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Update Your Data Model

On this page

  • Breaking Changes
  • Breaking vs. Non-Breaking Change Quick Reference
  • Development Mode and Breaking Changes
  • Add an Object Type
  • Add a Default Value on a Property
  • Add a Required Property
  • Modify Existing Atlas Documents
  • Add an Optional Property
  • Remove an Object Type
  • Remove a Property
  • Change an Object's Name
  • Change a Property Name
  • Change a Property Type but Keep the Name
  • Change a Property's Status Between Optional and Required
  • Using the Partner Collection Strategy

When developing an application using Atlas Device Sync, you may need to make changes to your schema at some point, such as when you need to:

  • Add a new property to an already-synced object

  • Remove a property from an already-synced object

  • Change the type stored in a property

  • Update an optional property to required

To make it easier to understand how schema changes affect your app, we characterize them as "breaking" versus "non-breaking" changes.

Atlas App Services provides for non-breaking schema changes to synced realms, allowing old clients to sync with newer ones.

Breaking schema changes, however, take some planning and work, and should be avoided whenever possible.

Breaking schema changes are difficult because older clients (those that have not been updated to your new code and schema) still need access to the data via the old schema definition. Clients that are updated need to work with the new schema changes.

Note

Make breaking changes via the Atlas App Services UI

Because breaking or destructive schema changes require special handling, App Services does not support making these changes via App Services CLI or automated deploy with GitHub. Instead, you should make breaking changes in the App Services UI.

A breaking change is a change that you make in your server-side schema that requires additional action to handle. A breaking server-side schema change requires you to terminate sync in the backend, and then re-enable sync. Breaking schema changes result in clients being unable to open a realm, or the appearance of data loss when server-side documents are unable to sync to client-side applications. They prevent applications from automatically recovering from a client reset.

A non-breaking change is a change that you can make in your server-side schema or your object model without requiring additional handling in your app. Also known as additive changes, they apply automatically to synced realms.

Note

Applying non-breaking changes to the client may require migration

When you only make non-breaking changes to the server-side schema, no additional action is required. However, if you then try to apply these changes to your client object model, you may need to perform a migration. If the client device has an existing realm file, you must perform a migration. For details, see the Modify an Object Schema page in your preferred SDK.

The following diagram shows the types of schema changes you can make and the process you go through to perform the change. Each of these changes is explained in more detail in the table and sections below.

Flow chart of schema changes

This table summarizes each type of change and whether it is a breaking or non-breaking change.

Type of Change
Server-Side Schema
Client-Side Object Model
Add an object type
Non-breaking
Non-breaking
Non-breaking
Non-breaking
Non-breaking
Non-breaking
Non-breaking
Non-breaking
Breaking
Non-breaking
Breaking
Non-breaking
Breaking*
Breaking*
Breaking*
Breaking*
Breaking
Breaking
Breaking
Breaking

Tip

Rename a property

While renaming a property or object is a breaking change, some of the Realm SDKs offer a workaround to remap the property name. See Change a Property Name for more details.

Applies to App Services Apps created after September 13, 2023.

App Services Apps in Development Mode that were created after September 13, 2023 can make breaking changes from their client code to synced object schemas.

Refer to Development Mode for details about making breaking changes in Development Mode.

Development Mode is not suitable for production use. If you use Development Mode, make sure to disable it before moving your app to production.

You can add an object type to either the server-side schema or the client object model without doing any additional handling.

If you want to add an object type to both the server-side schema and the client object model, you can add the object type to the object model, and use Development Mode to let App Services handle the server-side schema updates. Or you can manually add the object type to both the model and the schema.

Note

These changes may trigger a resync

When you add a new object type, we retrieve the documents for the collection and re-insert them into App Services to get the new values. This is expected behavior, but it does cause a temporary halt to propagating changes while this process is underway.

You can add a default value on an object's required property. When you insert an Atlas document missing this required property into the collection, the Realm clients set the property to the default value. However, the same property on the Atlas document remains empty until a client makes changes to the property or updates the document directly in Atlas.

For more information on how default values are helpful when modifying existing Atlas documents, see Add a Required Property.

Warning

Ensure the default value type and property's type are the same

The default field does not have type validation. If the default field's type and the property's type are not the same, the error will indicate that the document is missing a required field.

You can add a required property to the client's object model, and use Development Mode to let App Services infer the server-side schema updates. Or you can manually add the required property to both the client model and the Atlas schema. However, you should consider making the property optional to avoid the need to modify existing Atlas documents.

Note

Required Properties Missing in a Schema Subset Will Default to Zero

Clients can open the Realm with a schema subset that doesn't include a required property. The server populates the missing required value field with a zero or blank value (like 0, "", or 0.0 depending on the property type) when the document syncs.

When you add a new required property, you must update existing documents with the new property or they do not sync to the client. This may give client users the impression that the data has been lost. Resolve this issue by adding the new property to each impacted document and populating it with a value. After you update the documents to match the client schema, they sync to the client application.

When you add a new required property, App Services retrieves the documents for the collection that have new values per the updated schema. App Services iterates through those documents and re-inserts them to get the new values. This is expected behavior, but it does cause a temporary halt to propagating changes while this process is underway.

Important

App Services uses a __realm_sync.unsynced_documents collection to track unsynced documents. When you add a required property, the re-sync process can push this collection over the limit of 100,000 documents. In this case, you must terminate and re-enable sync, even though the change you're making is a non-breaking change.

