Mobile Data Storage
Mobile apps need to store data locally — user preferences, cached content, offline data, and more. Different storage mechanisms suit different needs. Choosing correctly affects performance, reliability, and user experience.
Storage Options
UserDefaults (iOS) / SharedPreferences (Android) store small key-value pairs. Perfect for settings, feature flags, and authentication tokens. They're fast and simple but not designed for large amounts of data.
// iOS
UserDefaults.standard.set("dark", forKey: "theme")
let theme = UserDefaults.standard.string(forKey: "theme")
SQLite provides a full relational database on the device. It handles complex queries, relationships between data, and large datasets efficiently. Both platforms include SQLite support.
Realm offers an object database that's often easier than SQLite. You work with objects directly rather than writing SQL. It also provides built-in synchronization features.
Core Data (iOS) is Apple's persistence framework built on SQLite. It adds object mapping, change tracking, and iCloud sync. Powerful but has a learning curve.
File storage works for documents, images, and large binary data. Each platform provides directories for different purposes — documents that should persist, caches that can be cleared, and temporary files.
Choosing the Right Storage
Match storage to data characteristics:
Settings and tokens belong in UserDefaults/SharedPreferences. Small, simple, frequently accessed.
Structured data like user profiles, messages, or product catalogs fits SQLite or Realm. You need queries, relationships, and efficient updates.
Large files like images, videos, or documents go in file storage. Databases aren't designed for binary blobs.
Temporary data uses cache directories. The system may delete these when storage runs low, so don't store anything important there.
Synchronization Strategies
Most apps need to sync local data with a server. Common patterns include:
Pull on launch fetches fresh data when the app opens. Simple but can feel slow.
Background sync updates data periodically or when conditions are right (Wi-Fi, charging). Keeps data fresh without user action.
Real-time sync uses WebSockets or push notifications to update immediately when server data changes.
Conflict resolution handles cases where local and server data diverge. Timestamps, version numbers, or operational transforms help determine which changes win.
The right strategy depends on how fresh data needs to be and how often it changes.