In this lab, you'll learn how to update the number of instances in a deployment has and how to roll out an update of your application on Kubernetes with zero downtime. A key requirement of a container orchestration system is to automate the management steps of applications. Scaling up/down and handling application updates with support for rollback are a couple of essential use cases (but not the only ones).
For this lab, you need a running deployment of the guestbook application from the previous lab.
Scale apps with replicas
A replica is a copy of a pod that contains a running service. By having multiple replicas of a pod, you can ensure your deployment has the available resources to handle increasing load on your application.
kubectl provides a scale subcommand to change the size of an existing deployment. Let's increase our capacity from a single running instance of guestbook up to 10 instances using:
If you remember back to the architecture overview in the previous exercise, the kubectl scale command updates the desired state in the etcd database in Kubernetes. The watchers and controllers will now try to make reality match the desired state of 10 replicas by starting 9 new pods with the same configuration as the first.
To see your changes being rolled out, you can run:
kubectl rollout status deployment guestbook
The rollout might occur so quickly that you may only see deployment "guestbook" successfully rolled out for the output.
$ kubectl rollout status deployment guestbook
Waiting for rollout to finish: 1 of 10 updated replicas are available...
Waiting for rollout to finish: 2 of 10 updated replicas are available...
Waiting for rollout to finish: 3 of 10 updated replicas are available...
Waiting for rollout to finish: 4 of 10 updated replicas are available...
Waiting for rollout to finish: 5 of 10 updated replicas are available...
Waiting for rollout to finish: 6 of 10 updated replicas are available...
Waiting for rollout to finish: 7 of 10 updated replicas are available...
Waiting for rollout to finish: 8 of 10 updated replicas are available...
Waiting for rollout to finish: 9 of 10 updated replicas are available...
deployment "guestbook" successfully rolled out
Once the rollout has finished, ensure your pods are running by using:
kubectl get pods
You should see output listing 10 replicas of your deployment:
Kubernetes allows you to do a rolling upgrade of your application to a new container image. Kubernetes allows you to easily update the running image but also allows you to easily undo a rollout if a problem is discovered during or after deployment.
In the previous lab, we used an image with a v1 tag. For our upgrade, we'll use the image with the v2 tag.
To update and roll back:
Using kubectl, you can now update your deployment to use the v2 image. kubectl allows you to change details about existing resources with the set subcommand. We can use it to change the image being used.
kubectl set image deployment/guestbook guestbook=ibmcom/guestbook:v2
Check the status of the rollout. The rollout might occur so quickly that you may only see deployment "guestbook" successfully rolled out for the output.
kubectl rollout status deployment/guestbook
$ kubectl rollout status deployment/guestbook
Waiting for rollout to finish: 2 out of 10 new replicas have been updated...
Waiting for rollout to finish: 3 out of 10 new replicas have been updated...
Waiting for rollout to finish: 3 out of 10 new replicas have been updated...
Waiting for rollout to finish: 3 out of 10 new replicas have been updated...
Waiting for rollout to finish: 4 out of 10 new replicas have been updated...
Waiting for rollout to finish: 4 out of 10 new replicas have been updated...
Waiting for rollout to finish: 4 out of 10 new replicas have been updated...
Waiting for rollout to finish: 4 out of 10 new replicas have been updated...
Waiting for rollout to finish: 4 out of 10 new replicas have been updated...
Waiting for rollout to finish: 5 out of 10 new replicas have been updated...
Waiting for rollout to finish: 5 out of 10 new replicas have been updated...
Waiting for rollout to finish: 5 out of 10 new replicas have been updated...
Waiting for rollout to finish: 6 out of 10 new replicas have been updated...
Waiting for rollout to finish: 6 out of 10 new replicas have been updated...
Waiting for rollout to finish: 6 out of 10 new replicas have been updated...
Waiting for rollout to finish: 7 out of 10 new replicas have been updated...
Waiting for rollout to finish: 7 out of 10 new replicas have been updated...
Waiting for rollout to finish: 7 out of 10 new replicas have been updated...
Waiting for rollout to finish: 7 out of 10 new replicas have been updated...
Waiting for rollout to finish: 8 out of 10 new replicas have been updated...
Waiting for rollout to finish: 8 out of 10 new replicas have been updated...
Waiting for rollout to finish: 8 out of 10 new replicas have been updated...
Waiting for rollout to finish: 8 out of 10 new replicas have been updated...
Waiting for rollout to finish: 9 out of 10 new replicas have been updated...
Waiting for rollout to finish: 9 out of 10 new replicas have been updated...
Waiting for rollout to finish: 9 out of 10 new replicas have been updated...
Waiting for rollout to finish: 1 old replicas are pending termination...
Waiting for rollout to finish: 1 old replicas are pending termination...
Waiting for rollout to finish: 1 old replicas are pending termination...
Waiting for rollout to finish: 9 of 10 updated replicas are available...
Waiting for rollout to finish: 9 of 10 updated replicas are available...
Waiting for rollout to finish: 9 of 10 updated replicas are available...
deployment "guestbook" successfully rolled out
Test the application as before, by accessing <public-IP>:<nodeport> (use the same as the previous lab) in the browser to confirm your new code is active.
Remember, to get the "nodeport" and "public-ip" use:
kubectl describe service guestbook
and (replace mycluster below if you used a different name for your cluster):
ibmcloud ks workers -c mycluster
To verify that you're running "v2" of guestbook, press down shift while clicking on reload to discard the cached content and look at the title of the page, it should now be Guestbook - v2
If you want to undo your latest rollout, use:
kubectl rollout undo deployment guestbook
You can then use kubectl rollout status deployment/guestbook to see the status.
When doing a rollout, you see references to old replicas and new replicas. The old replicas are the original 10 pods deployed when we scaled the application. The new replicas come from the newly created pods with the different image. All of these pods are owned by the Deployment. The deployment manages these two sets of pods with a resource called a ReplicaSet. We can see the guestbook ReplicaSets with:
kubectl get replicasets -l app=guestbook
$ kubectl get replicasets -l app=guestbook
NAME DESIRED CURRENT READY AGE
guestbook-5f5548d4f 10 10 10 21m
guestbook-768cc55c78 0 0 0 3h
Before we continue, let's delete the application so we can learn about a different way to achieve the same results by using resource files instead of providing command line options.
To remove the deployment, use:
kubectl delete deployment guestbook
To remove the service,use:
kubectl delete service guestbook
Deeper dive on configuring resources
In the last section of this lab, you will again deploy the guestbook application, along with other objects for a multi-tier application using resource files.
Note that a pod could have multiple containers, each with its own name. Each image can be changed individually or all at once by referring to the name. In the case of our guestbook Deployment, the container name is also guestbook. Multiple containers can be updated at the same time. (.)
Although it was convenient to create the guestbook deployment using the cli, in practice most applications and other kubernetes objects are created using configuration files in .yaml format. For example, the file is an example configuration file that would deploy the guestbook image with a total of 3 instances. This file shows the key parts needed for each kubenetes object. After the API version and resource type, there is a metadata section which specfies the name of the resource and a set of labels. Then, there is a spec section which defines the desired state. First, there's the definition of the object for the deployment. Within the replica set, there is a template for the pod controlled by the set. Within this template, you can find labels applied at the pod level and the spec for the container(s) that will be deployed with each pod.