How do i use a kubernetes liveness command for application health checks?

amurleopard

A liveness command to check the health of an application. It all very well having the container running in a pod but application checks need to be in place too.

Let's break down the code below:

create a container running the busybox image
get it to execute command to create file
wait 30 seconds
remove the file (to trigger simulate a failure)
wait 300 seconds.

apiVersion: v1
kind: Pod
metadata:
  labels:
    test: liveness
  name: liveness-exec
spec:
  containers:
  - name: liveness
    image: k8s.gcr.io/busybox
    args:
    - /bin/sh
    - -c
    - touch /tmp/healthy; sleep 30; rm -rf /tmp/healthy; sleep 600
    livenessProbe:
      exec:
        command:
        - cat
        - /tmp/healthy
      initialDelaySeconds: 3
      periodSeconds: 5

Create the pod.

$ kubectl create -f livenesscmd.yaml

Looking at the events, you can see the pod being creating going into a unhealthy state, and then starting a new container.

$ kubectl describe pod liveness

Events:
  Type     Reason                 Age                From               Message
  ----     ------                 ----               ----               -------
  Normal   Scheduled              1m                 default-scheduler  Successfully assigned liveness-exec to minikube
  Normal   SuccessfulMountVolume  1m                 kubelet, minikube  MountVolume.SetUp succeeded for volume "default-token-fpffp"
  Warning  Unhealthy              33s (x3 over 43s)  kubelet, minikube  Liveness probe failed: cat: can't open '/tmp/healthy': No such file or directory
  Normal   Pulling                2s (x2 over 1m)    kubelet, minikube  pulling image "k8s.gcr.io/busybox"
  Normal   Pulled                 2s (x2 over 1m)    kubelet, minikube  Successfully pulled image "k8s.gcr.io/busybox"
  Normal   Created                2s (x2 over 1m)    kubelet, minikube  Created container
  Normal   Killing                2s                 kubelet, minikube  Killing container with id docker://liveness:Container failed liveness probe.. Container will be killed and recreated.
  Normal   Started                1s (x2 over 1m)    kubelet, minikube  Started container

As well as the Liveness command, there are two other types of Liveness checks, Liveness HTTP Request and TCP Liveness Probe.