Failback an application

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.

Once your unhealthy Kubernetes cluster is back up and running, the Portworx nodes in that cluster will not immediately rejoin the cluster. They will stay in Out of Quorum state until you explicitly Activate this cluster domain.

After this domain is marked as Active you can failback the applications if you want.

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. The cluster domain for this source cluster isus-east-1a.
• Destination Cluster is the Kubernetes cluster where the applications will be failed over. The cluster domain for this destination cluster isus-east-1b.
• The Zookeeper application is being failed over to the destination cluster.

Reactivate your source cluster domain

Once your source cluster is operational, perform the following steps from your destination cluster to active your source cluster domain:

1. Run the following command to activate the source cluster domain:

   storkctl activate clusterdomain us-east-1a

   Cluster Domain activate operation started successfully for us-east-1a

2. Verify if the source cluster domain is activated:

   storkctl get clusterdomainsstatus

   NAME                            LOCAL-DOMAIN   ACTIVE                                     INACTIVE   CREATED
   px-dr-cluster                   us-east-1a     us-east-1a (InSync), us-east-1b (InSync)              29 Nov 22 22:09 UTC

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. Copy and paste the following spec into a file called reverse-migrationschedule.yaml on your destination cluster:

   apiVersion: stork.libopenstorage.org/v1alpha1
   kind: MigrationSchedule
   metadata:
     name: reversemigrationschedule
     namespace: <migrationnamespace> 
   spec:
     template:
       spec:
         clusterPair: <your-clusterpair-name>
         includeResources: true
         startApplications: false
         includeVolumes: false
         namespaces:
         - <app-namespace1>
         - <app-namespace2>
     schedulePolicyName: <your-schedule-policy>
     suspend: false

3. Apply your reverse-migrationschedule.yaml on your destination cluster:

   kubectl apply -f reverse-migrationschedule.yaml

4. Verify if at least one migration cycle has been successfully completed:

   storkctl get migration -n <migrationnamespace>

   NAME                                                  CLUSTERPAIR                 STAGE   STATUS       VOLUMES   RESOURCES   CREATED               ELAPSED                               TOTAL BYTES TRANSFERRED
   reversemigrationschedule-interval-2023-02-01-201747   <your-remote-clusterpair>   Final   Successful   0/0       4/4         01 Feb 23 20:17 UTC   Volumes () Resources (21.71709746s)   0

5. Suspend the reverse migration schedule:

   storkctl suspend  migrationschedule reversemigrationschedule -n <migrationnamespace>

Stop the application on the destination cluster

Stop the applications from running by changing the replica count of your deployments and statefulsets to 0:

storkctl deactivate migration -n <migrationnamespace>

Restart the application on the source cluster

1. After you have stopped the applications on the destination cluster, start the applications on the source cluster by editing the replica count:

   storkctl activate migration -n <migrationnamespace>

2. Verify if your application pods and associated resources are migrated:

   kubectl 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

3. Run the following command on the destination to resume the migration schedule:

   storkctl resume migrationschedule migrationschedule -n <migrationnamespace>

   MigrationSchedule migrationschedule resumed successfully

4. Verify if the migration schedule is active:

   storkctl get migrationschedule -n <migrationnamespace>

   NAME                POLICYNAME                CLUSTERPAIR              SUSPEND   LAST-SUCCESS-TIME     LAST-SUCCESS-DURATION
   migrationschedule   <your-schedule-policy>   <your-clusterpair-name>   false     01 Dec 23 22:25 UTC   10s

   The false value for the SUSPEND field shows that the migration schedule for your policy is active on the source cluster. Hence, your application has successfully failed back to your source cluster.