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Sync Settings
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This page explains the settings available when you enable or configure Device Sync.
Available Settings
Sync Type
Atlas Device Sync has two sync modes: Flexible Sync and the older Partition-Based Sync. We recommend using Flexible Sync. For information about Partition-Based Sync, refer to Partition-Based Sync.
Flexible Sync lets you define a query in the client and sync only the objects that match the query. With client-side subscriptions, client applications can:
Maintain queries
React to changes
Add, change, or delete queries
Development Mode
Development Mode is a configuration setting that allows Device Sync to infer and update schemas based on client-side data models. This streamlines development, but should not be used in production.
Development Mode speeds up development by allowing you to design schemas directly in client application code.
When you sync a realm file, Atlas App Services maps every synced object type to its own collection in the database specified by Database Name (Development Mode Only). If you update the object model on the client side, App Services updates the collection schema to match. This lets you update objects in your client code as you develop your app.
You can use data access rules with Development Mode. Note that schema changes ignore data access rules. This means that any client can update the backend schema by changing the client model.
To learn more about how the Realm Object Schemas map to the App Services Schemas when using Development Mode, refer to Data Model Mapping.
For more information about modifying synced object schemas, including how to handle breaking changes, see: Update Your Data Model.
Important
Disable Development Mode for Production Apps
Development mode is a development utility that is not suitable for production use. Make sure that you turn off Development Mode before you make your app accessible in a production environment.
Cluster to Sync
The name of the Atlas cluster data source where you want the synced data stored.
Flexible Sync requires MongoDB 5.0. You cannot use Sync with a serverless instance. In the UI, clusters display as gray and not-selectable when they do not meet the requirements for Flexible Sync.
Database Name (Development Mode Only)
When you enable Development Mode, you specify a database to store synced objects. App Services creates new collections in this Development Mode database for every type of synced object.
Example
Specify a Development Mode database of myapp. Your iOS client has a
Person model. You sync a realm that contains an instance of the
Person object. Development Mode creates a server-side schema associated
with the model. The object syncs to the myapp.Person collection.
App Services continues creating new server-side schemas and collections
for each new object type. If you later add a Dog object, that object
will sync to a new myapp.Dog collection that App Services will
create.
Queryable Fields
When you configure Flexible Sync, you specify field names that your client application can query. Fields that can be used in a subscription query are called queryable fields.
You can automatically specify queryable fields by enabling Development Mode. Development Mode allows App Services to automatically mark fields as queryable as they are used.
Queryable fields apply across all collections in an App's Schema. You can use rules and permissions to configure more granular access control on individual collections.
The field names you provide are arbitrary strings. If an object type has a field whose name matches a field name you provided (and meets other eligibility criteria), that field becomes available to Device Sync to query.
Example
In a to-do list app, you might set assignee or owner
as queryable fields. On the client side, you can then query for tasks
whose assignee or owner matches the logged-in user.
Eligible Field Types
Flexible Sync only supports top-level primitive fields with a scalar type as queryable fields. You can also include arrays of these primitives as queryable fields. Flexible Sync does not support embedded objects or arrays of objects as queryable fields.
Tip
See also: Realm Query Language - Flexible Sync Limitations
For information on the queries you can perform on these fields, see: Flexible Sync RQL Limitations
Reserved Field Names
App Services reserves some keywords for the Realm Query Language and other purposes. You cannot use reserved keywords as field names.
App Services reserves the following keywords with any capitalization:
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Example
You cannot use descending, Descending, DESCENDING, or
DeScEnDiNG as a field name.
App Services also reserves the following keywords with the given exact capitalization:
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Example
You cannot use true or TRUE, since both capitalizations are
specifically reserved, but you can use True or tRUE as a field name.
Performance and Storage
Each queryable field adds additional metadata storage to your Atlas cluster and may lead to degraded write performance. You should have as few queryable fields as needed by your application. A good rule of thumb is to have at most 10 queryable fields.
Tip
See also:
Learn more about optimizing performance and storage when using Flexible Sync.
Consequences of Adding or Removing Queryable Fields
You can update your Sync configuration to add or remove queryable field names, but be aware of the following:
When you add a queryable field, devices can only sync on that field once the device has caught up to the point in time in Device Sync History where the field was added.
