Task 3 - Deploy and Scale Deployments

Objective: Master the Creation and Management of Deployments

Description: This module zeroes in on Deployments as the primary mechanism for deploying applications on Kubernetes. You will delve into the creation of Deployments, learn how to scale them effectively, and update applications with zero downtime. Through hands-on lab exercises, you will experience deploying a multi-replica application and conducting rolling updates to ensure seamless application transitions.

Deploying an Application with a Deployment

We’ve previously seen how to use kubectl create deployment Kubernetes-bootcamp –image=gcr.io/google-samples/kubernetes-bootcamp:v1 to create a deployment directly from the command line. However, Kubernetes also supports deploying applications using YAML or JSON manifests. This approach provides greater flexibility than using the kubectl CLI alone and facilitates version control of your deployment configurations.

By defining deployments in YAML or JSON files, you can specify detailed configurations, manage them through source control systems, and apply changes systematically. This method enhances the maintainability and reproducibility of your deployments within a Kubernetes environment.

Deployment kubernetes-bootcamp application

cat << EOF | tee kubernetes-bootcamp.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: kubernetes-bootcamp
spec:
  replicas: 1 # Default value for replicas when not specified
  selector:
    matchLabels:
      app: kubernetes-bootcamp
  template:
    metadata:
      labels:
        app: kubernetes-bootcamp
    spec:
      containers:
      - name: kubernetes-bootcamp
        image: gcr.io/google-samples/kubernetes-bootcamp:v1
EOF
kubectl create -f kubernetes-bootcamp.yaml

replicas: 1: Specifies that only one replica of the Pod should be running.
selector: Defines how the Deployment finds which Pods to manage using matchLabels.
template: Describes the Pod template used by the Deployment to create new Pods.
metadata.labels: Sets the label app: kubernetes-bootcamp on the Pod, matching the selector in the Deployment spec.
spec.containers: Lists the containers to run in the Pod.
image: gcr.io/google-samples/kubernetes-bootcamp:v1: Specifies the container image to use.

Scale your deployment

After deploying the Kubernetes Bootcamp application, you might find the need to scale your deployment to accommodate an increased load or enhance availability. Kubernetes allows you to scale a deployment, increasing the number of replicas from 1 to 10, for instance. This ensures your application can handle a higher load. There are several methods to scale a deployment, each offering unique benefits.

1. Using kubectl scale

Benefits

Immediate: This command directly changes the number of replicas in the deployment, making it a quick way to scale.
Simple: Easy to remember and use for ad-hoc scaling operations.

Check the existing Deployment

kubectl get deployment

expected output

NAME                  READY   UP-TO-DATE   AVAILABLE   AGE
kubernetes-bootcamp   1/1     1            1           63m

Scaling the deployment:

kubectl scale deployment kubernetes-bootcamp --replicas=10

expected output

deployment.apps/kubernetes-bootcamp scaled

3. Check the Deployment status

kubectl rollout status deployment kubernetes-bootcamp

Expected output

Waiting for deployment "kubernetes-bootcamp" rollout to finish: 1 of 10 updated replicas are available...
Waiting for deployment "kubernetes-bootcamp" rollout to finish: 2 of 10 updated replicas are available...
Waiting for deployment "kubernetes-bootcamp" rollout to finish: 3 of 10 updated replicas are available...
Waiting for deployment "kubernetes-bootcamp" rollout to finish: 4 of 10 updated replicas are available...
Waiting for deployment "kubernetes-bootcamp" rollout to finish: 5 of 10 updated replicas are available...
Waiting for deployment "kubernetes-bootcamp" rollout to finish: 6 of 10 updated replicas are available...
Waiting for deployment "kubernetes-bootcamp" rollout to finish: 7 of 10 updated replicas are available...
Waiting for deployment "kubernetes-bootcamp" rollout to finish: 8 of 10 updated replicas are available...
Waiting for deployment "kubernetes-bootcamp" rollout to finish: 9 of 10 updated replicas are available...
deployment "kubernetes-bootcamp" successfully rolled out

4. Verify the Deployment

kubectl get deployment kubernetes-bootcamp

After scaling, you should observe that the number of Pods has increased to meet the deployment’s requirements.

expected output

NAME                  READY   UP-TO-DATE   AVAILABLE   AGE
kubernetes-bootcamp   10/10   10           10          64m

