See https://spot.io/resources/kubernetes-autoscaling/5-kubernetes-deployment-strategies-roll-out-like-the-pros/

In this step we look at deploying different application images

• doritoes/k8s-php-demo-app:blue
• doritoes/k8s-php-demo-app:green
• doritoes/k8s-php-demo-app:orange

Strategies supported by Kubernetes out of the box:

• rolling deployment (default) - replaces pods running the older version with the new version one-by-one
• recreate deployment - terminates all pods and replaces them with the new version

Strategies that requires customization or specialized tools:

• ramped slow rollout
• best-effort controlled rollout
• blue/green deployment
• canary deployment
• shadow deployment
• A/B testing

Open a second terminal or ssh session and run the watch command

watch -d -n 1 'kubectl get pods,deploy'

You will watch the status of your deployment here.

Update k8s-deployment-web.yml to use the new image tag

image: doritoes/k8s-php-demo-app:green

Here is the entire modified file:

k8s-deployment-web.yml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: web-deployment
spec:
  replicas: 2
  selector:
    matchLabels:
      app: web
  template:
    metadata:
      labels:
        app: web
    spec:
      nodeSelector:
        my-role: worker # restrict scheduling to the nodes with the label my-role: worker
      containers:
      - name: web
        image: doritoes/k8s-php-demo-app:green
        ports:
        - containerPort: 8080
        livenessProbe:
          httpGet:
            path: /liveness.php
            port: 8080
          initialDelaySeconds: 30
          periodSeconds: 10
        readinessProbe: # Add the readinessProbe section
          httpGet:
            path: /readiness.php
            port: 8080
          initialDelaySeconds: 5
          periodSeconds: 5

This process will perform a one-by-one replacement. Health checks (liveness and readiness) ensure new pods are healthy before taking old ones offline.

Start the update

kubectl apply -f k8s-deployment-web.yml

Observe the rollout

kubectl rollout status deployment/web-deployment

Watch the status in the other session. What can you see, and what useful information is missing?

Examine the deployment and see which image is running:

kubectl describe deployment/web-deployment

Here is a way to get the images running on all pods:

kubectl get pods --all-namespaces -o jsonpath='{range .items[*]}{"\n"}{.metadata.name}{":\t"}{range .spec.containers[*]}{.image}{", "}{end}{end}' |\
sort

Load the web app now (either by nodePort or by updating the HAProxy config).

You may need to press F5 or Control F5 to refresh the style sheet. The color of the app should now be green

If the rollout is stalled, use these commands to investigate potential issues:

kubectl describe deployment web-deployment
kubectl get pods
kubectl logs <pod-name>

Roll back the update using

kubectl rollout undo deployment/web-deployment 

Observe the rollback

kubectl rollout status deployment/web-deployment

Watch the status in the other session.

Examine the deployment and see which image is running:

kubectl describe deployment/web-deployment

Load the web app again and Control-F5/reload to refresh the CSS formatting.

The color should be back to blue.

You can customize the rolling deployment behavior within your Deployment spec with the maxSurge and maxUnavailable properties.

For example here is adding the ability to add up to 25% of the desired number of pods and ensuring you never drop before the desired number of pods.

spec:
  replicas: 2 
  strategy:
    type: RollingUpdate
    rollingUpdate:
      maxSurge: 25%  # Can exceed the desired number of pods
      maxUnavailable: 0  # No downtime during the update 
  # ... rest of your deployment specification ...
 

Continue to Step 7 - Load Balancing

Or, back to Step 5 - Application Pods or Start