Lets start understanding what is Deployment in Kubernetes?
The main role of deployment is to provide declarative updates to both the pod and the replicasets. With Deployments you can roll updates to your existing cluster without any downtime. Deployment is always in conjunction with replica sets. Which maintain the desired number of counts for the pods. If one of the pods fails, Replica Sets creates a new one. Below are some of the key features of deployment:
- Easily deploy a Replica Sets
- Update pods
- Rollback to previous deployment versions
- Scale deployment
- Pause and resume deployment
Deployment strategies
Deployment strategies are used to replace old pods by new ones.
Rolling update:
Pods are updated in a rolling fashion. This is defined by setting.spec.strategy.type to RollingUpdate. We can set maxUnavailable and maxSurge but, by default, it makes sure that only 25 percent of your pods are unavailable so we don’t have to change it if it’s not necessary.
So, in layman words, the Rolling Update is used to create a new pod with new updates and then delete the old pod one at a time, so there is no downtime in our application.
Now let’s get our hands dirty with a practical to Rolling Updates in K8s cluster:
Here we are using an example of nginx image in a pod. We would first deploy a pod with older image, and then we would update the pod with new image with rolling updates.
You can create a deployment using Imperative and Declarative way:
1) Declarative Way: By defining into a YAML file We would create a nginx-deployment.yaml file and add the contents below
apiVersion: apps/v1
kind: Deployment
metadata:
name: nginx-deployment
labels:
app: nginx
spec:
replicas: 10
selector:
matchLabels:
app: nginx
template:
metadata:
labels:
app: nginx
spec:
containers:
– name: nginx
image: nginx:1.14.2
ports:
– containerPort: 80
2) Imperative Way: By using imperative commands.
Note: You cannot add replicas in the imperative way, you can create a deployment and scale it or you can output the command into .yaml file and edit it manually. We would discuss both the ways.
A) Creating a deployment and then scaling the replicas:
Creating Deployment with 1 Replicas
kubectl create deployment –image=nginx:1.14.2 nginx-deployment
Scaling it to 10 Replicas:
kubectl scale deployment nginx-deployment –replicas=10
You can verify your deployment using the kubectl describe deployment nginx-deployment command.
You can scale down from 10 to 1 replicas also by kubectl scale deployment nginx.deployment –replicas=1
You can also scale by editing the nginx-deployment.yaml file. And editing the replicas=1
apiVersion: apps/v1
kind: Deployment
metadata:
name: nginx-deployment
labels:
app: nginx
spec:
replicas: 1
selector:
matchLabels:
app: nginx
template:
metadata:
labels:
app: nginx
spec:
containers:
– name: nginx
image: nginx:1.14.2
ports:
– containerPort: 80
Now let us start the rolling update
We would change the nginx version 1.14.2 to 1.19. Hence, we are rolling from older to newer version. By default the RollingUpdateStrategy is 25% max unavailable, 25% maxsurge.
Hence the pod with newer specifications is created first, than the older pod is evicted, this process is done one by one without downtime.
We would edit the deployment and change the image version to 1.19.
kubectl edit deployment nginx-deployment
And change the image of nginx to nginx:1.19
Now using the rollout command check the status of the pods. You will see new pods are created first with new configuration and older are deleted later one by one. This is the rolling update feature of Kubernetes.
With kubectl rollout history deployment nginx-deployment .You can observe the revisions of the update and can rollout back to previous version also.
Lets rollback to previous version using kubectl rollout undo deployment nginx-deployment
You can see the revision has changed from 2 to 3 that is previous version.
You can verify the previous deployment, by reading into the deployment file by
kubectl edit deployment nginx-deployment
and you would see the previous version been undo.
Deleting Deployment:
kubectl delete deployment nginx-deployment
This will delete all pods and replicasets under it.
