Client Resets - .NET SDK

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Using the DiscardLocalResetHandler

Using the ManualRecoveryHandler

When using Atlas Device Sync, it is possible that your client app will need to handle a client reset. While not a common event, you need to perform a client reset when Sync is terminated on the server. For example, when a developer deploys a change that includes a destructive schema change. To fix this, you must stop Sync, modify the schema, and then re-enable Sync. This process causes the server to send a client reset message to the clients.

By default, the SDK handles client reset messages automatically, but only on app startup. Until the app is restarted, users will receive no new data from the server, and any changes on the device are lost when the app is restarted and the reset happens.

Warning

A client reset erases all local data and downloads a new copy of the data stored in MongoDB Atlas. Performing a client reset loses all local changes made since the client last successfully synced.

The .NET SDK provides the option to specify a client reset strategy in your PartitionSyncConfiguration and FlexibleSyncConfiguration. The ClientResetHandler property can be set to either the DiscardLocalResetHandler method (for PartitionSyncConfiguration), or the ManualRecoveryHandler method for either configuration type. If you do not specify a ClientResetHandler, the default is ManualRecoveryHandler.

Using the DiscardLocalResetHandler

When you specify a DiscardLocalResetHandler on a PartitionSyncConfiguration object, the SDK automatically performs a client reset with minimal code and minimal disruption to your application workflow. This client reset mode restores your local realm file to a syncable state without closing the realm and while maintaining notifications.

Be aware that this strategy permanently deletes all local unsynced changes made since the last successful sync. For a better user experience, consider using the OnBeforeReset and OnAfterReset callback methods. Within the OnBeforeReset callback method, notify the user that a reset is about to happen, and then in the OnAfterReset callback method, notify the user when the reset is complete.

If the client reset operation cannot complete when using the DiscardLocalResetHandler method -- as in the case of a destructive schema change -- the client reset process reverts to manual mode. You specify the method for handling a manual reset with the ManualResetFallback callback method.

Example

The following example shows how you might configure the OnBeforeReset, OnAfterReset, and ManualResetFallback callbacks.

private void SetupRealm()
{
    var config = new PartitionSyncConfiguration("myPartition", user);
    config.ClientResetHandler = new DiscardLocalResetHandler()
    {
        OnBeforeReset = HandleBeforeResetCallback,
        OnAfterReset = HandleAfterResetCallback,
        ManualResetFallback = HandleManualResetCallback
    };
    var realm = await Realm.GetInstanceAsync(config);
}
private void HandleBeforeResetCallback(Realm beforeFrozen)
{
    // Notify the user that a reset is going to happen.
    // This method may also be useful if you want to make a backup
    // of the existing Realm before it is reset by the system.
}
private void HandleAfterResetCallback(Realm beforeFrozen, Realm after)
{
    // Notify the user when the reset is complete.
    // This method may also be useful if you want to merge in changes
    // that were in the "before" Realm into the re-created Realm
}
private void HandleManualResetCallback(ClientResetException clientResetException)
{
    // An error occurred. Use this method to perform a manual client reset.
    // Prompt user to perform client reset immediately. If they don't do it,
    // they won't receive any data from the server until they restart the app
    // and all changes they make will be discarded when the app restarts.
    var didUserConfirmReset = ShowUserAConfirmationDialog();
    if (didUserConfirmReset)
    {
        // Close the Realm before doing the reset. It must be 
        // deleted as part of the reset.
        realm.Dispose();
        // perform the client reset
        var didReset = clientResetException.InitiateClientReset();
        if (didReset)
        {
            // Navigate the user back to the main page or reopen the
            // the Realm and reinitialize the current page.
        }
        else
        {
            // Reset failed - notify user that they'll need to
            // restart the app.
        }
    }
}

Using the ManualRecoveryHandler

To handle manual client resets, set the ClientResetHandler property of the FlexibleSyncConfiguration object to a ManualRecoveryHandler(ClientResetException) method. Be sure to use the overload of the ManualRecoveryHandler that takes a ClientResetException parameter.

Within the ManualRecoveryHandler, dispose the existing realm, and then call the InitiateClientReset() method.

Example

The following example demonstrates this:

private void SetupRealm()
{
    app = App.Create(myRealmAppId);
    user = app.LogInAsync(
        Credentials.EmailPassword("valerie@mongodb.com", "astral")).Result;
    var config = new FlexibleSyncConfiguration(user);
    config.ClientResetHandler =
        new ManualRecoveryHandler(HandleSessionError);
    var realm = await Realm.GetInstanceAsync(config);
}
private void HandleSessionError(ClientResetException clientResetException)
{
    Console.WriteLine($"Client Reset requested: {clientResetException.Message}");
    // Prompt user to perform a client reset immediately. If they don't do it,
    // they won't receive any data from the server until they restart the app
    // and all changes they make will be discarded when the app restarts.
    var didUserConfirmReset = ShowUserAConfirmationDialog();
    if (didUserConfirmReset)
    {
        // Close the Realm before doing the reset. It must be 
        // deleted as part of the reset.
        fsRealm.Dispose();
        // perform the client reset
        var didReset = clientResetException.InitiateClientReset();
        if (didReset)
        {
            // Navigate the user back to the main page or reopen the
            // the Realm and reinitialize the current page.
        }
        else
        {
            // Reset failed - notify user that they'll need to
            // restart the app.
        }
    }
}

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