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---
title: Extend the Kubernetes API with CustomResourceDefinitions
reviewers:
- deads2k
- jpbetz
- liggitt
- roycaihw
- sttts
content_type: task
min-kubernetes-server-version: 1.16
weight: 20
---
<!-- overview -->
This page shows how to install a
[custom resource](/docs/concepts/extend-kubernetes/api-extension/custom-resources/)
into the Kubernetes API by creating a
[CustomResourceDefinition](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#customresourcedefinition-v1-apiextensions-k8s-io).
## {{% heading "prerequisites" %}}
{{< include "task-tutorial-prereqs.md" >}} {{< version-check >}}
If you are using an older version of Kubernetes that is still supported, switch to
the documentation for that version to see advice that is relevant for your cluster.
<!-- steps -->
## Create a CustomResourceDefinition
When you create a new CustomResourceDefinition (CRD), the Kubernetes API Server
creates a new RESTful resource path for each version you specify. The CRD can be
either namespaced or cluster-scoped, as specified in the CRD's `scope` field. As
with existing built-in objects, deleting a namespace deletes all custom objects
in that namespace. CustomResourceDefinitions themselves are non-namespaced and
are available to all namespaces.
For example, if you save the following CustomResourceDefinition to `resourcedefinition.yaml`:
```yaml
apiVersion: apiextensions.k8s.io/v1
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
# list of versions supported by this CustomResourceDefinition
versions:
- name: v1
# Each version can be enabled/disabled by Served flag.
served: true
# One and only one version must be marked as the storage version.
storage: true
schema:
openAPIV3Schema:
type: object
properties:
spec:
type: object
properties:
cronSpec:
type: string
image:
type: string
replicas:
type: integer
# 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
```
and create it:
```shell
kubectl apply -f resourcedefinition.yaml
```
Then a new namespaced RESTful API endpoint is created at:
```
/apis/stable.example.com/v1/namespaces/*/crontabs/...
```
This endpoint URL can then be used to create and manage custom objects.
The `kind` of these objects will be `CronTab` from the spec of the
CustomResourceDefinition object you created above.
It might take a few seconds for the endpoint to be created.
You can watch the `Established` condition of your CustomResourceDefinition
to be true or watch the discovery information of the API server for your
resource to show up.
## Create custom objects
After the CustomResourceDefinition object has been created, you can create
custom objects. Custom objects can contain custom fields. These fields can
contain arbitrary JSON.
In the following example, the `cronSpec` and `image` custom fields are set in a
custom object of kind `CronTab`. The kind `CronTab` comes from the spec of the
CustomResourceDefinition object you created above.
If you save the following YAML to `my-crontab.yaml`:
```yaml
apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
name: my-new-cron-object
spec:
cronSpec: "* * * * */5"
image: my-awesome-cron-image
```
and create it:
```shell
kubectl apply -f my-crontab.yaml
```
You can then manage your CronTab objects using kubectl. For example:
```shell
kubectl get crontab
```
Should print a list like this:
```none
NAME AGE
my-new-cron-object 6s
```
Resource names are not case-sensitive when using kubectl, and you can use either
the singular or plural forms defined in the CRD, as well as any short names.
You can also view the raw YAML data:
```shell
kubectl get ct -o yaml
```
You should see that it contains the custom `cronSpec` and `image` fields
from the YAML you used to create it:
```yaml
apiVersion: v1
items:
- apiVersion: stable.example.com/v1
kind: CronTab
metadata:
annotations:
kubectl.kubernetes.io/last-applied-configuration: |
{"apiVersion":"stable.example.com/v1","kind":"CronTab","metadata":{"annotations":{},"name":"my-new-cron-object","namespace":"default"},"spec":{"cronSpec":"* * * * */5","image":"my-awesome-cron-image"}}
creationTimestamp: "2021-06-20T07:35:27Z"
generation: 1
name: my-new-cron-object
namespace: default
resourceVersion: "1326"
uid: 9aab1d66-628e-41bb-a422-57b8b3b1f5a9
spec:
cronSpec: '* * * * */5'
image: my-awesome-cron-image
kind: List
metadata:
resourceVersion: ""
selfLink: ""
```
## Delete a CustomResourceDefinition
When you delete a CustomResourceDefinition, the server will uninstall the RESTful API endpoint
and delete all custom objects stored in it.
```shell
kubectl delete -f resourcedefinition.yaml
kubectl get crontabs
```
```none
Error from server (NotFound): Unable to list {"stable.example.com" "v1" "crontabs"}: the server could not find the requested resource (get crontabs.stable.example.com)
```
If you later recreate the same CustomResourceDefinition, it will start out empty.
