: true },
        { name: 'status', type: 'string' },
        { name: 'total', type: 'number' },
        { name: 'created_at', type: 'number' },
      ],
    }),
    tableSchema({
      name: 'customers',
      columns: [
        { name: 'name', type: 'string' },
        { name: 'email', type: 'string', isIndexed: true },
      ],
    }),
  ],
  migrations: [
    {
      toVersion: 2,
      steps: [
        addColumns({ table: 'orders', columns: [{ name: 'notes', type: 'string' }] }),
      ],
    },
  ],
});

// Model
import { Model } from '@nozbe/watermelondb';
import { field, date, relation } from '@nozbe/watermelondb/decorators';

export class Order extends Model {
  static table = 'orders';
  static associations = { customers: { type: 'belongs_to', key: 'customer_id' } };

  @field('customer_id') customerId: string;
  @field('status') status: string;
  @field('total') total: number;
  @date('created_at') createdAt: Date;
  @relation('customers', 'customer_id') customer;
}

// Repository
class OrderRepository {
  async getByStatus(status: string): Promise<Order[]> {
    return database.get<Order>('orders')
      .query(Q.where('status', status), Q.sortBy('created_at', 'desc'))
      .fetch();
  }

  async create(order: Partial<Order>): Promise<Order> {
    return database.write(async () => {
      return database.get<Order>('orders').create(order);
    });
  }

  observeByCustomer(customerId: string) {
    return database.get<Order>('orders')
      .query(Q.where('customer_id', customerId), Q.sortBy('created_at', 'desc'))
      .observe();  // Reactive
  }
}

Implementation — Document/NoSQL

Flutter — Isar

import 'package:isar/isar.dart';

// Collection
@collection
class Order {
  Id id = Isar.autoIncrement;  // Auto-increment
  late String remoteId;
  late String customerName;
  late double total;
  late DateTime createdAt;
  String? notes;
  late List<String> tags;    // List supported natively
  late Address? address;     // Embedded object

  @Index()
  late String status;        // Indexed
}

@embedded
class Address {
  late String street;
  late String city;
  late String zip;
}

// Usage
class OrderRepository {
  final Isar db;

  Stream<List<Order>> watchByStatus(String status) {
    return db.orders
      .where()
      .statusEqualTo(status)
      .watch(fireImmediately: true);
  }

  Future<void> saveOrder(Order order) async {
    await db.writeTxn(() => db.orders.put(order));
  }

  Future<List<Order>> search(String query) async {
    return db.orders
      .where()
      .customerNameContains(query, caseSensitive: false)
      .findAll();
  }
}

Flutter — Hive

import 'package:hive_flutter/hive_flutter.dart';

@HiveType(typeId: 0)
class Order extends HiveObject {
  @HiveField(0)
  late String id;

  @HiveField(1)
  late String customerName;

  @HiveField(2)
  late double total;

  @HiveField(3)
  late String status;
}

// Usage
class OrderBox {
  static const boxName = 'orders';

  static Future<void> open() async {
    await Hive.openBox<Order>(boxName);
  }

  static Box<Order> get box => Hive.box<Order>(boxName);

  static void save(Order order) => box.put(order.id, order);
  static Order? get(String id) => box.get(id);
  static List<Order> getAll() => box.values.toList();
  static Stream<BoxEvent> watch() => box.watch();  // Reactive
}

Key-Value Storage

Secure Storage

// Flutter — flutter_secure_storage
final storage = FlutterSecureStorage();
await storage.write(key: 'auth_token', value: token);
final token = await storage.read(key: 'auth_token');

// RN — react-native-keychain
await Keychain.setGenericPassword('token', token, { service: 'auth' });
const credentials = await Keychain.getGenericPassword({ service: 'auth' });

Preferences

// Android — SharedPreferences (non-sensitive only)
val prefs = context.getSharedPreferences("app_prefs", Context.MODE_PRIVATE)
prefs.edit().putString("theme", "dark").apply()
val theme = prefs.getString("theme", "light")

// iOS — UserDefaults
UserDefaults.standard.set("dark", forKey: "theme")
let theme = UserDefaults.standard.string(forKey: "theme")

// RN — AsyncStorage
await AsyncStorage.setItem('theme', 'dark');
const theme = await AsyncStorage.getItem('theme');

File Storage

Flutter — path_provider

import 'package:path_provider/path_provider.dart';

Future<String> get cacheDir async {
  final dir = await getTemporaryDirectory();
  return dir.path;
}

Future<String> get documentsDir async {
  final dir = await getApplicationDocumentsDirectory();
  return dir.path;
}

Future<void> cacheImage(String url, Uint8List bytes) async {
  final dir = await getTemporaryDirectory();
  final file = File('${dir.path}/${url.hashCode}.jpg');
  await file.writeAsBytes(bytes);
}

iOS — FileManager

let fileManager = FileManager.default
let documentsURL = fileManager.urls(for: .documentDirectory, in: .userDomainMask)[0]
let cacheURL = fileManager.urls(for: .cachesDirectory, in: .userDomainMask)[0]
let tempURL = fileManager.temporaryDirectory

// Exclude documents from backup
var resourceValues = URLResourceValues()
resourceValues.isExcludedFromBackup = true
try documentsURL.setResourceValues(resourceValues)

// Write
let data = try JSONEncoder().encode(orders)
try data.write(to: documentsURL.appendingPathComponent("orders.json"))

Android — Context Files

// Internal storage (private to app)
val file = File(context.filesDir, "orders.json")
file.writeText(json)

// Cache
val cacheFile = File(context.cacheDir, "image_cache")
cacheFile.writeBytes(bytes)

// External storage (requires permission)
val externalDir = context.getExternalFilesDir(null)

