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Creating a Kubernetes Exec ReadinessProbe

Introduction

This post demonstrates an exec readinessProbe. It uses a check for DNS to establish readiness. Other kinds of exec probes could be the existance of a file, access to a database, etc. A Kubernetes NetworkPolicy is used to demonstrate the probes functionality. A CNI that supports Kubernetes NetworkPolicy such as NSX-T, Calico, Weave is required for the NetworkPolicy test. Another test could be to just use an invalid URL.

Create the Pod and ReadinessProbe

This pod is just a simple busybox image with a exec readinessProbe that performs an nslookup on google.com. If the nslookup fails the readinessProbe fails and flags the pod as not ready.

kind: Pod
apiVersion: v1
metadata:
  name: my-pod
  namespace: default
spec:
  containers:
  - name: c1
    image: busybox
    imagePullPolicy: IfNotPresent
    command:
    - sleep
    - '3600'
    readinessProbe:
      periodSeconds: 5
      exec:
        command:
        - '/bin/sh'
        - '-c'
        - 'nslookup www.google.com'

Now create the pod.

kubectl apply -f my-pod.yml
pod/my-pod created

Now check the status. Notice the READY value is 1/1.

kubectl get po
NAME      READY     STATUS    RESTARTS   AGE
my-pod    1/1       Running   0          53s

Verify the Probe’s Functionality

This test creates a NetworkPolicy that blocks all pods egress traffic from the default namespace. After creating the policy notice the READY status of the pod changes from 1/1 to 0/1.

kind: NetworkPolicy
apiVersion: networking.k8s.io/v1
metadata:
  name: my-policy
  namespace: default
spec:
  podSelector: {}
  policyTypes:
  - Egress

Create the NetworkPolicy

kubectl create -f my-netp.yml
networkpolicy.networking.k8s.io/my-policy created

Wait a few seconds and then check the pod READY status.

kubectl get po
NAME      READY     STATUS    RESTARTS   AGE
my-pod    0/1       Running   0          1m

Conclusion

This is a pretty simple demonstration of using readinessProbes to verify if a Pod is ready for use. Typically readinessProbes are used in conjunction with a Kubernetes Service.

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