## Specifying a structural schema
CustomResources store structured data in custom fields (alongside the built-in
fields `apiVersion`, `kind` and `metadata`, which the API server validates
implicitly). With [OpenAPI v3.0 validation](#validation) a schema can be
specified, which is validated during creation and updates, compare below for
details and limits of such a schema.
With `apiextensions.k8s.io/v1` the definition of a structural schema is
mandatory for CustomResourceDefinitions. In the beta version of
CustomResourceDefinition, the structural schema was optional.
A structural schema is an [OpenAPI v3.0 validation schema](#validation) which:
1. specifies a non-empty type (via `type` in OpenAPI) for the root, for each specified field of an object node (via `properties` or `additionalProperties` in OpenAPI) and for each item in an array node (via `items` in OpenAPI), with the exception of:
* a node with `x-kubernetes-int-or-string: true`
* a node with `x-kubernetes-preserve-unknown-fields: true`
2. for each field in an object and each item in an array which is specified within any of `allOf`, `anyOf`, `oneOf` or `not`, the schema also specifies the field/item outside of those logical junctors (compare example 1 and 2).
3. does not set `description`, `type`, `default`, `additionalProperties`, `nullable` within an `allOf`, `anyOf`, `oneOf` or `not`, with the exception of the two pattern for `x-kubernetes-int-or-string: true` (see below).
4. if `metadata` is specified, then only restrictions on `metadata.name` and `metadata.generateName` are allowed.
Non-structural example 1:
```yaml
allOf:
- properties:
foo:
...
```
conflicts with rule 2. The following would be correct:
```yaml
properties:
foo:
...
allOf:
- properties:
foo:
...
```
Non-structural example 2:
```yaml
allOf:
- items:
properties:
foo:
...
```
conflicts with rule 2. The following would be correct:
```yaml
items:
properties:
foo:
...
allOf:
- items:
properties:
foo:
...
```
Non-structural example 3:
```yaml
properties:
foo:
pattern: "abc"
metadata:
type: object
properties:
name:
type: string
pattern: "^a"
finalizers:
type: array
items:
type: string
pattern: "my-finalizer"
anyOf:
- properties:
bar:
type: integer
minimum: 42
required: ["bar"]
description: "foo bar object"
```
is not a structural schema because of the following violations:
* the type at the root is missing (rule 1).
* the type of `foo` is missing (rule 1).
* `bar` inside of `anyOf` is not specified outside (rule 2).
* `bar`'s `type` is within `anyOf` (rule 3).
* the description is set within `anyOf` (rule 3).
* `metadata.finalizers` might not be restricted (rule 4).
In contrast, the following, corresponding schema is structural:
```yaml
type: object
description: "foo bar object"
properties:
foo:
type: string
pattern: "abc"
bar:
type: integer
metadata:
type: object
properties:
name:
type: string
pattern: "^a"
anyOf:
- properties:
bar:
minimum: 42
required: ["bar"]
```
Violations of the structural schema rules are reported in the `NonStructural` condition in the CustomResourceDefinition.
### Field pruning
CustomResourceDefinitions store validated resource data in the cluster's persistence store, {{< glossary_tooltip term_id="etcd" text="etcd">}}. As with native Kubernetes resources such as {{< glossary_tooltip text="ConfigMap" term_id="configmap" >}}, if you specify a field that the API server does not recognize, the unknown field is _pruned_ (removed) before being persisted.
{{< note >}}
CRDs converted from `apiextensions.k8s.io/v1beta1` to `apiextensions.k8s.io/v1` might lack structural schemas, and `spec.preserveUnknownFields` might be `true`.
For legacy CustomResourceDefinition objects created as
`apiextensions.k8s.io/v1beta1` with `spec.preserveUnknownFields` set to
`true`, the following is also true:
* Pruning is not enabled.
* You can store arbitrary data.
For compatibility with `apiextensions.k8s.io/v1`, update your custom
resource definitions to:
1. Use a structural OpenAPI schema.
2. Set `spec.preserveUnknownFields` to `false`.
{{< /note >}}
If you save the following YAML to `my-crontab.yaml`:
```yaml
apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
name: my-new-cron-object
spec:
cronSpec: "* * * * */5"
image: my-awesome-cron-image
someRandomField: 42
```
and create it:
```shell
kubectl create --validate=false -f my-crontab.yaml -o yaml
```
your output is similar to:
```console
apiVersion: stable.example.com/v1
kind: CronTab
metadata:
creationTimestamp: 2017-05-31T12:56:35Z
generation: 1
name: my-new-cron-object
namespace: default
resourceVersion: "285"
uid: 9423255b-4600-11e7-af6a-28d2447dc82b
spec:
cronSpec: '* * * * */5'
image: my-awesome-cron-image
```
Notice that the field `someRandomField` was pruned.
