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Microsoft patterns & practices

CQRS Journey - Migrations

07/25/2012
http://aka.ms/cqrs

The most up-to-date version of the Release Notes is available online.

Migrating from the V1 to the V2 release

If you have been running the V1 release and have data that you would like to preserve as you migrate to the V2 release, the following steps describe how you can perform this migration if you are hosting the V1 release in Windows Azure.

Note: You should create a backup of the Conference database before you begin the migration.

  1. Make sure that the V1 release is running in your Windows Azure production environment.
  2. Deploy the V2 release to your Windows Azure staging environment. The V2 release has a global MaintenanceMode property that is initially set to true. In this mode, the application displays a message to the user that the site is currently undergoing maintenance.
  3. When you are ready, swap the V2 release (still in maintenance mode) into your Windows Azure production environment.
  4. Leave the V1 release (now running in the staging environment) to run for a few minutes to ensure that all in-flight messages complete their processing.
  5. Run the migration program to migrate the data (see below).
  6. After the data migration completes successfully, change the MaintenanceMode property to false.
  7. The V2 release is now live in Windows Azure.

Note: You can change the value of the MaintenanceMode property in the Windows Azure Management Portal.

Running the migration program to migrate the data

Before beginning the data migration process, ensure that you have a backup of the data from your SQL Database database.

The MigrationToV2 utility uses the same Settings.xml file as the other projects in the Conference solution in addition to its own App.config file to specify the Windows Azure storage account and SQL connection strings.

The Settings.xml file contains the names of the new Windows Azure tables that the V2 release uses. If you are migrating data from V1 to V2 ensure that the name of the EventSourcing table is different from the name of the table used by the V1 release. The name of the table used by the V1 release is hardcoded in the Program.cs file in the MigrationToV2 project:

var originalEventStoreName = "ConferenceEventStore";

The name of the new table for V2 is in the Settings.xml file:

<EventSourcing>
	<ConnectionString>...</ConnectionString>
	<TableName>ConferenceEventStoreApplicationDemoV2</TableName>
</EventSourcing>

Note: The migration utility assumes that the V2 event sourcing table is in the same Windows Azure storage account as the V1 event sourcing table. If this is not the case, you will need to modify the MigrationToV2 application code.

The App.config file contains the DbContext.ConferenceManagement connection string. The migration utility uses this connection string to connect to the SQL Database instance that contains the SQL tables used by the application. Ensure that this connection string points to the Windows Azure SQL Database that contains your production data. You can verify which SQL Database instance your production environment uses by looking in the active ServiceConfiguration.csfg file.

Note: If you are running the application locally using the Debug configuration, the DbContext.ConferenceManagement connection string will point to local SQL Express database.

Note: To avoid data transfer charges, you should run the migration utility inside a Windows Azure worker role instead of on-premise. The solution includes an empty, configured Windows Azure worker role in the MigrationToV2.Azure with diagnostics that you can use for this purpose. For information about how to run an application inside a Windows Azure role instance, see Using Remote Desktop with Windows Azure Roles.

Note: Migration from V1 to V2 is not supported if you are using the DebugLocal configuration.

If the data migration fails

If the data migration process fails for any reason, then before you retry the migration you should:

  1. Restore the SQL Database back to its state before you ran the migration utility.
  2. Delete the two new Windows Azure tables defined in Settings.xml in the EventSourcing and MessageLog sections.

Migrating from the V2 to the V3 release

If you have been running the V2 release and have data that you would like to preserve as you migrate to the V3 release, the following steps describe how you can perform this migration if you are hosting the V2 release in Windows Azure.

Note: You should create a backup of the Conference database before you begin the migration.

  1. Make sure that the V2 release is running in your Windows Azure production environment.

  2. Deploy the V3 release to your Windows Azure staging environment. In the V3 release, the command processor worker role has a MaintenanceMode property that is initially set to true.

  3. Start the ad hoc MigrationToV3.InHouseProcessor utility to rebuild the read models for the V3 deployment.

  4. Change the MaintenanceMode property of the command processor worker role in the V2 release (running in the production slot) to true.
    At this point, the application is still running, but the registrations cannot progress. You should wait until the status of the worker role instance shows as Ready in the Windows Azure portal (this may take some time).

  5. Change the MaintenanceMode property of the command processor worker role in the V3 release (running in the staging slot) to false and allow the MigrationToV3.InHouseProcessor utility to start handling the V2 events. The migration utility prompts you to start handling these V2 events when you are ready. This change is faster than changing the value of the MaintenanceMode property in the V2 release. When this change is complete, the V2 release web roles are using the data processed by the V3 version of the worker role. This configuration change also triggers the database migration.

  6. In the Windows Azure portal, perform a VIP swap to make the V3 web roles visible externally.

  7. Shutdown the V2 deployment that is now running in the staging slot.

  8. The V3 release is now live in Windows Azure.

Note: You can change the value of the MaintenanceMode property in the Windows Azure Management Portal.