Using Custom Resources (CRD)

Overview

Custom Resources extend the Kubernetes API. A CustomResourceDefinition (CRD) is used to define these custom resources.

Documentation

Extend the Kubernetes API with CustomResourceDefinitions.

Example CRD

In this example, we define a CronTab resource as described in the Kubernetes documentation.

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

Steps to Use CRDs

Create the CRD Definition

Define your custom resource in a YAML file using the structure above. Save this file as crontab-crd.yaml.
Apply the CRD

Use kubectl to create the CRD in your cluster:
```
kubectl apply -f crontab-crd.yaml
```

Define a Custom Resource

Once the CRD is applied, you can define custom resources based on it. Example:

apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
  name: my-new-cron-object
spec:
  cronSpec: "* * * * */5"
  image: my-awesome-cron-image
  replicas: 1

Save this as my-new-cron-object.yaml.

Create the Custom Resource

Apply the custom resource to your cluster:
```
kubectl apply -f my-new-cron-object.yaml
```
Interact with the Custom Resource

Use standard Kubernetes commands to interact with your custom resource: To get the resource:
```
kubectl get crontab
```
To describe the resource:
```
kubectl describe crontab my-new-cron-object
```

Conclusion

Custom Resources in Kubernetes are a powerful way to introduce new API objects tailored to your application's needs, enhancing the flexibility and functionality of your Kubernetes cluster.

Documentation

Custom Resources Defination