This example turned off client-side validation to demonstrate the API server's behavior, by adding the `--validate=false` command line option.
Because the [OpenAPI validation schemas are also published](#publish-validation-schema-in-openapi-v2)
to clients, `kubectl` also checks for unknown fields and rejects those objects well before they would be sent to the API server.
#### Controlling pruning
By default, all unspecified fields for a custom resource, across all versions, are pruned. It is possible though to opt-out of that for specifc sub-trees of fields by adding `x-kubernetes-preserve-unknown-fields: true` in the [structural OpenAPI v3 validation schema](#specifying-a-structural-schema).
For example:
```yaml
type: object
properties:
json:
x-kubernetes-preserve-unknown-fields: true
```
The field `json` can store any JSON value, without anything being pruned.
You can also partially specify the permitted JSON; for example:
```yaml
type: object
properties:
json:
x-kubernetes-preserve-unknown-fields: true
type: object
description: this is arbitrary JSON
```
With this, only `object` type values are allowed.
Pruning is enabled again for each specified property (or `additionalProperties`):
```yaml
type: object
properties:
json:
x-kubernetes-preserve-unknown-fields: true
type: object
properties:
spec:
type: object
properties:
foo:
type: string
bar:
type: string
```
With this, the value:
```yaml
json:
spec:
foo: abc
bar: def
something: x
status:
something: x
```
is pruned to:
```yaml
json:
spec:
foo: abc
bar: def
status:
something: x
```
This means that the `something` field in the specified `spec` object is pruned, but everything outside is not.
### IntOrString
Nodes in a schema with `x-kubernetes-int-or-string: true` are excluded from rule 1, such that the following is structural:
```yaml
type: object
properties:
foo:
x-kubernetes-int-or-string: true
```
Also those nodes are partially excluded from rule 3 in the sense that the following two patterns are allowed (exactly those, without variations in order to additional fields):
```yaml
x-kubernetes-int-or-string: true
anyOf:
- type: integer
- type: string
...
```
and
```yaml
x-kubernetes-int-or-string: true
allOf:
- anyOf:
- type: integer
- type: string
- ... # zero or more
...
```
With one of those specification, both an integer and a string validate.
In [Validation Schema Publishing](#publish-validation-schema-in-openapi-v2),
`x-kubernetes-int-or-string: true` is unfolded to one of the two patterns shown above.
### RawExtension
RawExtensions (as in `runtime.RawExtension` defined in
[k8s.io/apimachinery](https://github.com/kubernetes/apimachinery/blob/03ac7a9ade429d715a1a46ceaa3724c18ebae54f/pkg/runtime/types.go#L94))
holds complete Kubernetes objects, i.e. with `apiVersion` and `kind` fields.
It is possible to specify those embedded objects (both completely without constraints or partially specified) by setting `x-kubernetes-embedded-resource: true`. For example:
```yaml
type: object
properties:
foo:
x-kubernetes-embedded-resource: true
x-kubernetes-preserve-unknown-fields: true
```
Here, the field `foo` holds a complete object, e.g.:
```yaml
foo:
apiVersion: v1
kind: Pod
spec:
...
```
Because `x-kubernetes-preserve-unknown-fields: true` is specified alongside, nothing is pruned. The use of `x-kubernetes-preserve-unknown-fields: true` is optional though.
With `x-kubernetes-embedded-resource: true`, the `apiVersion`, `kind` and `metadata` are implicitly specified and validated.
## Serving multiple versions of a CRD
See [Custom resource definition versioning](/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definition-versioning/)
for more information about serving multiple versions of your
CustomResourceDefinition and migrating your objects from one version to another.
<!-- discussion -->
## Advanced topics
### Finalizers
*Finalizers* allow controllers to implement asynchronous pre-delete hooks.
Custom objects support finalizers similar to built-in objects.
You can add a finalizer to a custom object like this:
```yaml
apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
finalizers:
- stable.example.com/finalizer
```
Identifiers of custom finalizers consist of a domain name, a forward slash and the name of
the finalizer. Any controller can add a finalizer to any object's list of finalizers.
