Webhooks for Custom Resource Definitions (CRDs) in Kubernetes

1. Introduction

Webhooks in Kubernetes allow external systems to be notified of events related to resources. They are particularly useful when working with Custom Resource Definitions (CRDs), enabling custom validation and admission controls.

This lesson will cover webhook types, how to set them up, and best practices for using webhooks with CRDs in Kubernetes.

2. Webhook Types

Kubernetes supports two main types of webhooks:

• Admission Webhooks: Used for validating or mutating incoming requests to the API server.
• Validation Webhooks: Validate the object creation or update against custom criteria.
• Mutation Webhooks: Modify the object before it is persisted to etcd.

3. Setting Up Webhooks

To set up webhooks for CRDs, follow these steps:

1. Create a Service for your webhook endpoint.
2. Deploy your webhook server (this can be a simple HTTP server).
3. Create a ValidatingWebhookConfiguration or MutatingWebhookConfiguration resource that points to your service.
4. Test the webhook by creating or updating a CRD resource.

4. Example Implementation

apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingWebhookConfiguration
metadata:
  name: example-webhook
webhooks:
  - name: validate.example.com
    clientConfig:
      service:
        name: example-webhook-service
        namespace: default
        path: "/validate"
      caBundle: 
    rules:
      - operations: ["CREATE", "UPDATE"]
        apiGroups: ["example.com"]
        apiVersions: ["v1"]
        resources: ["examples"]
    admissionReviewVersions: ["v1"]
    sideEffects: None
            

5. Best Practices

• Use HTTPS for secure communication.
• Implement proper error handling in your webhook server.
• Use a timeout for webhook calls to prevent blocking the API server.
• Test your webhooks extensively in a development environment.

6. FAQ