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Programming with Kubernetes

Understanding CRDs

Explore the role of CustomResourceDefinitions in extending Kubernetes APIs. Understand their schema, versioning, scope, and how they dynamically register with the kube-apiserver without downtime. Learn the differences between CRDs and ConfigMaps to manage your custom Kubernetes resources effectively.

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More about CRDs

With CRDs, we can define our own API schema and plumb them to the kube-apiserver at any time. No broken changes in the kube-apiserver are made. We don’t need to restart or recompile the kube-apiserver. They work pretty much like add-on plugins, in the sense that we can apply or remove them whenever we want. Super handy!

What does a CRD look like?

To better understand CRDs, let’s see what one looks like.

YAML
apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: crontabs.stable.example.com
spec:
  group: stable.example.com
  versions:
    - name: v1
      served: true
      storage: true
      schema:
        openAPIV3Schema:
          type: object
          properties:
            spec:
              type: object
              properties:
                cronSpec:
                  type: string
                image:
                  type: string
                replicas:
                  type: integer
  scope: Namespaced
  names:
    plural: crontabs
    singular: crontab
    kind: CronTab
    shortNames:
    - ct

Now, let’s try to break down the definitions above so that we can better understand every field in a CRD. These fields are quite different in other built-in Kubernetes objects, and some of them are quite important and worth discussing.

  • apiVersion (line 1): This field specifies the apiVersion that we’ll use for the CustomResourceDefinition. The CustomResourceDefinition is a Kubernetes built-in API, which has its own API versions. Normally, we use the apiextensions.k8s.io/v1 API. If you’re using a lower version of Kubernetes, it could be apiextensions.k8s.io/v1beta1 instead.

  • kind (line 2): This field indicates the resource kind. Of course, here, we want to create a CustomResourceDefinition.

  • metadata.name (line 4): This field is quite important, because it specifies the name of the resource. ...