The first delete request on an object with finalizers sets a value for the
`metadata.deletionTimestamp` field but does not delete it. Once this value is set,
entries in the `finalizers` list can only be removed. While any finalizers remain it is also
impossible to force the deletion of an object.
When the `metadata.deletionTimestamp` field is set, controllers watching the object execute any
finalizers they handle and remove the finalizer from the list after they are done. It is the
responsibility of each controller to remove its finalizer from the list.
The value of `metadata.deletionGracePeriodSeconds` controls the interval between polling updates.
Once the list of finalizers is empty, meaning all finalizers have been executed, the resource is
deleted by Kubernetes.
### Validation
Custom resources are validated via
[OpenAPI v3 schemas](https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.0.md#schemaObject),
by x-kubernetes-validations when the [Validation Rules feature](#validation-rules) is enabled, and you
can add additional validation using
[admission webhooks](/docs/reference/access-authn-authz/admission-controllers/#validatingadmissionwebhook).
Additionally, the following restrictions are applied to the schema:
- These fields cannot be set:
- `definitions`,
- `dependencies`,
- `deprecated`,
- `discriminator`,
- `id`,
- `patternProperties`,
- `readOnly`,
- `writeOnly`,
- `xml`,
- `$ref`.
- The field `uniqueItems` cannot be set to `true`.
- The field `additionalProperties` cannot be set to `false`.
- The field `additionalProperties` is mutually exclusive with `properties`.
The `x-kubernetes-validations` extension can be used to validate custom resources using [Common
Expression Language (CEL)](https://github.com/google/cel-spec) expressions when the [Validation
rules](#validation-rules) feature is enabled and the CustomResourceDefinition schema is a
[structural schema](#specifying-a-structural-schema).
The `default` field can be set when the [Defaulting feature](#defaulting) is enabled,
which is the case with `apiextensions.k8s.io/v1` CustomResourceDefinitions.
Defaulting is in GA since 1.17 (beta since 1.16 with the `CustomResourceDefaulting`
[feature gate](/docs/reference/command-line-tools-reference/feature-gates/)
enabled, which is the case automatically for many clusters for beta features).
Refer to the [structural schemas](#specifying-a-structural-schema) section for other
restrictions and CustomResourceDefinition features.
The schema is defined in the CustomResourceDefinition. In the following example, the
CustomResourceDefinition applies the following validations on the custom object:
- `spec.cronSpec` must be a string and must be of the form described by the regular expression.
- `spec.replicas` must be an integer and must have a minimum value of 1 and a maximum value of 10.
Save the CustomResourceDefinition to `resourcedefinition.yaml`:
```yaml
apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
name: crontabs.stable.example.com
spec:
group: stable.example.com
versions:
- name: v1
served: true
storage: true
schema:
# openAPIV3Schema is the schema for validating custom objects.
openAPIV3Schema:
type: object
properties:
spec:
type: object
properties:
cronSpec:
type: string
pattern: '^(\d+|\*)(/\d+)?(\s+(\d+|\*)(/\d+)?){4}$'
image:
type: string
replicas:
type: integer
minimum: 1
maximum: 10
scope: Namespaced
names:
plural: crontabs
singular: crontab
kind: CronTab
shortNames:
- ct
```
and create it:
```shell
kubectl apply -f resourcedefinition.yaml
```
A request to create a custom object of kind CronTab is rejected if there are invalid values in its fields.
In the following example, the custom object contains fields with invalid values:
- `spec.cronSpec` does not match the regular expression.
- `spec.replicas` is greater than 10.
If you save the following YAML to `my-crontab.yaml`:
```yaml
apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
name: my-new-cron-object
spec:
cronSpec: "* * * *"
image: my-awesome-cron-image
replicas: 15
```
and attempt to create it:
```shell
kubectl apply -f my-crontab.yaml
```
then you get an error:
```console
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", "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
```
If the fields contain valid values, the object creation request is accepted.
Save the following YAML to `my-crontab.yaml`:
```yaml
apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
name: my-new-cron-object
spec:
cronSpec: "* * * * */5"
image: my-awesome-cron-image
replicas: 5
```
And create it:
```shell
kubectl apply -f my-crontab.yaml
crontab "my-new-cron-object" created
```
## Validation rules
{{< feature-state state="alpha" for_k8s_version="v1.23" >}}
Validation rules are in alpha since 1.23 and validate custom resources when the
`CustomResourceValidationExpressions` [feature
gate](/docs/reference/command-line-tools-reference/feature-gates/) is enabled.
This feature is only available if the schema is a
[structural schema](#specifying-a-structural-schema).
