Realm Swift
Mobile-first object store database for Swift (and Objective-C). Realm stores Swift objects directly — no ORM, no SQL mapping. Objects are live — querying returns live results that auto-update. Integrated with MongoDB Atlas Device Sync for offline-first agent apps that sync automatically when online. Alternative to Core Data with simpler API and Atlas Sync for cloud.
Score Breakdown
⚙ Agent Friendliness
🔒 Security
AES-256 encryption at rest. Atlas Sync uses TLS. MongoDB maintains. SOC2 compliance for Atlas. Thread safety enforced by Realm runtime.
⚡ Reliability
Best When
You need offline-first iOS agent apps with automatic cloud sync to MongoDB Atlas and want object-oriented data access without SQL.
Avoid When
You need complex relational queries or cross-platform data portability — SQLite or PostgreSQL are better for relational agent data.
Use Cases
- • Build offline-first agent iOS apps where agent data persists locally and syncs to MongoDB Atlas when connectivity is available
- • Store and query complex agent object graphs directly without ORM mapping — Realm's live objects simplify agent data access
- • Use Realm's reactive notifications to update agent UI when underlying data changes without manual polling
- • Build agent apps with conflict-free sync between multiple devices using Atlas Device Sync's operational transformation
- • Embed a fast local database in agent iOS apps with zero SQL knowledge — Realm's object-oriented API is simple and type-safe
Not For
- • Complex SQL queries and analytics — Realm is an object store; for relational queries, use SQLite or Core Data
- • Teams that need SQL portability — Realm's format is proprietary; migrating to another database requires data transformation
- • Apps without Atlas Sync requirements — if you just need local SQLite, use SQLite.swift or FMDB; Realm's complexity is justified by Atlas Sync
Interface
Authentication
Atlas Device Sync requires MongoDB Atlas authentication. App Services user auth with email/password, API keys, JWT, or OAuth providers. On-device Realm has no auth — file system protection.
Pricing
Local Realm is free and open source. Atlas Sync requires MongoDB Atlas account (free tier available).
Agent Metadata
Known Gotchas
- ⚠ Realm objects are live and thread-confined — passing a Realm object from one thread to another throws 'Object is not on the same thread' exception; use ThreadSafeReference to pass across threads
- ⚠ All writes must happen inside write transactions — calling .set() outside a write block throws 'Realm not in a write transaction' exception; always wrap mutations in realm.write {}
- ⚠ Schema migrations require incrementing the schemaVersion — if Realm detects a schema change without matching migration block, it throws; migrations cannot be skipped between versions
- ⚠ Notifications from Realm changes arrive on the thread that opened the Realm — UI updates require opening Realm on the main thread or using @ObservedResults on main thread
- ⚠ Atlas Device Sync conflict resolution is last-write-wins for most properties — agent apps with collaborative editing need careful data modeling to avoid silent data loss on sync
- ⚠ Realm file encryption: AES-256 encryption key must be the same on every open — losing the key makes the Realm file permanently unreadable; key management is critical for encrypted agent stores
Alternatives
Full Evaluation Report
Detailed scoring breakdown, competitive positioning, security analysis, and improvement recommendations for Realm Swift.
Scores are editorial opinions as of 2026-03-06.