CustomResourceDefinition

CustomResourceDefinition (CRD) is an ingenious mechanism introduced in v1.7 that allows you to extend the Kubernetes API without tinkering with code to manage custom objects. Practically speaking, it's an upgraded version of ThirdPartyResources (TPR), which was deprecated in v1.8.

API Version Comparison Table

Kubernetes Version

CRD API Version

v1.8+

apiextensions.k8s.io/v1beta1

CRD Example

The example below crafts a custom API: /apis/stable.example.com/v1/namespaces/<namespace>/crontabs/….

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
  # versions to use for REST API: /apis/<group>/<version>
  versions:
  - name: v1beta1
    # Each version can be enabled/disabled by the Served flag.
    served: true
    # One and only one version must be marked as the storage version.
    storage: true
  - name: v1
    served: true
    storage: false
  # 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 a shorter string to match your resource on the CLI
    shortNames:
    - ct

Once the API is set up, you can proceed to create specific CronTab objects.

$ 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

Finalizers are used to implement asynchronous pre-deletion hooks for controllers, which can be specified via metadata.finalizers.

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

Once the finalizer is indicated, the operation to delete an object by a client will merely set metadata.deletionTimestamp instead of performing a direct deletion. This triggers controllers that are listening to the CRD to perform cleanup operations before deletion, remove its own finalizer from the list, and then initiate a new deletion operation. Only then is the object to be deleted truly eliminated.

Validation

Starting from v1.8, an experimental validation mechanism based on OpenAPI v3 schema was added to validate the conformity of resources submitted by users in advance. To use this function, you need to configure the kube-apiserver's --feature-gates=CustomResourceValidation=true.

For instance, the CRD below necessitates:

spec.cronSpec to be a string that matches a regular expression
spec.replicas to be an integer between 1 and 10

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

For example, when creating the following CronTab:

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

You'll encounter a validation failure error:

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

From v1.10 onwards, CRD also supports two subresources /status and /scale in the beta version, and they are enabled by default from v1.11.

To use in v1.10, you need to enable --feature-gates=CustomResourceSubresources=true on the kube-apiserver.

# 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 Controllers

When extending the Kubernetes API using CRD, it's generally also necessary to implement a controller for the new resources to monitor their changes and make further processing.

https://github.com/kubernetes/sample-controller provides an example of a CRD controller, including

How to register Foo resources
How to create, delete, and query Foo objects
How to monitor changes of Foo resources

Kubebuilder

As demonstrated above, building a CRD controller from scratch is quite challenging considering the level of understanding required for Kubernetes's API. Integrating RBAC, constructing images, and continuous integration and deployment all demand a large amount of work.

kubebuilder exists to solve this issue, providing an easy-to-use framework for creating CRD controllers and directly generating the resource files needed for image building, continuous integration, and deployment.

Installation

# Install kubebuilder
VERSION=1.0.1
wget https://github.com/kubernetes-sigs/kubebuilder/releases/download/v${VERSION}/kubebuilder_${VERSION}_linux_amd64.tar.gz
tar zxvf kubebuilder_${VERSION}_linux_amd64.tar.gz
sudo mv kubebuilder_${VERSION}_linux_amd64 /usr/local/kubebuilder
export PATH=$PATH:/usr/local/kubebuilder/bin

# Install dep kustomize
go get -u github.com/golang/dep/cmd/dep
go get github.com/kubernetes-sigs/kustomize

How to Use

Initialize the Project

mkdir -p $GOPATH/src/demo
cd $GOPATH/src/demo
kubebuilder init --domain k8s.io --license apache2 --owner "The Kubernetes Authors"

Create API

kubebuilder create api --group ships --version v1beta1 --kind Sloop

Then, depending on your actual needs, modify pkg/apis/ship/v1beta1/sloop_types.go and pkg/controller/sloop/sloop_controller.go to add business logic.

Run Local Test

make install
make run

If you run into the error ValidationError(CustomResourceDefinition.status): missing required field "storedVersions" in io.k8s.apiextensions-apiserver.pkg.apis.apiextensions.v1beta1.CustomResourceDefinitionStatus], manually modify config/crds/ships_v1beta1_sloop.yaml:
```yaml status: acceptedNames: kind: "" plural: "" conditions: [] storedVersions: []
Then run kubectl apply -f config/crds to create the CRD.

You can then create resources with Kind as Sloop using ships.k8s.io/v1beta1, such as

kubectl apply -f config/samples/ships_v1beta1_sloop.yaml

Build Image and Deploy Controller

# Replace IMG with your own
export IMG=feisky/demo-crd:v1
make docker-build
make docker-push
make deploy

kustomize no longer supports wildcards, so the above make deploy may encounter a Load from path ../rbac/*.yaml failed error. The solution is to manually modify config/default/kustomization.yaml:
resources:
../rbac/rbac_role.yaml
../rbac/rbac_role_binding.yaml
../manager/manager.yaml
Then execute kustomize build config/default | kubectl apply -f - to deploy. By default, it's deployed to the demo-system namespace.

Documentation and Testing

# run unit tests
make test

# generate docs
kubebuilder docs

References

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