Mocking the Kubernetes client in Go for Unit Testing

Yes, we can mimic K8s Client!

Photo by Frank Eiffert on Unsplash

Writing unit tests is always a pain for developers. The main reason for this is that, usually, the unit tests (functional unit tests) must not use any physical components/running instances of the application. For example, the unit tests for an SDK of an API should not have any running instance of the API. Which is why, mocking the application instances becomes critical and that is the trick too!

In this article, we shall understand how to mock the Kubernetes client using the fake clientset of client-go package. Lets’s begin!

Create a package called client and a file called client.go which defines a struct called Client which holds the Kubernetes Clientset.

Kubernetes Clientset

• contains the clients for groups.
• is the struct used to implement the Kubernetes interface calledInterface.
• NewForConfig() is the constructor of the Kubernetes Interface interface that returns the Clientset object.

Moving on, let’s define a public struct called Client in client.go

Create a method called CreatePod that is called upon the Client struct to create pods in a given namespace as shown in the same client.go file

The complete client.go file looks like this

Now, let us implement a main.go to consume the above CreatePod method. For this, we need to

• build the clientset

• load the Client struct

• define a pod resource object

• invoke the CreatePod call upon the client object

The complete main.go file looks like this

When we run this main.go file against a running cluster, we end up creating a pod called test-pod in the default namespace.

Now, let us write a unit test for the same using the Go’s testing package. Here, inorder to create the Kubernetes Clientset, we use the NewSimpleClientSet() constructor from the fake package of client-go/kubernetes package instead of NewForConfig() constructor.

What is the difference?

NewForConfig() constructor returns the actual ClientSet that has the clients for every Kubernetes groups and operates upon an actual cluster. Whereas, NewSimpleClientSet() constructor returns clientset that will respond with the provided objects. It’s backed by a very simple object tracker that processes creates, updates and deletions as-is, without applying any validations and/or defaults.

What is important to note here is that the Clientset of the fake package also implements the Kubernetes Interface. Meant to be embedded into a struct to get a default implementation. This makes faking out just the method you want to test easier.

Our main_test.go would like this

Now, if we run this test file using go test command, we will successfully execute the CreatePod() method even without running this against any running K8s cluster & by bypassing the Get() call by mimicing it to return the same object as sent as a parameter to it. Output will be something as shown

go test
I0520 00:02:03.789351   23893 pod.go:23] Pod test-pod is succesfully created
PASS
ok      github.com/kubernetes-sdk-for-go-101    0.681s

You can keep this article as reference and build upon this, a test framework or a unit test package for your applications employing Kubernetes client-go. I hope this was helpful. If you have any doubts, concerns or comments, then please feel free to leave them below.

Thanks & regards,
Samarth Deyagond (aka Teddy Winters)