Validation rules use the [Common Expression Language (CEL)](https://github.com/google/cel-spec)
to validate custom resource values. Validation rules are included in
CustomResourceDefinition schemas using the `x-kubernetes-validations` extension.
The Rule is scoped to the location of the `x-kubernetes-validations` extension in the schema.
And `self` variable in the CEL expression is bound to the scoped value.
All validation rules are scoped to the current object: no cross-object or stateful validation rules are supported.
For example:
```yaml
...
openAPIV3Schema:
type: object
properties:
spec:
type: object
x-kubernetes-validations:
- rule: "self.minReplicas <= self.replicas"
message: "replicas should be greater than or equal to minReplicas."
- rule: "self.replicas <= self.maxReplicas"
message: "replicas should be smaller than or equal to maxReplicas."
properties:
...
minReplicas:
type: integer
replicas:
type: integer
maxReplicas:
type: integer
required:
- minReplicas
- replicas
- maxReplicas
```
will reject a request to create this custom resource:
```yaml
apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
name: my-new-cron-object
spec:
minReplicas: 0
replicas: 20
maxReplicas: 10
```
with the response:
```
The CronTab "my-new-cron-object" is invalid:
* spec: Invalid value: map[string]interface {}{"maxReplicas":10, "minReplicas":0, "replicas":20}: replicas should be smaller than or equal to maxReplicas.
```
`x-kubernetes-validations` could have multiple rules.
The `rule` under `x-kubernetes-validations` represents the expression which will be evaluated by CEL.
The `message` represents the message displayed when validation fails. If message is unset, the above response would be:
```
The CronTab "my-new-cron-object" is invalid:
* spec: Invalid value: map[string]interface {}{"maxReplicas":10, "minReplicas":0, "replicas":20}: failed rule: self.replicas <= self.maxReplicas
```
Validation rules are compiled when CRDs are created/updated.
The request of CRDs create/update will fail if compilation of validation rules fail.
Compilation process includes type checking as well.
The compilation failure:
- `no_matching_overload`: this function has no overload for the types of the arguments.
e.g. Rule like `self == true` against a field of integer type will get error:
```
Invalid value: apiextensions.ValidationRule{Rule:"self == true", Message:""}: compilation failed: ERROR: \<input>:1:6: found no matching overload for '_==_' applied to '(int, bool)'
```
- `no_such_field`: does not contain the desired field.
e.g. Rule like `self.nonExistingField > 0` against a non-existing field will return the error:
```
Invalid value: apiextensions.ValidationRule{Rule:"self.nonExistingField > 0", Message:""}: compilation failed: ERROR: \<input>:1:5: undefined field 'nonExistingField'
```
- `invalid argument`: invalid argument to macros.
e.g. Rule like `has(self)` will return error:
```
Invalid value: apiextensions.ValidationRule{Rule:"has(self)", Message:""}: compilation failed: ERROR: <input>:1:4: invalid argument to has() macro
```
Validation Rules Examples:
| Rule | Purpose |
| ---------------- | ------------ |
| `self.minReplicas <= self.replicas && self.replicas <= self.maxReplicas` | Validate that the three fields defining replicas are ordered appropriately |
| `'Available' in self.stateCounts` | Validate that an entry with the 'Available' key exists in a map |
| `(size(self.list1) == 0) != (size(self.list2) == 0)` | Validate that one of two lists is non-empty, but not both |
| <code>!('MY_KEY' in self.map1) &#124;&#124; self['MY_KEY'].matches('^[a-zA-Z]*$')</code> | Validate the value of a map for a specific key, if it is in the map |
| `self.envars.filter(e, e.name = 'MY_ENV').all(e, e.value.matches('^[a-zA-Z]*$')` | Validate the 'value' field of a listMap entry where key field 'name' is 'MY_ENV' |
| `has(self.expired) && self.created + self.ttl < self.expired` | Validate that 'expired' date is after a 'create' date plus a 'ttl' duration |
| `self.health.startsWith('ok')` | Validate a 'health' string field has the prefix 'ok' |
| `self.widgets.exists(w, w.key == 'x' && w.foo < 10)` | Validate that the 'foo' property of a listMap item with a key 'x' is less than 10 |
| `type(self) == string ? self == '100%' : self == 1000` | Validate an int-or-string field for both the the int and string cases |
| `self.metadata.name.startsWith(self.prefix)` | Validate that an object's name has the prefix of another field value |
| `self.set1.all(e, !(e in self.set2))` | Validate that two listSets are disjoint |
| `size(self.names) == size(self.details) && self.names.all(n, n in self.details)` | Validate the 'details' map is keyed by the items in the 'names' listSet |
Xref: [Supported evaluation on CEL](https://github.com/google/cel-spec/blob/v0.6.0/doc/langdef.md#evaluation)
- If the Rule is scoped to the root of a resource, it may make field selection into any fields
declared in the OpenAPIv3 schema of the CRD as well as `apiVersion`, `kind`, `metadata.name` and
`metadata.generateName`. This includes selection of fields in both the `spec` and `status` in the
same expression:
```yaml
...
