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Readiness Probe vs Liveness Probe

Hands-on example​

Assume:

  • A working Kubernetes cluster
  • kubectl is configured

Initial state​

No Pods or Services exist.

kubectl get pods
kubectl get svc
No resources found in default namespace.
No resources found in default namespace.

Step 1: Create a Pod with a readiness probe​

kubectl apply -f - <<EOF
apiVersion: v1
kind: Pod
metadata:
  name: ready-demo
  labels:
    app: ready-demo
spec:
  containers:
  - name: app
    image: nginx
    readinessProbe:
      httpGet:
        path: /
        port: 80
      initialDelaySeconds: 5
EOF
pod/ready-demo created

Expose it.

kubectl expose pod ready-demo --port=80
service/ready-demo exposed

Check endpoints.

kubectl get endpoints ready-demo
NAME         ENDPOINTS           AGE
ready-demo   10.244.0.15:80      10s

Step 2: Break readiness without killing the container​

Delete the nginx index file.

kubectl exec ready-demo -- rm /usr/share/nginx/html/index.html

Check Pod status.

kubectl get pod ready-demo
NAME         READY   STATUS    RESTARTS   AGE
ready-demo   0/1     Running   0          30s

Check endpoints again.

kubectl get endpoints ready-demo
NAME         ENDPOINTS   AGE
ready-demo   <none>     30s

What changed:

  • Pod is removed from Service endpoints

What did not change:

  • Container is still running

Step 3: Create a Pod with a liveness probe​

kubectl apply -f - <<EOF
apiVersion: v1
kind: Pod
metadata:
  name: live-demo
spec:
  containers:
  - name: app
    image: nginx
    livenessProbe:
      httpGet:
        path: /
        port: 80
      initialDelaySeconds: 5
EOF
pod/live-demo created

Delete the same file.

kubectl exec live-demo -- rm /usr/share/nginx/html/index.html

Wait and check.

kubectl get pod live-demo
NAME        READY   STATUS    RESTARTS   AGE
live-demo   1/1     Running   1          40s

What changed:

  • Container was restarted

What did not change:

  • Pod identity stayed the same

Key observation​

  • Readiness controls traffic
  • Liveness controls restarts