Lab 8 - Probes.md

Probes

A probe is a diagnostic performed periodically by the kubelet on a container. To perform a diagnostic, the kubelet either executes code within the container, or makes a network request

Task 1: Liveness probes

Many applications running for long periods of time eventually transition to broken states, and cannot recover except by being restarted. Kubernetes provides liveness probes to detect and remedy such situations.

vi liveness.yaml

apiVersion: v1
kind: Pod
metadata:
  labels:
    app: lns
  name: liveness-pod
spec:
  containers:
  - name: liveness
    image: busybox
    args:
    - /bin/sh
    - -c
    - touch /liveness; sleep 6000
    livenessProbe:
      exec:
        command:
        - cat
        - /liveness
      initialDelaySeconds: 5
      periodSeconds: 5

Add a live watch on the pods in another tab

kubectl get pod -w

Toggle to the first tab and execute the below commands

kubectl apply -f liveness.yaml

kubectl get pods	  

kubectl describe pod liveness-pod

Now login to the container and delete the file

kubectl exec -it liveness-pod -- sh 

rm -f liveness

exit

kubectl get pods

kubectl describe pod liveness-pod

You can see the pod restarted

Task 2: Readiness probe

Sometimes, applications are temporarily unable to serve traffic. For example, an application might need to load large data or configuration files during startup, or depend on external services after startup. In such cases, you don't want to kill the application, but you don't want to send it requests either. Kubernetes provides readiness probes to detect and mitigate these situations

vi readiness.yaml

apiVersion: v1
kind: Pod
metadata:
  labels:
    app: rns
  name: readiness-pod
spec:
  containers:
  - name: readiness
    image: nginx
    args:
    - /bin/bash
    - -c
    - service nginx start; touch /readiness; sleep 6000
    readinessProbe:
      exec:
        command:
        - cat
        - /readiness
      initialDelaySeconds: 5
      periodSeconds: 5

kubectl apply -f readiness.yaml

kubectl get pods

Expose the pod

kubectl expose pod readiness-pod --name readiness-svc --port 80 

Describe the svc to check the endpoints or execute the below command.

kubectl get ep readiness-svc

Login inside pod and delete the file

kubectl exec -it readiness-pod bash 

rm -f /readiness

exit

View the Pod events and see that the readiness probe has failed

kubectl describe pods readiness-pod

Now describe service

kubectl describe svc readiness-svc

End point is gone

Replace the pod

kubectl replace -f readiness.yaml --force

Check for endpoints

kubectl describe svc readiness-svc

Endpoints are back