Adding Admission Webhooks to an Ansible-based Operator

For general background on what admission webhooks are, why to use them, and how to build them, please refer to the official Kubernetes documentation on Extensible Admission Controllers

This guide will assume that you understand the above content, and that you have an existing admission webhook server. You will likely need to make a few modifications to the webhook server container.

When integrating an admission webhook server into your Ansible-based Operator, we recommend that you deploy it as a sidecar container alongside your operator. This allows you to make use of the proxy server that the operator deploys, as well as the cache that backs it. The sidecar will be defined in the deploy/operator.yaml and it will look like:

# This deploys the webhook
- name: webhook
  # Replace this with the built image name
  image: "REPLACE_WEBHOOK_IMAGE"
  imagePullPolicy: "Always"
  volumeMounts:
  - mountPath: /etc/tls/
    name: webhook-cert

Ensuring the webhook server uses the caching proxy

When an Ansible-based Operator runs, it creates a Kubernetes proxy server and serves it on http://localhost:8888. This proxy server does not require any authorization, so all you need to do to make use of the proxy is ensure that your Kubernetes client is pointing at http://localhost:8888 and that it does not attempt to verify SSL. If you use the default in-cluster configuration, you will be hitting the real API server and will not get caching for free.

Deploying the webhook server

To deploy the webhook server as a sidecar alongside your operator, all you need to do is add the container specification to your deploy/operator.yaml. You may also need to add a volume for mounting in TLS secrets, as your webhook server is required to have a valid SSL configuration. Below is a sample updated container specification that deploys a webhook:

containers:
  - name: my-operator
    # Replace this with the built image name
    image: "REPLACE_IMAGE"
    imagePullPolicy: "Always"
    volumeMounts:
    - mountPath: /tmp/ansible-operator/runner
      name: runner
    env:
      - name: WATCH_NAMESPACE
        valueFrom:
          fieldRef:
            fieldPath: metadata.namespace
      - name: POD_NAME
        valueFrom:
          fieldRef:
            fieldPath: metadata.name
      - name: OPERATOR_NAME
        value: "validating-operator"
      - name: ANSIBLE_GATHERING
        value: explicit
  # This deploys the webhook
  - name: webhook
    # Replace this with the built image name
    image: "REPLACE_WEBHOOK_IMAGE"
    imagePullPolicy: "Always"
    volumeMounts:
    - mountPath: /etc/tls/
      name: webhook-cert
volumes:
  - name: runner
    emptyDir: {}
  # This assumes there is a secret called webhook-cert containing TLS certificates
  # Projects like cert-manager can create these certificates
  - name: webhook-cert
    secret:
      secretName: webhook-cert

This will run your webhook server alongside the operator, but Kubernetes will not yet call the webhooks before resources can be created. In order to let Kubernetes know about your webhooks, you must create specific API resources.

Making Kubernetes call your webhooks

In order to make your webhooks callable at all, first you must create a Service that points at your webhook server. Below is a sample service that creates a Service named my-operator-webhook, that will send traffic on port 443 to port 5000 in a Pod that matches the selector name=my-operator. Modify these values to match your environment.

apiVersion: v1
kind: Service
metadata:
  name: my-operator-webhook
spec:
  ports:
  - name: webhook
    port: 443
    protocol: TCP
    # Change targetPort to match the port your server is listening on
    targetPort: 5000
  selector:
    # Change this selector to match the labels on your operator pod
    name: my-operator
  type: ClusterIP

Now that you have a Service directing traffic to your webhook server, you will need to create MutatingWebhookConfiguration or ValidatingWebhookConfiguration objects (depending on what type of webhook you have deployed), which will tell Kubernetes to send certain API requests through your webhooks before writing to etcd.

Below are examples of both MutatingWebhookConfiguration and ValidatingWebhookConfiguration objects, which will tell Kubernetes to call the my-operator-webhook service when samples.example.com Example resources are created. The mutating webhook is served on the /mutating path in my example webhook server, and the validating webhook is served on /validating. Update these values as needed to reflect your environment and desired behavior. These objects are thoroughly documented in the official Kubernetes documentation on Extensible Admission Controllers

---
apiVersion: admissionregistration.k8s.io/v1
kind: MutatingWebhookConfiguration
metadata:
  name: mutating.example.com
webhooks:
- name: "mutating.example.com"
  rules:
  - apiGroups:   ["samples.example.com"]
    apiVersions: ["*"]
    operations:  ["CREATE"]
    resources:   ["examples"]
    scope:       "Namespaced"
  clientConfig:
    service:
      # Replace this with the namespace your service is in
      namespace: REPLACE_NAMESPACE
      name: my-operator-webhook
      path: /mutating
  admissionReviewVersions: ["v1"]
  sideEffects: None
---
apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingWebhookConfiguration
metadata:
  name: validating.example.com
webhooks:
- name: validating.example.com
  rules:
  - apiGroups:   ["samples.example.com"]
    apiVersions: ["*"]
    operations:  ["CREATE"]
    resources:   ["examples"]
    scope:       "Namespaced"
  clientConfig:
    service:
      # Replace this with the namespace your service is in
      namespace: REPLACE_NAMESPACE
      name: my-operator-webhook
      path: /validating
  admissionReviewVersions: ["v1"]
  failurePolicy: Fail
  sideEffects: None

If these resources are configured properly you will now have an admissions webhook that can reject or mutate incoming resources before they are written to the Kubernetes database.

Summary

To deploy an existing admissions webhook to validate or mutate your Kubernetes resources alongside an Ansible-based Operator, you must

1. Configure your admissions webhook to use the proxy server running on http://localhost:8888 in the operator pod
2. Add the webhook container to your operator deployment
3. Create a Service pointing to your webhook
4. Make sure your webhook is reachable via the Service over https
5. Create MutatingWebhookConfiguration or ValidatingWebhookConfiguration mapping the resource you want to mutate/validate to the Service you created

Last modified June 3, 2020: [v0.18.x] docs: fix v1.17 k8s doc links (#3166) (d576b2eb)