openAPIV3Schema:
type: object
x-kubernetes-validations:
- rule: "self.status.availableReplicas >= self.spec.minReplicas"
properties:
spec:
type: object
properties:
minReplicas:
type: integer
...
status:
type: object
properties:
availableReplicas:
type: integer
```
- If the Rule is scoped to an object with properties, the accessible properties of the object are field selectable
via `self.field` and field presence can be checked via `has(self.field)`. Null valued fields are treated as
absent fields in CEL expressions.
```yaml
...
openAPIV3Schema:
type: object
properties:
spec:
type: object
x-kubernetes-validations:
- rule: "has(self.foo)"
properties:
...
foo:
type: integer
```
- If the Rule is scoped to an object with additionalProperties (i.e. a map) the value of the map
are accessible via `self[mapKey]`, map containment can be checked via `mapKey in self` and all entries of the map
are accessible via CEL macros and functions such as `self.all(...)`.
```yaml
...
openAPIV3Schema:
type: object
properties:
spec:
type: object
x-kubernetes-validations:
- rule: "self['xyz'].foo > 0"
additionalProperties:
...
type: object
properties:
foo:
type: integer
```
- If the Rule is scoped to an array, the elements of the array are accessible via `self[i]` and also by macros and
functions.
```yaml
...
openAPIV3Schema:
type: object
properties:
...
foo:
type: array
x-kubernetes-validations:
- rule: "size(self) == 1"
items:
type: string
```
- If the Rule is scoped to a scalar, `self` is bound to the scalar value.
```yaml
...
openAPIV3Schema:
type: object
properties:
spec:
type: object
properties:
...
foo:
type: integer
x-kubernetes-validations:
- rule: "self > 0"
```
Examples:
|type of the field rule scoped to | Rule example |
| -----------------------| -----------------------|
| root object | `self.status.actual <= self.spec.maxDesired`|
| map of objects | `self.components['Widget'].priority < 10`|
| list of integers | `self.values.all(value, value >= 0 && value < 100)`|
| string | `self.startsWith('kube')`|
The `apiVersion`, `kind`, `metadata.name` and `metadata.generateName` are always accessible from the root of the
object and from any x-kubernetes-embedded-resource annotated objects. No other metadata properties are accessible.
Unknown data preserved in custom resources via `x-kubernetes-preserve-unknown-fields` is not accessible in CEL
expressions. This includes:
- Unknown field values that are preserved by object schemas with x-kubernetes-preserve-unknown-fields.
- Object properties where the property schema is of an "unknown type". An "unknown type" is recursively defined as:
- A schema with no type and x-kubernetes-preserve-unknown-fields set to true
- An array where the items schema is of an "unknown type"
- An object where the additionalProperties schema is of an "unknown type"
Only property names of the form `[a-zA-Z_.-/][a-zA-Z0-9_.-/]*` are accessible.
Accessible property names are escaped according to the following rules when accessed in the expression:
| escape sequence | property name equivalent |
| ----------------------- | -----------------------|
| `__underscores__` | `__` |
| `__dot__` | `.` |
|`__dash__` | `-` |
| `__slash__` | `/` |
| `__{keyword}__` | [CEL RESERVED keyword](https://github.com/google/cel-spec/blob/v0.6.0/doc/langdef.md#syntax) |
Note: CEL RESERVED keyword needs to match the exact property name to be escaped (e.g. int in the word sprint would not be escaped).
Examples on escaping:
|property name | rule with escaped property name |
| ----------------| ----------------------- |
| namespace | `self.__namespace__ > 0` |
| x-prop | `self.x__dash__prop > 0` |
| redact__d | `self.redact__underscores__d > 0` |
| string | `self.startsWith('kube')` |
Equality on arrays with `x-kubernetes-list-type` of `set` or `map` ignores element order, i.e. [1, 2] == [2, 1].
