Atlas Device Sync automatically synchronizes data between client applications and an Atlas App Services backend. When a client device is online, Sync asynchronously synchronizes data in a background thread between the device and your backend App.
When you select Flexible Sync for your backend App configuration, your client implementation must include subscriptions to queries on queryable fields. Flexible Sync works by synchronizing data that matches query subscriptions you maintain in the client application.
A subscription set contains a set of queries. Realm Flexible Sync returns documents matching those queries, where the user has the appropriate permissions to read and/or read and write the documents. If documents match the query, but the client does not have the permission to read or write them, they do not sync to the client application.
You can form queries using Realm Query Language.
Flexible Sync does not support all the query operators available in Realm Query Language and the SDK's query engine. See Flexible Sync RQL Limitations for details.
Subscription sets are based on a specific type of Realm object. You might have multiple subscriptions if you have many types of Realm objects.
To use Flexible Sync in your client application, open a synced realm with a flexible sync configuration. Then, manage subscriptions to determine which documents to sync.
The syntax to read, write, and watch for changes on a synced realm is identical to the syntax for non-synced realms. While you work with local data, a background thread efficiently integrates, uploads, and downloads changesets.
When Using Sync, Avoid Writes on the Main Thread
The fact that Realm performs sync integrations on a background thread means that if you write to your realm on the main thread, there's a small chance your UI could appear to hang as it waits for the background sync thread to finish a write transaction. Therefore, it's a best practice never to write on the main thread when using Sync.
Every write transaction for a subscription set has a performance cost. If you need to make multiple updates to a Realm object during a session, consider keeping edited objects in memory until all changes are complete. This improves sync performance by only writing the complete and updated object to your realm instead of every change.