Sync Overview - Kotlin SDK
Overview
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 use Sync in your client application, your implementation must match the Sync Mode you select in your backend App configuration. The Sync Mode options are:
Flexible Sync (recommended): lets you define a query in the client and sync only the objects that match the query.
Partition-Based Sync: documents in a synced cluster form a "partition" by having the same value for a field designated as "partition key". All documents in a partition have the same read/write permissions for a given user.
You can only use one Sync Mode for your application.
Tip
See also:
Partition-Based Sync
When you select Partition-Based Sync for your backend App configuration, your client implementation must include a partition value. This is the value of the partition key field you select when you configure Partition-Based Sync.
The partition value determines which data the client application can access.
You must provide a partition value when you open a synced realm.
Flexible Sync
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.
Note
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 object type. 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 an initial set of subscriptions to determine which documents to sync.
Group Updates for Improved Performance
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.