Realm — High-Performance Mobile Database

Introduction

Realm is an object-oriented mobile database built from the ground up as an alternative to SQLite and Core Data. It stores native objects directly, eliminating the object-relational mapping layer. Realm is available for Swift, Kotlin, Java, C#, and JavaScript (React Native), providing a consistent data layer across mobile platforms.

What Realm Does

• Stores native language objects without manual serialization or ORM boilerplate
• Delivers lazy-loaded query results with zero-copy reads for fast performance
• Provides live objects that update automatically when underlying data changes
• Supports cross-platform data models for iOS, Android, and React Native
• Optionally syncs with MongoDB Atlas Device Sync for real-time cloud backup

Architecture Overview

Realm uses a custom C++ storage engine based on a memory-mapped B+ tree. Data is written in a multiversion concurrency control (MVCC) format, allowing readers and writers to operate without blocking each other. Platform-specific SDKs wrap the core engine and expose idiomatic APIs. Queries are lazy — results are not materialized until accessed.

Self-Hosting & Configuration

• Add the Realm SDK via Swift Package Manager, CocoaPods, Gradle, or npm
• No server is needed for local-only usage — data lives in a .realm file on device
• Configure file location, encryption key, and schema version at Realm initialization
• Enable Atlas Device Sync by connecting to a MongoDB Atlas backend (optional)
• Schema migrations are defined in code and run automatically on version bumps

Key Features

• Zero-copy architecture reads data directly from memory-mapped files
• MVCC concurrency allows simultaneous reads and writes without locks
• Live objects and collection notifications power reactive UI patterns
• AES-256 encryption protects the database file at rest
• Compact file format keeps storage footprint small on mobile devices

Comparison with Similar Tools

• SQLite — relational with SQL queries; Realm uses native object APIs and avoids ORM
• Core Data — Apple-only framework on top of SQLite; Realm is cross-platform and simpler
• Room (Android) — SQLite abstraction for Android; Realm offers reactive objects and cross-platform support
• WatermelonDB — built on SQLite for React Native; Realm has its own engine and broader platform coverage

FAQ

Q: Is Realm free to use? A: The local database SDKs are free and open source under Apache 2.0. Atlas Device Sync is a paid MongoDB service.

Q: Can Realm handle large datasets on mobile? A: Yes. The lazy query engine and memory-mapped storage handle millions of objects efficiently on device.

Q: Does Realm support SQL queries? A: No. Realm uses its own query syntax with type-safe predicate builders in each language SDK.

Q: What happens to data when the app is deleted? A: The .realm file lives in the app sandbox and is removed with the app unless backed up to a cloud sync service.

Sources

• https://github.com/realm/realm-swift
• https://www.mongodb.com/docs/atlas/device-sdks/