On this page
A client reset error is a scenario where a client realm cannot sync data with the application backend. Clients in this state may continue to run and save data locally but cannot send or receive sync changesets until they perform a client reset. The Realm SDKs provide methods to automatically handle client resets under most scenarios.
By default, the client reset process attempts to recover unsynced changes that were successfully saved locally. When your client application has breaking schema changes, or if Recovery Mode is disabled on the server, the client reset process cannot recover unsynced data that may be persisted locally on the device.
Client reset scenarios occur when the server's history is incompatible with the client's history. The most common causes of client resets are:
Atlas App Services server crashes: A server may restore from a backup that has an earlier version. A client reset for this scenario would reset the client to the earlier version and lose any changes that were saved on the client but not yet synced with the server.
Disabling and re-enabling sync: Turning Sync off and then on again in the App Services UI causes all clients to require a client reset.
Client/Backend schema mismatch: If a client application attempts to synchronize with the backend while using a Realm Object schema that does not exist in the backend, that client must perform a client reset. This only applies in one direction: if the backend schema contains an Realm Object class not used in a client, that client does not need to client reset.
Client maximum offline time: When a client has not synchronized with the backend in more than client maximum offline time days, that client can no longer synchronize unsynced local changes with the backend. The client must discard all local changes since the last sync and download the current status of the realm from the backend.
Breaking schema changes: A breaking or destructive change, like changing a property type or a primary key, requires you to terminate and re-enable Sync. This creates a new synced realm file with a version unrelated to the client's file. In this scenario, the client reset process cannot complete automatically, and your app must provide a manual client reset handler.
Upgrading from a Shared to Dedicated cluster: When you upgrade from a shared to a dedicated cluster, you must terminate Sync on the old cluster. After you upgrade, you can re-enable Sync. Turning Sync off and then on again causes all clients to require a client reset.
Session role changes: When using Flexible Sync, changes to a user's Flexible Sync session role result in a client reset. The following scenarios all result in a client reset:
server-side modifications to the session role
changes to the value of any expansions in the "apply when", read, or write expressions
changes that qualify the user for a different session role
Breaking Schema Changes Require an App Schema Update
After a breaking schema change:
All clients must perform a manual client reset.
You must update client models affected by the breaking schema change.
The SDKs automatically detect the need for client resets. They can automatically perform a client reset in most cases, except in the event of a breaking schema change.
During an automatic client reset, the client:
Downloads a fresh copy of the realm from the backend.
Performs a diff to figure out the steps required to bring the original (local) copy of the realm to the same state as the fresh copy of the backend.
Applies that set of steps to transform the local realm into a state where it can sync with the backend.
If there are no schema changes, or only non-breaking schema changes, the SDK attempts to recover all local changes that have not yet synced to the backend. It also applies any inserts, updates, and deletes from the backend that haven't yet synced to the client.
If you have chosen to discard unsynced changes in the SDK, or if Recovery Mode cannot recover unsynced changes, the SDK can discard local changes that have not yet synced to the backend. Then it can apply any inserts, updates, and deletes from the backend that haven't yet synced to the client. To fall back to discard local changes, choose your preferred SDK's version of the
recoverOrDiscardclient reset mode.
Discards the fresh copy. The app continues to use the original copy of the realm with the diff applied.
In a breaking schema change, or if automatic client reset fails, the client reset falls back to a manual client reset handler that your app must define. The Realm SDKs cannot automatically perform a client reset when you make a breaking schema change.
Automatic client reset mode has several advantages compared to manual recovery:
Your application can perform a client reset without writing any custom logic, other than specifying the mode. You don't have to manually initiate the client reset or interact with the error object at all.
Your application can perform a client reset without closing any realms, disconnecting from the backend app, or manual restarts. This means you don't have to write any logic to handle these situations.
Application users receive notifications for changes as the local realm updates to match the state of the backend realm.
When Client Recovery is enabled in your Device Sync configuration - as it is by default - client applications can automatically recover unsynced changes. In most scenarios, the client application can detect that a client reset error has occurred and start an automated process to handle the client reset.
After the client reset, the app can open and operate as usual.
Client Recovery can recover unsynced data in client resets except where there has been a breaking schema change. Client Recovery applies Client Reset Recovery Rules when determining how to integrate unsynced data from the device.
You can choose to fall back to Discard Unsynced Changes in the
event that the client cannot recover unsynced data. In this case, local
data is discarded, but the client can automatically perform
the client reset. To fall back to discard local changes, choose your
preferred SDK's version of the
recoverOrDiscard client reset mode.
In a client reset that does not involve a breaking schema change, the Realm SDKs attempt to recover unsynced changes. The SDK integrates objects created locally that did not sync before the client reset. These rules determine how conflicts are resolved when both the backend and the client make changes to the same object:
If an object is deleted on the server, but is modified on the recovering client, the delete takes precedence and the client discards the update.
If an object is deleted on the recovering client, but not the server, then the client applies the delete instruction.
In the case of conflicting updates to the same field, the client update is applied.
Deletes Unsynced Local Changes Permanently
This client reset mode permanently deletes any changes made locally that have not yet synchronized to the backend. Do not use this client reset mode if your application needs to preserve unsynced changes.
The discard unsynced changes client reset mode automatically handles client resets without attempting to recover data from the client device. You might choose this mode if the Client Reset Recovery Rules do not work for your app, or if you don't need to save unsynced data. When this mode uses a diff to bring the local realm to the same state as the backend, unsynced changes are permanently deleted.
Discard Unsynced Changes mode cannot perform an automated client reset in the event of a breaking schema change.
In the event of a breaking schema change, Realm SDKs cannot automatically handle a client reset. You must define a manual client reset handler if you make breaking schema changes.
In this scenario, a manual client reset handler should do something like tell the user to update the app. In Realm SDK versions that automatically handle client resets, a manual client reset only occurs in error scenarios where no meaningful recovery can occur.
For more information on performing a client reset, check out the client reset examples for your SDK:
Recovery Mode is enabled by default in every Device Sync configuration. You can disable Recovery Mode, or re-enable it if you have previously disabled it.