Failback an application within asynchronous DR in OCP on bare metal

Failback is the process of moving the application and its data back to the source cluster once the source cluster is restored and operational again.

The following considerations are used in the examples on this page. Update them to the appropriate values for your environment:

• Source Cluster is the Kubernetes cluster which is down and where your applications were originally running.
• Destination Cluster is the Kubernetes cluster where the applications will be failed over.
• The Zookeeper application is being failed over to the destination cluster.

Prerequisite

• You must ensure that Stork version 24.2.0 or newer is installed on both the source and destination clusters.

  note

  If you are using a Stork version prior to 24.2.0, then you can follow this procedure to perform a failback.

• For operators deployed from the OpenShift OperatorHub, ensure that the operator and applications are in scaled down state on the destination cluster. Stork will leverage spec.replicas from most of the standard Kubernetes controllers such as Deployments, StatefulSets, and so on. However, for applications managed by an Operator, an ApplicationRegistration CR needs to be created which provides Stork with the necessary information required to perform a scale down of the application.

  For more information, see ApplicationRegistrations.

Create a reverse ClusterPair

note

Skip this section if you have created a bidirectional ClusterPair, and move to the next section.

You need to create a reverse ClusterPair if you had initially paired your clusters in a unidirectional manner (from source to destination), and now you should establish a pairing from the destination cluster back to the source cluster. The reverse ClusterPair enables reverse communication between the clusters (from destination to source), allowing for failback.

Run the following command from your destination cluster to create a reverse ClusterPair:

storkctl create clusterpair reverse-migration-cluster-pair \
--namespace <migrationnamespace> \
--src-kube-file <destination-kubeconfig-file> \
--dest-kube-file <source-kubeconfig-file> \
--use-existing-objectstorelocation \
--unidirectional

important

Ensure to provide the destination kubeconfig file with src-kube-file and source destination kubeconfi file with dest-kube-file as mentioned in the above command.

Reverse sync your clusters

If the destination cluster has been running applications for some time, it is possible that the state of your application on the destination cluster differs from your source cluster. This is due to the creation of new resources or changes in data within stateful applications on the destination cluster.

It is recommended to perform one migration from destination cluster to your source cluster before failing back your applications, so that you have the most up-to-date applications on your original source cluster.

As both of your clusters are accessible, follow the instructions to configure a reverse migration schedule:

1. Create a schedule policy on your destination cluster using the instructions in the Create a schedule policy section.

2. Create a migration schedule on your destination cluster using the storkctl create migrationschedule command. For more information on how to use the command, see Create MigrationSchedule with storkctl.

note

For operators deployed from the OpenShift OperatorHub, create a migration schedule with the --exclude-resource-types flag to exclude operator-related resources.

Example: storkctl create migrationschedule -c cluster-pair -n zookeeper reverse-migration-schedule --exclude-resource-types ClusterServiceVersion,operatorconditions,OperatorGroup,InstallPlan,Subscription --exclude-selectors olm.managed=true

Perform failback

You can perform a failback using the storkctl perform failback -m <reverse-migration-schedule> -n <reverse-migration-schedule-namespace> command.

You can also use one of the following flags to include or exclude a specific subset of the namespace for the migration, but not both at the same time.

• --include-namespaces - Includes a subset of namespaces for the migration.
• --exclude-namespaces - Excludes a subset of namespaces for the migration.

To start the failback operation, run the following command in the destination cluster:

storkctl perform failback -m <reverse-migration-schedule> -n <reverse-migration-schedule-namespace>

Example:

$ storkctl perform failback -m reverse-migration-schedule -n zookeeper

Started failback for MigrationSchedule zookeeper/reverse-migration-schedule
To check failback status use the command : `storkctl get failback failback-reverse-migration-schedule-2024-05-21-115006 -n zookeeper`

Check failback status

Run the following command to check the status of the failback operation. You can get the failback-action-name from the output of the storkctl perform failback command.

storkctl get failback <failback-action-name> -n <reverse-migration-schedule-namespace>

Example:

$ storkctl get failback failback-reverse-migration-schedule-2024-05-21-115006 -n zookeeper

NAME                                 CREATED               STAGE       STATUS       MORE INFO
failback-reverse-migration-schedule-2024-05-21-115006   21 May 24 11:50 UTC   Completed   Successful   Scaled up Apps in : 1/1 namespaces

If the status is failed, you can use the oc describe actions <failback-action-name> -n <reverse-migration-schedule-namespace> command to get more information about the failure.

Verify volumes and Kubernetes resources

To verify the volumes and Kubernetes resources that are migrated to the source cluster, run the following command:

oc get all -n <reverse-migration-schedule-namespace>

Example:

$ oc get all -n zookeeper

NAME                     READY   STATUS    RESTARTS   AGE
pod/zk-544ffcc474-6gx64   1/1     Running   0          18h

NAME                 TYPE        CLUSTER-IP     EXTERNAL-IP   PORT(S)    AGE
service/zk-service   ClusterIP   10.233.22.60   <none>        3306/TCP   18h

NAME                 READY   UP-TO-DATE   AVAILABLE   AGE
deployment.apps/zk   1/1     1            1           18h

NAME                            DESIRED   CURRENT   READY   AGE
replicaset.apps/zk-544ffcc474   1         1         1       18h