MongoDB Realm Mobile Sync

MongoDB Realm and Atlas Device Sync (EOL)

CRITICAL: Atlas Device Sync reached end-of-life September 30, 2025. All Device Sync and Device SDK features were shut down on that date. This skill provides historical context for customers still migrating away.

What Was Realm / Atlas Device Sync

Realm was a mobile-first database SDK with:

Realm Database (local): Embedded object database for iOS/Android (still available as standalone open-source)
Atlas Device Sync: Cloud sync between Realm local database and MongoDB Atlas (EOL Sep 30 2025)
Device SDKs: Swift, Kotlin, Java, Flutter, React Native, .NET, JavaScript (web)

EOL Timeline

Date	Event
January 2023	Atlas Device Sync deprecation announced
September 30, 2025	Atlas Device Sync shut down
September 30, 2025	All Device SDK sync features ceased
Ongoing	Realm Database (local, no sync) remains open-source

What Was Flexible Sync (Last Supported Mode)

Flexible Sync replaced Partition-Based Sync as the final architecture:

Subscriptions defined per-client based on query (e.g., realm.query<Task>().where("userId == $0", userId))
Server-side rules enforced permissions on which documents each client could sync
Offline-first: writes to local Realm database, synced when online

Migration Paths (Post-EOL)

Original use case	Recommended replacement
Mobile app offline-first with MongoDB	Custom sync layer (WebSocket / SSE / polling) + MongoDB driver
iOS app with Realm local database	Continue using Realm Database (standalone, no sync) or SQLite/SwiftData
Android app with Realm local database	Continue using Realm Database or Room/SQLite
Flutter offline-first	ObjectBox, Drift (SQLite), or Isar Database
React Native offline-first	WatermelonDB, MMKV + custom sync, or PouchDB + CouchDB
Multi-device real-time sync	Firebase Realtime Database, Supabase Realtime, or custom WebSocket layer

Realm Database (Local — Still Available)

The local Realm Database (without sync) remains available as open-source:

realm-swift — iOS/macOS Swift/Objective-C
realm-kotlin — Android/Kotlin Multiplatform
realm-js — Node.js, React Native
realm-dotnet — .NET/Xamarin

Use Realm local database for: fast local embedded storage without cloud sync requirements.

Common Migration Scenarios

Migrating Change Listeners to MongoDB Change Streams

What was a Realm change listener:

// Old Realm sync pattern
let tasks = realm.objects(Task.self)
let token = tasks.observe { changes in
    // Handle changes
}

Replace with a MongoDB change stream consumer in your backend:

// Backend: publish changes to mobile clients via SSE or WebSocket
const changeStream = db.tasks.watch(
  [{ $match: { "fullDocument.userId": userId } }],
  { fullDocument: "updateLookup" }
);
for await (const change of changeStream) {
  sse.send(change);  // Push to mobile client via SSE
}

Migrating Offline-First Writes

// Mobile: write to local store (MMKV, SQLite, etc.)
localDB.insert({ _id: uuid(), taskName: "Buy milk", synced: false });

// Background sync worker: push unsynced records to Atlas
const unsynced = localDB.query("SELECT * FROM tasks WHERE synced = 0");
for (const record of unsynced) {
  await mongodbApi.post('/tasks', record);
  localDB.update(record.id, { synced: 1 });
}

Migrating Atlas App Services Authentication

Atlas App Services Authentication providers were also EOL’d September 30, 2025. Migrate to:

Auth0 — drop-in OIDC replacement with mobile SDKs
Firebase Auth — Google’s mobile auth platform
AWS Cognito — AWS-native mobile auth
Clerk — Modern developer auth platform