Applications Here, Applications There! - Part 3 - Application Migration

In the first blog posts of this series, we introduced the basic concepts for Application Lifecycle on Red Hat Advanced Cluster Management and deployed an application to multiple clusters.

In the second blog post, we showed how a Blue/Green deployment can be made using Red Hat Advanced Cluster Management.

If you haven't read the previous blog posts yet, go ahead and read them. This post is a continuation and you will need the context from the previous ones.

We are using the same infrastructure we used in the previous posts. See the following diagram:

NOTE: Pay special attention to the different labels, as they will be used during the blog posts examples.

Application Migration on Advanced Cluster Management

In the previous post we upgraded the version of our application in Development and Production clusters using Blue/Green deployment technique.

Now, we are going to explore how Red Hat Advanced Cluster Management enables us to seamlessly move our applications between our different clusters.

Creating new PlacementRules and Subscription

We will create two new PlacementRules targeting clusters in the EU region and US region respectively. Additionally, a new Subscription will be used to deploy our reverse-words application in the region we want the application to run on.

1. Create a new Namespace to store the required manifests.

   oc --context hub create -f https://raw.githubusercontent.com/RHsyseng/acm-app-lifecycle-blog/master/acm-manifests/reversewords-region/00_namespace.yaml

2. Create the required PlacementRules targeting clusters located in EU and US regions.

   # PlacementRule targeting EU region clusters
   oc --context hub create -f https://raw.githubusercontent.com/RHsyseng/acm-app-lifecycle-blog/master/acm-manifests/reversewords-region/01_placement_rule_EU.yaml
   # PlacementRule targeting US region clusters
   oc --context hub create -f https://raw.githubusercontent.com/RHsyseng/acm-app-lifecycle-blog/master/acm-manifests/reversewords-region/02_placement_rule_US.yaml

3. Create the Subscription and Application.

   NOTE: The Subscription is currently configured to deploy the application using the PlacementRule matching clusters in EU region.

   oc --context hub create -f https://raw.githubusercontent.com/RHsyseng/acm-app-lifecycle-blog/master/acm-manifests/reversewords-region/03_subscription-region.yaml
   oc --context hub create -f https://raw.githubusercontent.com/RHsyseng/acm-app-lifecycle-blog/master/acm-manifests/reversewords-region/04_application-region.yaml

4. Now, we should see our application running in the cluster located in EU (which is our Development cluster).

   oc --context dev -n reverse-words-region get deployments,services,pods

   NAME                                  READY   UP-TO-DATE   AVAILABLE   AGE
   deployment.extensions/reverse-words   1/1     1            1           4m39s
   
   NAME                    TYPE           CLUSTER-IP      EXTERNAL-IP                                                                  PORT(S)          AGE
   service/reverse-words   LoadBalancer   172.30.79.111   a3115b78bce924ddc885d2b7dab766a6-1199935412.eu-central-1.elb.amazonaws.com   8080:30254/TCP   4m39s
   
   NAME                                 READY   STATUS    RESTARTS   AGE
   pod/reverse-words-68795d69ff-xmwc6   1/1     Running   0          4m39s

5. Run the same query against the cluster located in US (Production cluster). See that we don't have any pods running.

   oc --context pro -n reverse-words-region get deployments,services,pods

   No resources found in reverse-words-region namespace.

Migrating our Application

Let's say that due to some law enforcement, our application cannot run on EU servers anymore and we need to move it to US-based servers. We can do that with a single command.

If you remember, we created two PlacementRules, one matching EU servers and another matching US servers. We will patch our Subscription so it stops using EU based servers PlacementRule and starts using US based servers PlacementRule.

Changing the PlacementRule used by our Subscription will move our application from one region to the other automatically.

1. Patch the Subscription.

   The following patch updates the PlacementRule used by the Subscription to us-region-clusters.

   oc --context hub -n reverse-words-region patch subscription.apps.open-cluster-management.io/reversewords-region-app-subscription -p '{"spec":{"placement":{"placementRef":{"name":"us-region-clusters"}}}}' --type=merge

2. Our application will be moved from EU cluster to US cluster automatically. See the commands and outputs:

   The application will not run on EU (development cluster) anymore.

   oc --context dev -n reverse-words-region get deployments,services,pods

   No resources found in reverse-words-region namespace.

   The application will be running on US (production cluster) now.

   oc --context pro -n reverse-words-region get deployments,services,pods

   NAME                                  READY   UP-TO-DATE   AVAILABLE   AGE
   deployment.extensions/reverse-words   1/1     1            1           92s
   
   NAME                    TYPE           CLUSTER-IP       EXTERNAL-IP                                                              PORT(S)          AGE
   service/reverse-words   LoadBalancer   172.30.177.196   a90273a7fa3ea4015989fac522b6b36e-709976322.us-west-2.elb.amazonaws.com   8080:30375/TCP   2m33s
   
   NAME                                 READY   STATUS    RESTARTS   AGE
   pod/reverse-words-68795d69ff-jlktw   1/1     Running   0          92s

As you just saw, by using PlacementRules you can migrate your applications between clusters easily. We just used a region-based PlacementRule, but the PlacementRule can be based on any labels configured on your clusters.

What's next

In the next blog post of this series we will be demonstrating how Red Hat Advanced Cluster Management can help us in a Disaster Recovery scenario.