Concatenation on arrays with x-kubernetes-list-type use the semantics of the list type:
- `set`: `X + Y` performs a union where the array positions of all elements in `X` are preserved and
non-intersecting elements in `Y` are appended, retaining their partial order.
- `map`: `X + Y` performs a merge where the array positions of all keys in `X` are preserved but the values
are overwritten by values in `Y` when the key sets of `X` and `Y` intersect. Elements in `Y` with
non-intersecting keys are appended, retaining their partial order.
Here is the declarations type mapping between OpenAPIv3 and CEL type:
| OpenAPIv3 type | CEL type |
| -------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------- |
| 'object' with Properties | object / "message type" |
| 'object' with AdditionalProperties | map |
| 'object' with x-kubernetes-embedded-type | object / "message type", 'apiVersion', 'kind', 'metadata.name' and 'metadata.generateName' are implicitly included in schema |
| 'object' with x-kubernetes-preserve-unknown-fields | object / "message type", unknown fields are NOT accessible in CEL expression |
| x-kubernetes-int-or-string | dynamic object that is either an int or a string, `type(value)` can be used to check the type |
| 'array | list |
| 'array' with x-kubernetes-list-type=map | list with map based Equality & unique key guarantees |
| 'array' with x-kubernetes-list-type=set | list with set based Equality & unique entry guarantees |
| 'boolean' | boolean |
| 'number' (all formats) | double |
| 'integer' (all formats) | int (64) |
| 'null' | null_type |
| 'string' | string |
| 'string' with format=byte (base64 encoded) | bytes |
| 'string' with format=date | timestamp (google.protobuf.Timestamp) |
| 'string' with format=datetime | timestamp (google.protobuf.Timestamp) |
| 'string' with format=duration | duration (google.protobuf.Duration) |
xref: [CEL types](https://github.com/google/cel-spec/blob/v0.6.0/doc/langdef.md#values), [OpenAPI
types](https://swagger.io/specification/#data-types), [Kubernetes Structural Schemas](https://kubernetes.io/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions/#specifying-a-structural-schema).
#### Validation functions {#available-validation-functions}
Functions available include:
- CEL standard functions, defined in the [list of standard definitions](https://github.com/google/cel-spec/blob/v0.7.0/doc/langdef.md#list-of-standard-definitions)
- CEL standard [macros](https://github.com/google/cel-spec/blob/v0.7.0/doc/langdef.md#macros)
- CEL [extended string function library](https://pkg.go.dev/github.com/google/cel-go@v0.11.2/ext#Strings)
- Kubernetes [CEL extension library](https://pkg.go.dev/k8s.io/apiextensions-apiserver@v0.24.0/pkg/apiserver/schema/cel/library#pkg-functions)
#### Transition rules
A rule that contains an expression referencing the identifier `oldSelf` is implicitly considered a
_transition rule_. Transition rules allow schema authors to prevent certain transitions between two
otherwise valid states. For example:
```yaml
type: string
enum: ["low", "medium", "high"]
x-kubernetes-validations:
- rule: "!(self == 'high' && oldSelf == 'low') && !(self == 'low' && oldSelf == 'high')"
message: cannot transition directly between 'low' and 'high'
```
Unlike other rules, transition rules apply only to operations meeting the following criteria:
- The operation updates an existing object. Transition rules never apply to create operations.
- Both an old and a new value exist. It remains possible to check if a value has been added or
removed by placing a transition rule on the parent node. Transition rules are never applied to
custom resource creation. When placed on an optional field, a transition rule will not apply to
update operations that set or unset the field.
- The path to the schema node being validated by a transition rule must resolve to a node that is
comparable between the old object and the new object. For example, list items and their
descendants (`spec.foo[10].bar`) can't necessarily be correlated between an existing object and a
later update to the same object.
Errors will be generated on CRD writes if a schema node contains a transition rule that can never be
applied, e.g. "*path*: update rule *rule* cannot be set on schema because the schema or its parent
schema is not mergeable".
Transition rules are only allowed on _correlatable portions_ of a schema.
A portion of the schema is correlatable if all `array` parent schemas are of type `x-kubernetes-list-type=map`; any `set`or `atomic`array parent schemas make it impossible to unambiguously correlate a `self` with `oldSelf`.
