What is CRD in Kubernetes

 CRD in Kubernetes stands for Custom Resource Definition. It allows users to extend Kubernetes' API by defining their own custom resource types. These custom resources can be used to store and manage application-specific or domain-specific configuration data, similar to how native Kubernetes resources like Pods, Services, and Deployments are used.

 

Key Concepts of CRD

1. Custom Resource (CR):

o A custom resource is an extension of Kubernetes’ API that allows users to define their own resource types.

o Example: A CRD can define a resource called Backup to manage database backups.

2. Custom Resource Definition (CRD):

o A CRD is a Kubernetes resource that specifies the structure and behavior of a custom resource.

o By creating a CRD, users can introduce new resource types into Kubernetes that are managed in a way similar to native Kubernetes resources.

 

How CRDs Work

1. Define the Custom Resource:

o A CRD describes the schema of the custom resource, including its fields and properties.

o It is written in YAML or JSON and applied to the cluster using kubectl.

2. API Endpoint Creation:

o Once the CRD is created, Kubernetes dynamically creates a new API endpoint for the custom resource.

o For example, a CRD named Backup with a group example.com will create an endpoint like /apis/example.com/v1/backups.

3. Managing Custom Resources:

o After defining a CRD, you can create, update, and delete instances of the custom resource using kubectl or the Kubernetes API.

 

Example of a CRD

Here is an example of a Backup custom resource definition:

apiVersion: apiextensions.k8s.io/v1

kind: CustomResourceDefinition

metadata:

  name: backups.example.com

spec:

  group: example.com

  names:

    plural: backups

    singular: backup

    kind: Backup

    shortNames:

      - bk

  scope: Namespaced

  versions:

    - name: v1

      served: true

      storage: true

      schema:

        openAPIV3Schema:

          type: object

          properties:

            spec:

              type: object

              properties:

                database:

                  type: string

                schedule:

                  type: string

 

Why Use CRDs?

1. Extensibility:

o Allows you to integrate domain-specific logic into Kubernetes without modifying the core code.

2. Declarative Management:

o Custom resources are managed declaratively using Kubernetes manifests.

3. Automation:

o Works seamlessly with Kubernetes controllers to automate tasks based on the custom resource.

 

Use Cases of CRDs

1. Custom Application Configurations:

o Define application-specific configurations that are not covered by native Kubernetes resources.

2. Operators:

o Many Kubernetes Operators (software that automates the management of complex applications) use CRDs to define custom resources.

3. Workflow Automation:

o Manage workflows like CI/CD pipelines or database backups with CRDs.

 

Commands to Work with CRDs

1. Create a CRD:

2. kubectl apply -f custom-resource-definition.yaml

3. List CRDs:

4. kubectl get crd

5. View a Specific CRD:

6. kubectl describe crd backups.example.com

7. Delete a CRD:

8. kubectl delete crd backups.example.com

CRDs are a powerful feature in Kubernetes for extending its functionality to meet specific application requirements.