2.2 主要概念 - CustomResourceDefinition

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2023-12-01

CustomResourceDefinition（CRD）是 v1.7 + 新增的无需改变代码就可以扩展 Kubernetes API 的机制，用来管理自定义对象。它实际上是 ThirdPartyResources（TPR）的升级版本，而 TPR 已经在 v1.8 中删除。

API 版本对照表

Kubernetes 版本	CRD API 版本
v1.8+	apiextensions.k8s.io/v1beta1

CRD 示例

下面的例子会创建一个 /apis/stable.example.com/v1/namespaces/<namespace>/crontabs/… 的自定义 API：

apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
metadata:
  # name must match the spec fields below, and be in the form: <plural>.<group>
  name: crontabs.stable.example.com
spec:
  # group name to use for REST API: /apis/<group>/<version>
  group: stable.example.com
  # version name to use for REST API: /apis/<group>/<version>
  version: v1
  # either Namespaced or Cluster
  scope: Namespaced
  names:
    # plural name to be used in the URL: /apis/<group>/<version>/<plural>
    plural: crontabs
    # singular name to be used as an alias on the CLI and for display
    singular: crontab
    # kind is normally the CamelCased singular type. Your resource manifests use this.
    kind: CronTab
    # shortNames allow shorter string to match your resource on the CLI
    shortNames:
    - ct

API 创建好后，就可以创建具体的 CronTab 对象了

$ cat my-cronjob.yaml
apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
  name: my-new-cron-object
spec:
  cronSpec: "* * * * /5"
  image: my-awesome-cron-image
$ kubectl create -f my-crontab.yaml
crontab "my-new-cron-object" created
$ kubectl get crontab
NAME                 KIND
my-new-cron-object   CronTab.v1.stable.example.com
$ kubectl get crontab my-new-cron-object -o yaml
apiVersion: stable.example.com/v1
kind: CronTab
metadata:
  creationTimestamp: 2017-07-03T19:00:56Z
  name: my-new-cron-object
  namespace: default
  resourceVersion: "20630"
  selfLink: /apis/stable.example.com/v1/namespaces/default/crontabs/my-new-cron-object
  uid: 5c82083e-5fbd-11e7-a204-42010a8c0002
spec:
  cronSpec: '* * * * /5'
  image: my-awesome-cron-image

Finalizer

Finalizer 用于实现控制器的异步预删除钩子，可以通过 metadata.finalizers 来指定 Finalizer。

apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
  finalizers:
  - finalizer.stable.example.com

Finalizer 指定后，客户端删除对象的操作只会设置 metadata.deletionTimestamp 而不是直接删除。这会触发正在监听 CRD 的控制器，控制器执行一些删除前的清理操作，从列表中删除自己的 finalizer，然后再重新发起一个删除操作。此时，被删除的对象才会真正删除。

Validation

v1.8 开始新增了实验性的基于 OpenAPI v3 schema 的验证（Validation）机制，可以用来提前验证用户提交的资源是否符合规范。使用该功能需要配置 kube-apiserver 的 --feature-gates=CustomResourceValidation=true。

比如下面的 CRD 要求

spec.cronSpec 必须是匹配正则表达式的字符串
spec.replicas 必须是从 1 到 10 的整数

apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
metadata:
  name: crontabs.stable.example.com
spec:
  group: stable.example.com
  version: v1
  scope: Namespaced
  names:
    plural: crontabs
    singular: crontab
    kind: CronTab
    shortNames:
    - ct
  validation:
   # openAPIV3Schema is the schema for validating custom objects.
    openAPIV3Schema:
      properties:
        spec:
          properties:
            cronSpec:
              type: string
              pattern: '^(d+|*)(/d+)?(s+(d+|*)(/d+)?){4}$'
            replicas:
              type: integer
              minimum: 1
              maximum: 10

这样，在创建下面的 CronTab 时

apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
  name: my-new-cron-object
spec:
  cronSpec: "* * * *"
  image: my-awesome-cron-image
  replicas: 15

会报验证失败的错误：

The CronTab "my-new-cron-object" is invalid: []: Invalid value: map[string]interface {}{"apiVersion":"stable.example.com/v1", "kind":"CronTab", "metadata":map[string]interface {}{"name":"my-new-cron-object", "namespace":"default", "deletionTimestamp":interface {}(nil), "deletionGracePeriodSeconds":(*int64)(nil), "creationTimestamp":"2017-09-05T05:20:07Z", "uid":"e14d79e7-91f9-11e7-a598-f0761cb232d1", "selfLink":"","clusterName":""}, "spec":map[string]interface {}{"cronSpec":"* * * *", "image":"my-awesome-cron-image", "replicas":15}}:
validation failure list:
spec.cronSpec in body should match '^(d+|*)(/d+)?(s+(d+|*)(/d+)?){4}$'
spec.replicas in body should be less than or equal to 10

Subresources

v1.10 开始 CRD 还支持 /status 和 /scale 等两个子资源（Alpha）。使用前需要在 kube-apiserver 开启 --feature-gates=CustomResourceSubresources=true。

# resourcedefinition.yaml
apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
metadata:
  name: crontabs.stable.example.com
spec:
  group: stable.example.com
  version: v1
  scope: Namespaced
  names:
    plural: crontabs
    singular: crontab
    kind: CronTab
    shortNames:
    - ct
  # subresources describes the subresources for custom resources.
  subresources:
    # status enables the status subresource.
    status: {}
    # scale enables the scale subresource.
    scale:
      # specReplicasPath defines the JSONPath inside of a custom resource that corresponds to Scale.Spec.Replicas.
      specReplicasPath: .spec.replicas
      # statusReplicasPath defines the JSONPath inside of a custom resource that corresponds to Scale.Status.Replicas.
      statusReplicasPath: .status.replicas
      # labelSelectorPath defines the JSONPath inside of a custom resource that corresponds to Scale.Status.Selector.
      labelSelectorPath: .status.labelSelector

$ kubectl create -f resourcedefinition.yaml
$ kubectl create -f- <<EOF
apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
  name: my-new-cron-object
spec:
  cronSpec: "* * * * */5"
  image: my-awesome-cron-image
  replicas: 3
EOF
$ kubectl scale --replicas=5 crontabs/my-new-cron-object
crontabs "my-new-cron-object" scaled
$ kubectl get crontabs my-new-cron-object -o jsonpath='{.spec.replicas}'
5

CRD 控制器

在使用 CRD 扩展 Kubernetes API 时，通常还需要实现一个新建资源的控制器，监听改资源的变化情况，并作进一步的处理。

https://github.com/kubernetes/sample-controller 提供了一个 CRD 控制器的示例，包括

如何注册资源 Foo
如何创建、删除和查询 Foo 对象
如何监听 Foo 资源对象的变化情况