5. Listing the Pods from Deployment

kubectl get pod -l app=kubernetes-bootcamp

expected output

NAME                                  READY   STATUS    RESTARTS   AGE
kubernetes-bootcamp-bcbb7fc75-5fjhc   1/1     Running   0          44s
kubernetes-bootcamp-bcbb7fc75-5kjd7   1/1     Running   0          44s
kubernetes-bootcamp-bcbb7fc75-5r649   1/1     Running   0          30m
kubernetes-bootcamp-bcbb7fc75-bmzbv   1/1     Running   0          44s
kubernetes-bootcamp-bcbb7fc75-fn29h   1/1     Running   0          44s
kubernetes-bootcamp-bcbb7fc75-fp2d9   1/1     Running   0          44s
kubernetes-bootcamp-bcbb7fc75-jfdvf   1/1     Running   0          44s
kubernetes-bootcamp-bcbb7fc75-nh9sn   1/1     Running   0          44s
kubernetes-bootcamp-bcbb7fc75-q7sqc   1/1     Running   0          44s
kubernetes-bootcamp-bcbb7fc75-t4tkm   1/1     Running   0          44s

This output confirms the deployment now runs 10 replicas of the Kubernetes Bootcamp application, demonstrating successful scaling.

6. delete the deployment

kubectl delete deployment kubernetes-bootcamp

2. use kubectl apply

Benefits

If the intended resource to update is in yaml file, we can directly edit the yaml file with any editor, then use kubectl apply to update.

Version Controlled: Can be version-controlled if using a local YAML file, allowing for tracking of changes and rollbacks.
Reviewable: Changes can be reviewed by team members before applying if part of a GitOps workflow.

The kubectl apply -f command is more flexible and is recommended for managing applications in production. It updates resources with the changes defined in the YAML file but retains any modifications that are not specified in the file.It’s particularly suited for scenarios where you might want to maintain manual adjustments or unspecified settings.

create Kubernetes-bootcamp deployment with yaml file

cat << EOF | tee kubernetes-bootcamp.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: kubernetes-bootcamp
spec:
  replicas: 1 # Default value for replicas when not specified
  selector:
    matchLabels:
      app: kubernetes-bootcamp
  template:
    metadata:
      labels:
        app: kubernetes-bootcamp
    spec:
      containers:
      - name: kubernetes-bootcamp
        image: gcr.io/google-samples/kubernetes-bootcamp:v1
EOF
kubectl apply -f kubernetes-bootcamp.yaml

Verify the deployment

kubectl get deployment

expected output

NAME                  READY   UP-TO-DATE   AVAILABLE   AGE
kubernetes-bootcamp   1/1     1            1           3s

modify replicas in yaml file

Now, use editor like vim to change the replicas=1 in the yaml file to replicas=10. Alternatively you can also use sed to change it

sed -i 's/replicas: 1/replicas: 10/' kubernetes-bootcamp.yaml

Apply the change with kubectl apply

kubectl apply -f kubernetes-bootcamp.yaml

Verify the result

kubectl get deployment kubernetes-bootcamp

expected outcome

NAME                  READY   UP-TO-DATE   AVAILABLE   AGE
kubernetes-bootcamp   10/10   10           10          99s

clean up

kubectl delete -f kubernetes-bootcamp.yaml

3. use kubectl edit

This approach involves manually editing the resource definition in a text editor (invoked by kubectl edit), where you can change any part of the resource. After saving and closing the editor, Kubernetes applies the changes. This method requires no separate kubectl apply, as kubectl edit directly applies the changes once the file is saved and closed.

Benefit: Ablility to review and modify other configuration which is not in YAML file

create the deployment

kubectl create deployment kubernetes-bootcamp --image=gcr.io/google-samples/kubernetes-bootcamp:v1 --replicas=1

Modify replicas with kubectl edit

kubectl edit deployment kubernetes-bootcamp

above command show you into VI EDITOR with opened yaml file, locate the text

spec:
  progressDeadlineSeconds: 600
  replicas: 1

then change replicas from 1 to 10, save the change and exit the EDITOR with press Ctrl-C follow :wq!.

Verify the deployment

kubectl get deployment kubernetes-bootcamp

delete deployment

kubectl delete deployment kubernetes-bootcamp

4. Kubectl patch

kubectl patch directly updates specific parts of a resource without requiring you to manually edit a file or see the entire resource definition. It’s particularly useful for making quick changes, like updating an environment variable in a Pod or changing the number of replicas in a deployment.

Benefit: Ideal for scripts and automation because you can specify the exact change in a single command line.

Create Deployment

kubectl create deployment kubernetes-bootcamp --image=gcr.io/google-samples/kubernetes-bootcamp:v1 --replicas=1

Verify the Deployment

kubectl get deployment kubernetes-bootcamp

expected result

NAME                  READY   UP-TO-DATE   AVAILABLE   AGE
kubernetes-bootcamp   1/1     1            1           33s

use kubectl patch to change replicas

kubectl patch deployment kubernetes-bootcamp --type='json' -p='[{"op": "replace", "path": "/spec/replicas", "value":10}]'

Verify the Deployment

kubectl get deployment kubernetes-bootcamp

expected result

NAME                  READY   UP-TO-DATE   AVAILABLE   AGE
kubernetes-bootcamp   10/10   10           10          97s

Delete the Deployment

kubectl delete deployment kubernetes-bootcamp

5. use kubectl replace

The kubectl replace -f command replaces a resource with the new state defined in the YAML file. If the resource doesn’t exist, the command fails. This command requires that the resource be defined completely in the file being applied because it replaces the existing configuration with the new one provided.

