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Kubernetes Custom Resources

Custom resources allow to extend Kubernetes API with additional types not available in the default Kubernetes distribution.

Like native types, such as Pod, Deployment or Service, custom resources become first-class citizens in a Kubernetes cluster and can be queried via API and manipulated by kubectl.

Creating Custom Resource Definition

Before a custom resource can be created, we need to let Kubernetes know what the new resource kind is, what its spec looks like, how to validate its fields and so on.

Kubernetes provides an easy declarative way of doing so via a special resource called CustomResourceDefinition, or CRD. Let's create a CRD spec that describes a new resources called Nginx:

$ cat ~/workshop/crd/assets/crd.yaml
apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
metadata:
  name: nginxes.training.example.com
spec:
  group: training.example.com
  version: v1
  scope: Namespaced
  names:
    kind: Nginx
    plural: nginxes
    singular: nginx
    shortNames:
      - ng
  validation:
    openAPIV3Schema:
      required: ["spec"]
      properties:
        spec:
          required: ["version"]
          properties:
            version:
              type: "string"

Registering Custom Resource

Now that the spec for our new resource has been defined, we need to register it with the Kubernetes API server.

To do that, simply create the CRD resource:

$ kubectl create -f ~/workshop/crd/assets/crd.yaml

CRDs themselves are global (not namespaced) resources and all currently registered custom types can be viewed using:

$ kubectl get crd

Defining Custom Resource

Now that the custom type has been registered, we can create our custom resource.

Let's define its spec:

$ cat ~/workshop/crd/assets/nginx.yaml
apiVersion: training.example.com/v1
kind: Nginx
metadata:
  name: mynginx
spec:
  version: 1.17.5

Creating Custom Resource

Since custom resources are treated exactly like built-in resources, to create an instance of a new custom resource we can use:

$ kubectl create -f ~/workshop/crd/assets/nginx.yaml

We can also view our new custom resource in exactly the same way as the core resource:

$ kubectl get ng

Or delete it:

$ kubectl delete ng/mynginx

Custom Resource Controller

On their own custom resources already provide a lot of value as they allow to store and retrieve structured data using native Kubernetes tools.

It is also possible to create a custom controller that can monitor custom resources and take specific actions, thus unlocking true declarative capabilities of custom resources.

When combined together, custom resources and custom controllers, constitute what is called the Operator pattern.

Let's create a custom contoller that will be watching our Nginx resources and launching respective nginx pods.

Defining Custom Types

First we need to define the custom types for our Nginx resource and write some boilerplate code to make sure they're registered with Kubernetes API server.

The initial project structure looks like this:

$ tree ~/workshop/crd/controller/pkg/apis
~/workshop/crd/controller/pkg/apis
└── nginxcontroller
    ├── register.go
    └── v1
        ├── doc.go
        ├── register.go
        └── types.go
$ cat ~/workshop/crd/controller/pkg/apis/nginxcontroller/register.go
$ cat ~/workshop/crd/controller/pkg/apis/nginxcontroller/v1/doc.go
$ cat ~/workshop/crd/controller/pkg/apis/nginxcontroller/v1/register.go
$ cat ~/workshop/crd/controller/pkg/apis/nginxcontroller/v1/types.go

Note that the build tags are important as they tell Kubernetes code generator how to process these files.

Generating Client Stubs

Next we need to use Kubernetes code generator to generate native clients for our custom types:

$ cd ~/workshop/crd/controller
$ make generate

Note that you need https://github.com/kubernetes/code-generator installed in your GOPATH for this to work.

After the generator has finished, it has produced a new package under ~/workshop/crd/controller/pkg/generated that contains stubs for creating Kubernetes clients for interacting with our new Nginx resource and additional utility methods.

$ tree ~/workshop/crd/controller/pkg/generated

Writing Controller

Now we can use our new generated client stubs to initialize two Kubernetes clients, for core resources and for the custom resource:

$ cat ~/workshop/crd/controller/main.go

And implement the actual controller logic that subscribes to Nginx resources using the custom client and creates nginx pods:

$ cat ~/workshop/crd/controller/controller.go

Note that the controller does not subscribe to "delete" events in this case because the custom resources become owners of their respective pods via the ownerReference field and thus when they are deleted, they dependent pods are also deleted.

Deploying Controller

The controller is published to Gravitational's image registry so let's deploy it in our Kubernetes cluster:

$ cat ~/workshop/crd/assets/controller.yaml
$ kubectl create -f ~/workshop/crd/assets/controller.yaml

Once the controller pod is up and running, let's create an Nginx resource again:

$ kubectl create -f ~/workshop/crd/assets/nginx.yaml

And observe that the controller has spun up an nginx pod:

$ kubectl get nginx
$ kubectl get pods

Now attempt to delete the Nginx resource and see that the pod is deleted as well:

$ kubectl delete nginx/mynginx
$ kubectl get pods

Useful Resources

https://kubernetes.io/docs/concepts/extend-kubernetes/api-extension/custom-resources/ https://kubernetes.io/docs/tasks/access-kubernetes-api/custom-resources/custom-resource-definitions/ https://github.com/kubernetes/sample-controller https://supergiant.io/blog/custom-resources-and-custom-resource-definitions-crds-in-kubernetes/ https://medium.com/@trstringer/create-kubernetes-controllers-for-core-and-custom-resources-62fc35ad64a3