If you want to add an optional property to both the server-side schema and the client object model, you can add the optional property to the object model, and use Development Mode to let App Services infer the server-side schema updates. Or you can manually add the optional property to both the model and the schema.

Note

These changes may trigger a resync

When you add a new optional property, we retrieve the documents for the collection that have new values per the updated schema. We iterate through those documents and re-insert them into App Services to get the new values. This is expected behavior, but it does cause a temporary halt to propagating changes while this process is underway.

You can remove an object from the client's object model as a non-breaking change. If you remove the object from the server-side schema, it is a breaking change. For this reason, we recommend you remove the object type from the client-side object model only and leave it in place on the server-side schema.

You can remove an optional or required property from the client-side object model and leave it in place on the server-side schema. This is a non-breaking change to the object model.

If you remove a property from the server-side schema, it is a breaking change. For this reason, we recommend that you remove the property from the client-side object model only and leave it in place on the server-side schema.

To maintain backward compatibility, removing a property from a client-side object model does not delete the property from the database. Instead, new objects retain the removed property, and App Services sets the value to an appropriate empty value, such as null for nullable properties, a 0 for integer values, or an empty string for string values.

Changing an object's name on both the server-side schema and the client-side object model is a breaking change. However, some SDKs offer an API to map a new object name to an existing name in the schema. This allows you to rename an object on the client but not change the object name on the server. In this way, you avoid triggering a migration. Object name mapping is supported in the following SDKs:

  • Kotlin

  • Java

  • .NET

  • Flutter

If name mapping is not an option, consider implementing a partner collection strategy, in which you keep the existing collection and schema, and create a new collection with the new schema.

If you choose to change the object's name instead of using the partner collection strategy, you must terminate sync, manually update the schema, and re-enable sync. In addition, your client application must perform a client reset to restore Sync. In the default client reset mode, the client attempts to recover any unsynced changes before resetting.

Note

Development Mode does not automatically update your schema for breaking changes.

Changing a property's name on both the server-side schema and the client-side object model is a breaking change. However, some SDKs offer an API to map a new property name to an existing name in the schema. This allows you to rename a property on the client but not change the property name on the server. In this way, you avoid triggering a migration. Property name mapping is supported in the following SDKs:

Warning

Update existing documents

If you change a property name in the server-side schema, you must update existing documents with that new property name or they do not sync to the client. This may give client users the impression that the data has been lost.

If name mapping is not an option, consider implementing a partner collection strategy, in which you keep the existing collection and schema, and create a new collection with the new schema.

If you choose to change the property's name instead of using the partner collection strategy, you must terminate sync, manually update the schema, and re-enable sync. In addition, your client application must perform a client reset to restore Sync. In the default client reset mode, the client attempts to recover any unsynced changes before resetting.

When you terminate and re-enable sync, you must also update existing Atlas documents to enable them to Sync with your client applications. Without this additional handling, those documents do not Sync and it may appear in the client that the data has been lost. You could resolve this issue in two ways:

  • Change the old field name on each document to match the new schema

  • Add a new field to each document with a name that matches the new schema, and copy the value from the old field into it

After you've made these changes, the appropriate documents sync to the client application.

Changing a property's type is a breaking change to both the server-side schema and the client-side object model.

Warning

Update existing documents

If you change a property's type in the server-side schema, you must update existing documents with that new property type or they do not sync to the client. This may give client users the impression that the data has been lost.

Instead of changing a property's type, consider implementing the partner collection strategy, in which you keep the existing collection and schema, and create a new collection with the new schema.

If you choose to change the property's type instead of using the partner collection strategy, you must terminate sync, manually update the schema, and re-enable sync. In addition, your client application must perform a client reset to restore Sync. In the default client reset mode, the client attempts to recover any unsynced changes before resetting.

Note

Development Mode does not automatically update your schema for breaking changes.

When you terminate and re-enable sync, you must also update existing Atlas documents to enable them to Sync with your client applications. Without this additional handling, those documents do not Sync and it may appear in the client that the data has been lost. You could resolve this issue in two ways:

  • Change the old field type on each document to match the new schema

  • Add a new field to each document with the type that matches the new schema, and copy the value from the old field into it, transforming its type

After you've made these changes, the appropriate documents should once again sync to the client application.

Changing a property's status between optional and required is a breaking change to both the server-side schema and the client-side object model.

Warning

Update existing documents

If you change a property's status in the server-side schema, you must update existing documents with that new property type or they do not sync to the client. This may give client users the impression that the data has been lost.

Instead of changing a property's status, consider implementing the partner collection strategy, in which you keep the existing collection and schema, and create a new collection with the new schema.

If you choose to change the property's status instead of using the partner collection strategy, you must terminate sync, manually update the schema, and re-enable sync. In addition, your client application must perform a client reset to restore Sync. In the default client reset mode, the client attempts to recover any unsynced changes before resetting.

Note

Development Mode does not automatically update your schema for breaking changes.

A partner collection is a collection that contains the same data as the original collection, but has the new schema definition in place. Partner collections use database triggers to ensure that data flows in both directions, meaning that when one collection is written to, the other is also written to (with the data modifications required for the new schema).

To implement a breaking schema change using the partner collection strategy, see Make Breaking Schema Changes.

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