kubernetes Admission Controller原理介绍

metadata:
  name: test-net
  annotations:
    initializer.kubernetes.io/lxcfs: "true"   # 就是在pod的metadata里加上这个配置

kubernetes/plugin/pkg/admission/alwayspullimages/admission.go

func (a *AlwaysPullImages) Admit(attributes admission.Attributes, o admission.ObjectInterfaces) (err error) {
    // 你可以在attibutes里获取到对象的一切信息，用户信息等
	if shouldIgnore(attributes) { // 检查一下是不是你关注的object, 比如创建的一个configmap 那么显然可以忽视
		return nil
	}
	pod, ok := attributes.GetObject().(*api.Pod)

    // 这里把initContainer和Container的拉取策略都给改了
	for i := range pod.Spec.InitContainers {
		pod.Spec.InitContainers[i].ImagePullPolicy = api.PullAlways
	}

	for i := range pod.Spec.Containers {
		pod.Spec.Containers[i].ImagePullPolicy = api.PullAlways
	}

	return nil
}

# 还提供一个校验接口，看是不是真的都已经被改了
func (a *AlwaysPullImages) Validate(attributes admission.Attributes, o admission.ObjectInterfaces) (err error) {
	pod, ok := attributes.GetObject().(*api.Pod)
	for i := range pod.Spec.InitContainers {
		if pod.Spec.InitContainers[i].ImagePullPolicy != api.PullAlways {
			return admission.NewForbidden(attributes,
				field.NotSupported(field.NewPath("spec", "initContainers").Index(i).Child("imagePullPolicy"),
					pod.Spec.InitContainers[i].ImagePullPolicy, []string{string(api.PullAlways)},
				),
			)
		}
	}

    ...

	return nil
}

func Register(plugins *admission.Plugins) {
	plugins.Register(PluginName, func(config io.Reader) (admission.Interface, error) {
		return NewAlwaysPullImages(), nil
	})
}

type AlwaysPullImages struct {
	*admission.Handler
}

kubernetes/pkg/kubeapiserver/options/plugins.go

func RegisterAllAdmissionPlugins(plugins *admission.Plugins) {
	imagepolicy.Register(plugins)
    ...
}

const (
    // 特定的json patch格式
	addFirstLabelPatch string = `[
         { "op": "add", "path": "/metadata/labels", "value": {"added-label": "yes"}}
     ]`
	addAdditionalLabelPatch string = `[
         { "op": "add", "path": "/metadata/labels/added-label", "value": "yes" }
     ]`
)

// Add a label {"added-label": "yes"} to the object
func addLabel(ar v1beta1.AdmissionReview) *v1beta1.AdmissionResponse {
	obj := struct {
		metav1.ObjectMeta
		Data map[string]string
	}{}
	raw := ar.Request.Object.Raw
	err := json.Unmarshal(raw, &obj)
	if err != nil {
		klog.Error(err)
		return toAdmissionResponse(err)
	}

	reviewResponse := v1beta1.AdmissionResponse{}
	reviewResponse.Allowed = true
	if len(obj.ObjectMeta.Labels) == 0 {
		reviewResponse.Patch = []byte(addFirstLabelPatch) // 这里最需要注意的就是修改时是通过patch的方式
	} else {
		reviewResponse.Patch = []byte(addAdditionalLabelPatch)
	}
	pt := v1beta1.PatchTypeJSONPatch
	reviewResponse.PatchType = &pt
	return &reviewResponse
}

apiVersion: admissionregistration.k8s.io/v1beta1
kind: ValidatingWebhookConfiguration
metadata:
  name: <name of this configuration object>
webhooks:
- name: <webhook name, e.g., pod-policy.example.io>
  rules:                                            # 最好明确一下该hook关心哪些api，防止带来不必要的额外开销。
  - apiGroups:
    - ""
    apiVersions:
    - v1
    operations:
    - CREATE
    resources:
    - pods
    scope: "Namespaced"
  clientConfig:
    service:
      namespace: <namespace of the front-end service>  # webhook server的namespace
      name: <name of the front-end service>            # service name
    caBundle: <pem encoded ca cert that signs the server cert used by the webhook> # 因为需要通过https访问，所以要给apiserver配置ca
  admissionReviewVersions:
  - v1beta1
  timeoutSeconds: 1