Deletion and Recreation: Under the hood, replace effectively deletes and then recreates the resource, which can lead to downtime for stateful applications or services. This method does not preserve unspecified fields or previous modifications made outside the YAML file.

Usage: Use kubectl replace -f when you want to overwrite the resource entirely, and you are certain that the YAML file represents the complete and desired state of the resource.

create deployment with image=gcr.io/google-samples/kubernetes-bootcamp:v1

cat << EOF | tee kubernetes-bootcamp.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: kubernetes-bootcamp
spec:
  replicas: 5 # Default value for replicas when not specified
  selector:
    matchLabels:
      app: kubernetes-bootcamp
  template:
    metadata:
      labels:
        app: kubernetes-bootcamp
    spec:
      containers:
      - name: kubernetes-bootcamp
        image: gcr.io/google-samples/kubernetes-bootcamp:v1
        
EOF
kubectl create -f kubernetes-bootcamp.yaml

verify the deployment

kubectl get pod -l app=kubernetes-bootcamp

expected output

NAME                                   READY   STATUS        RESTARTS   AGE
kubernetes-bootcamp-5485cc6795-5wdrb   1/1     Running       0          4s
kubernetes-bootcamp-5485cc6795-79qkm   1/1     Running       0          4s
kubernetes-bootcamp-5485cc6795-ltqst   1/1     Running       0          4s
kubernetes-bootcamp-5485cc6795-szgqt   1/1     Running       0          4s
kubernetes-bootcamp-5485cc6795-xw7l6   1/1     Running       0          4s

the Pod has name from deployment template hash kubernetes-bootcamp-5485cc6795

Modify image in yaml file

Modify container image in kubernetes-bootcamp.yaml. Modify this will affect the Pod template hash

sed -i 's|image: gcr.io/google-samples/kubernetes-bootcamp:v1|image: jocatalin/kubernetes-bootcamp:v2|' kubernetes-bootcamp.yaml

apply the change

kubectl replace -f kubernetes-bootcamp.yaml

Verify deployment v

kubectl get pod -l app=kubernetes-bootcamp

Expected output

NAME                                   READY   STATUS        RESTARTS   AGE
kubernetes-bootcamp-5485cc6795-5wdrb   1/1     Terminating   0          78s
kubernetes-bootcamp-5485cc6795-79qkm   1/1     Terminating   0          78s
kubernetes-bootcamp-5485cc6795-ltqst   1/1     Terminating   0          78s
kubernetes-bootcamp-5485cc6795-szgqt   1/1     Terminating   0          78s
kubernetes-bootcamp-5485cc6795-xw7l6   1/1     Terminating   0          78s
kubernetes-bootcamp-7c6644499c-559fq   1/1     Running       0          7s
kubernetes-bootcamp-7c6644499c-9zdg2   1/1     Running       0          7s
kubernetes-bootcamp-7c6644499c-fb9dn   1/1     Running       0          6s
kubernetes-bootcamp-7c6644499c-l9xhn   1/1     Running       0          6s
kubernetes-bootcamp-7c6644499c-nzhj9   1/1     Running       0          7s

You should observe the creation of new Pods with a new hash in their names, indicating an update. Concurrently, all previous Pods with the old hash in their names will be deleted, suggesting that the entire resource has been recreated.

Delete the deployment

kubectl delete deployment kubernetes-bootcamp

Risk of Downtime: For some resources, using kubectl replace can cause downtime since it may delete and recreate the resource, depending on the type and changes made. It’s important to use this command with caution, especially for critical resources in production environments.

Summary

kubectl scale: Quickly scales the number of replicas for a deployment, ideal for immediate, ad-hoc adjustments.
kubectl edit: Offers an interactive way to scale by manually editing the deployment’s YAML definition in a text editor, providing a chance to review and adjust other configurations simultaneously.
kubectl patch: Efficiently updates the replicas count with a single command, suitable for scripts and automation without altering the rest of the deployment’s configuration.
kubectl replace -f: Replaces the entire deployment with a new configuration from a YAML file, used when you have a prepared configuration that includes the desired replicas count.
kubectl apply -f: Applies changes from a YAML file to the deployment, allowing for version-controlled and incremental updates, including scaling operations.