Unit test kubernetes client in Go

10 Jan 2020 · Three minute read · golang, kubernetes, mockmania

I write a lot of operations and integrations with Kubernetes those days. You can follow my journey in its dedicated section on this blog “Building Kubernetes”.

I had to write a function recently capable of filtering pods based on assigned annotations.

const (
	ProfefeEnabledAnnotation = "profefe.com/enable"

// GetSelectedPods returns all the pods with the profefe annotation enabled
// filtered by the selected labels
func GetSelectedPods(clientset kubernetes.Interface,
	namespace string,
	listOpt metav1.ListOptions) ([]v1.Pod, error) {

	target := []v1.Pod{}
	pods, err := clientset.CoreV1().Pods(namespace).List(listOpt)
	if err != nil {
		return target, err
	for _, pod := range pods.Items {
		enabled, ok := pod.Annotations[ProfefeEnabledAnnotation]
		if ok && enabled == "true" && pod.Status.Phase == v1.PodRunning {
			target = append(target, pod)
	return target, nil

This function is pretty easy, but it has a good amount of assertions that we can check. Even more when we have a so well scoped functions writing tests should be almost mandatory.

  • The returned list of pods should only contains pods with the ProfefeEnabledAnnotation set
  • The returned list of pods should only returns pods from the specified namespace
  • The returned list of pods should observe the filtering and label selection criteria specified by metav1.ListOptions

Covering those use cases will give us a solid foundation to avoid regression when this function will get more complicated (usually that’s the evolution for successful piece of code).

Kubernetes Client Mock

Kubernetes offers a simple and powerful fake client that has a very efficient mechanism to simulate the desired output from a specific request, in our case clientset.CoreV1().Pods(namespace).List(listOpt). You have to pass the slice of runtime.Object you desire when you create a new fake client. Awesome and easy.

clientset: fake.NewSimpleClientset(&v1.Pod{
    ObjectMeta: metav1.ObjectMeta{
        Name:        "influxdb-v2",
        Namespace:   "default",
        Annotations: map[string]string{},
}, &v1.Pod{
    ObjectMeta: metav1.ObjectMeta{
        Name:        "chronograf",
        Namespace:   "default",
        Annotations: map[string]string{},

For example this clientset will return two pods, one called influxdb-v2 and one called chronograf, but you can return what ever you need: Services, Deployments, Ingress, Custom Resource Definition or even a mix of everything.

In practice

I wrote a bunch of tests for kube-profefe that are using a fake client. You can get inspiration over there.


fake client is easy to use, so easy that since I added it in my tool chain for some functions like the one I described here I efficiently do TDD because it makes the iteration over my code way faster.