Migration Patterns

Destructive Migration

// Room — fallback when migration not possible
Room.databaseBuilder(context, AppDatabase::class.java, "app.db")
  .fallbackToDestructiveMigration()
  .build()
// Caution: destroys all existing data

Migration Testing

// Android — MigrationTestHelper
@Test
fun migrateFrom1To2_keepsData() {
  helper.createDatabase(TEST_DB_NAME, 1).apply {
    execSQL("INSERT INTO orders (id, customer_id, status, total) VALUES ('1', 'c1', 'pending', 50.0)")
  }
  val db = helper.runMigrationsAndValidate(
    TEST_DB_NAME, 2, true, MIGRATION_1_2
  )
  val cursor = db.query("SELECT * FROM orders WHERE id = '1'")
  assertThat(cursor.count).isEqualTo(1)  // Data preserved
  assertThat(cursor.getColumnIndex("notes")).isNotEqualTo(-1)  // New column exists
}

Non-Destructive Schema Changes

-- Safe changes (no data loss)
ALTER TABLE orders ADD COLUMN notes TEXT DEFAULT '';
CREATE INDEX idx_orders_status ON orders(status);

-- Destructive changes require migration logic
-- Bad: ALTER TABLE orders DROP COLUMN old_field;
-- Good: Create new table, copy data with transformation, drop old table

Reactive Queries

Room Flow

// Room returns Flow — automatically re-emits on table changes
@Dao
interface OrderDao {
  @Query("SELECT * FROM orders WHERE customer_id = :id ORDER BY created_at DESC")
  fun observeByCustomer(id: String): Flow<List<OrderEntity>>

  @Query("SELECT SUM(total) FROM orders WHERE status = :status")
  fun observeTotalByStatus(status: String): Flow<Double>
}

Core Data NSFetchedResultsController

// Automatic UI updates via NSFetchedResultsControllerDelegate
class OrdersViewController: UIViewController, NSFetchedResultsControllerDelegate {
  private lazy var frc = OrderEntity.fetchedResultsController(status: "pending")

  override func viewDidLoad() {
    frc.delegate = self
    try? frc.performFetch()
  }

  func controllerWillChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
    tableView.beginUpdates()
  }
  func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
    tableView.endUpdates()
  }
}

Drift Stream

// Drift queries return Stream — watch for real-time updates
final stream = db.watchOrders(customerId);
stream.listen((orders) {
  setState(() => this.orders = orders);
});

Batch Operations

// Room — batch insert in transaction
@Transaction
suspend fun insertBatch(orders: List<OrderEntity>) {
  orderDao.upsertAll(orders)  // Single transaction internally
}

// Manual transaction
@Transaction
suspend fun complexBatch() {
  orderDao.deleteByStatus("cancelled")
  orderDao.upsertAll(newOrders)
  customerDao.updateStats(totalOrders)
}

Anti-Patterns

• Storing tokens in UserDefaults/SharedPreferences: Plaintext on disk. Always use Keychain/EncryptedSharedPrefs
• No migration from v1: Impossible to update schema without data loss. Version from day one
• SELECT * without LIMIT: Loads entire table into memory. Always paginate
• Storing files in database: BLOBs in SQLite kill performance. Store file path, keep file on disk
• Synchronous DB access on main thread: ANR/frozen UI. Always async with coroutines/async-await
• No indexes on query columns: Full table scans on every query. Index WHERE, JOIN, and ORDER BY columns
• One storage solution for everything: Settings (K/V) + business data (SQLite) + files (FileManager) need different strategies
• Not closing database connections: Leaks file handles. Use dependency injection with scoped lifecycle
• No reactive queries: Manually re-fetching data creates stale UI. Use Flow/Combine/Stream/watch for automatic updates
• Over-indexing: Every index slows writes 5-15%. Index only what you query on
• Mutable state outside DB: In-memory list diverges from persisted state. DB is source of truth
• Not handling concurrent access: Multiple threads writing simultaneously causes corruption. Use transactions
• Ignore foreign keys: SQLite doesn't enforce by default. Execute PRAGMA foreign_keys = ON after connection

Performance Considerations

• Batch writes into transactions: 100 single inserts = 100x slower than 1 batch of 100
• Index columns used in WHERE, JOIN, ORDER BY, GROUP BY
• Composite indexes for multi-column queries — column order matters (most selective first)
• Project only needed columns — never SELECT *
• Use prepared statements — reuse query plan, prevent SQL injection
• In-memory cache (LRU) for frequently read, rarely changed data
• Cache TTL: 5min for user data, 1h for reference data, never for critical state
• L2 cache: serialize to disk for app restart recovery
• WatermelonDB/Isar lazy-load only visible records — ideal for large datasets
• SQLite WAL mode for concurrent read/write performance
• PRAGMA mmap_size for large databases on modern devices
• Monitor DB size growth — set up alerts for unexpected growth

References

• references/cloud-storage.md — Cloud Storage Integration
• references/file-system.md — Mobile File System
• references/local-storage.md — Local Data Storage
• references/mobile-storage-patterns.md — Mobile Storage Patterns
• references/preferences.md — Mobile Preferences
• references/sqlite.md — Mobile SQLite

Handoff

After storage setup, hand off to:

• mobile/universal/offline-first — Sync engine, conflict resolution
• mobile/universal/security — Database encryption, secure file storage
• mobile/universal/performance — Query optimization, cache strategy
• mobile/universal/testing — Migration testing, data integrity tests
• mobile/universal/networking — API response caching layer
• mobile/android — Room specifics, SharedPreferences
• mobile/ios — CoreData, Keychain
• mobile/flutter — Drift, Isar, Hive
• mobile/react-native — WatermelonDB, AsyncStorage