When you remove a queryable field, any devices still using that field will have their Device Sync session dropped and must perform a client reset. Clients not using the removed field won't receive any errors. To avoid triggering a client reset when you remove the queryable field, you should first remove usage of that field on the client-side.
Permissions
Atlas Device Sync enforces role-based data access rules for all requests to a synced cluster. Rules are dynamic JSON expressions that determine a user's ability to sync, view, and modify data.
For details, see Role-based Permissions.
Asymmetric Sync
Asymmetric Sync is a sync strategy for applications with heavy client-side insert-only workloads. You can enable it for one or more collections. It supports writing to any collection type, including an Atlas time-series collection.
For example, an IoT app that frequently logs sensor data has a significant write workload and no read workload. The device may also be offline for extended periods of time. Asymmetric Sync bypasses some of the processing required for bi-directional sync, significantly improving write speed to an Atlas collection.
Other use cases include writing immutable data, such as invoices from a retail app, or logging application events, neither of which requires conflict resolution.
You can apply Asymmetric Sync to individual collections. This means your app can use Asymmetric Sync to write some data, but bi-directional Flexible Sync on other collections.
Asymmetric Sync collections are only for writing data. You cannot use Flexible Sync queries against these collections. Instead, use Connect to MongoDB Data Sources.
After you have enabled Asymmetric Sync, you implement it in the client app via the client SDKs. Currently, the following Realm SDKs support Asymmetric Sync:
.NET SDK: Unidirectional Data Sync - .NET SDK
Node.js SDK: Define an Asymmetric Object
React Native SDK: Define an Asymmetric Object - React Native SDK
Swift SDK: Stream Data to Atlas - Swift SDK
Client Max Offline Time
Client Maximum Offline Time determines how long the client can be offline between sync sessions. Changing this value enables you to balance offline access with storage used in the synced Atlas cluster. For more information, refer to Client Maximum Offline Time.
Client Recovery
Client Recovery enables the client to attempt to automatically perform a client reset while recovering data on the device. For more information, refer to Recover Unsynced Changes.
Sync Configuration File Reference
You can find the Sync configuration file for your application in the sync
directory of an exported app:
app/ └── sync/ └── config.json
For example, the following Sync configuration applies to apps using Flexible Sync (recommended). If you are using the older Partition-Based Sync, refer to Partition-Based Sync Configuration.
{ "type": "flexible", "development_mode_enabled": <Boolean>, "service_name": "<Data Source Name>", "database_name": "<Development Mode Database Name>", "state": <"enabled" | "disabled">, "client_max_offline_days": <Number>, "is_recovery_mode_disabled": <Boolean>, "queryable_fields_names": [ <Array of String Field Names> ], "permissions": "<Deprecated, Do Not Use>" }
The deprecated permissions field might still appear in your exported app's
configuration. That might indicate your app has not automatically migrated to
the unified rule system yet. Please avoid
deleting this field until your app has been migrated.
Sync Config Object
Field | Description |
|---|---|
typeString | The sync mode. There are two Sync modes: Flexible Sync and the older Partition-Based Sync. We recommend using Flexible Sync. For more information about Partition-Based Sync, refer to Partition-Based Sync. Valid Options for a Flexible Sync Configuration:
|
development_mode_enabledBoolean | If true, Development Mode is enabled
for the application. While enabled, App Services automatically stores synced
objects in a specific database (specified in database_name) and
mirrors objects types in that database's collection schemas. |
service_nameString | The name of the Atlas cluster data source
to sync. You cannot use sync with a serverless instance. |
database_nameString | The name of a database in the synced cluster where App Services stores data in
Development Mode. App Services automatically
generates a schema for each synced type and maps each object type to a
collection within the database. |
stateString | The current state of the sync protocol for the application. Valid Options:
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client_max_offline_daysNumber | Controls how long the backend compaction
process waits before aggressively pruning metadata that some clients
require to synchronize from an old version of a realm. |
is_recovery_mode_disabledBoolean | If false, Recovery Mode is enabled
for the application. While enabled, Realm SDKs that support this feature
attempt to recover unsynced changes upon performing a client reset.
Recovery mode is enabled by default. |
queryable_fields_namesArray<String> | The names of the fields that your client
application can query to determine which data to synchronize. |
last_disabledNumber | The date and time that sync was last paused or disabled, represented by
the number of seconds since the Unix epoch (January 1, 1970, 00:00:00
UTC). |