Here are some examples for transition rules:
{{< table caption="Transition rules examples" >}}
| Use Case | Rule
| -------- | --------
| Immutability | `self.foo == oldSelf.foo`
| Prevent modification/removal once assigned | `oldSelf != 'bar' \|\| self == 'bar'` or `!has(oldSelf.field) \|\| has(self.field)`
| Append-only set | `self.all(element, element in oldSelf)`
| If previous value was X, new value can only be A or B, not Y or Z | `oldSelf != 'X' \|\| self in ['A', 'B']`
| Monotonic (non-decreasing) counters | `self >= oldSelf`
{{< /table >}}
#### Resource use by validation functions
When you create or update a CustomResourceDefinition that uses validation rules,
the API server checks the likely impact of running those validation rules. If a rule is
estimated to be prohibitively expensive to execute, the API server rejects the create
or update operation, and returns an error message.
A similar system is used at runtime that observes the actions the interpreter takes. If the interpreter executes
too many instructions, execution of the rule will be halted, and an error will result.
Each CustomResourceDefinition is also allowed a certain amount of resources to finish executing all of
its validation rules. If the sum total of its rules are estimated at creation time to go over that limit,
then a validation error will also occur.
You are unlikely to encounter issues with the resource budget for validation if you only
specify rules that always take the same amount of time regardless of how large their input is.
For example, a rule that asserts that `self.foo == 1` does not by itself have any
risk of rejection on validation resource budget groups.
But if `foo` is a string and you define a validation rule `self.foo.contains("someString")`, that rule takes
longer to execute depending on how long `foo` is.
Another example would be if `foo` were an array, and you specified a validation rule `self.foo.all(x, x > 5)`. The cost system always assumes the worst-case scenario if
a limit on the length of `foo` is not given, and this will happen for anything that can be iterated
over (lists, maps, etc.).
Because of this, it is considered best practice to put a limit via `maxItems`, `maxProperties`, and
`maxLength` for anything that will be processed in a validation rule in order to prevent validation errors during cost estimation. For example, given this schema with one rule:
```yaml
openAPIV3Schema:
type: object
properties:
foo:
type: array
items:
type: string
x-kubernetes-validations:
- rule: "self.all(x, x.contains('a string'))"
```
then the API server rejects this rule on validation budget grounds with error:
```
spec.validation.openAPIV3Schema.properties[spec].properties[foo].x-kubernetes-validations[0].rule: Forbidden:
CEL rule exceeded budget by more than 100x (try simplifying the rule, or adding maxItems, maxProperties, and
maxLength where arrays, maps, and strings are used)
```
The rejection happens because `self.all` implies calling `contains()` on every string in `foo`,
which in turn will check the given string to see if it contains `'a string'`. Without limits, this is a very
expensive rule.
If you do not specify any validation limit, the estimated cost of this rule will exceed the per-rule cost limit. But if you
add limits in the appropriate places, the rule will be allowed:
```yaml
openAPIV3Schema:
type: object
properties:
foo:
type: array
maxItems: 25
items:
type: string
maxLength: 10
x-kubernetes-validations:
- rule: "self.all(x, x.contains('a string'))"
```
The cost estimation system takes into account how many times the rule will be executed in addition to the
estimated cost of the rule itself. For instance, the following rule will have the same estimated cost as the
previous example (despite the rule now being defined on the individual array items):
```yaml
openAPIV3Schema:
type: object
properties:
foo:
type: array
maxItems: 25
items:
type: string
x-kubernetes-validations:
- rule: "self.contains('a string'))"
maxLength: 10
```
If a list inside of a list has a validation rule that uses `self.all`, that is significantly more expensive
than a non-nested list with the same rule. A rule that would have been allowed on a non-nested list might need lower limits set on both nested lists in order to be allowed. For example, even without having limits set,
the following rule is allowed:
```yaml
openAPIV3Schema:
type: object
properties:
foo:
type: array
items:
type: integer
x-kubernetes-validations:
- rule: "self.all(x, x == 5)"
```
But the same rule on the following schema (with a nested array added) produces a validation error:
```yaml
openAPIV3Schema:
type: object
properties:
foo:
type: array
items:
type: array
items:
type: integer
x-kubernetes-validations:
- rule: "self.all(x, x == 5)"
```
This is because each item of `foo` is itself an array, and each subarray in turn calls `self.all`. Avoid nested
lists and maps if possible where validation rules are used.
### Defaulting
{{< note >}}
To use defaulting, your CustomResourceDefinition must use API version `apiextensions.k8s.io/v1`.
{{< /note >}}
Defaulting allows to specify default values in the [OpenAPI v3 validation schema](#validation):
```yaml
apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
name: crontabs.stable.example.com
spec:
group: stable.example.com
versions:
- name: v1
served: true
storage: true
schema:
# openAPIV3Schema is the schema for validating custom objects.
openAPIV3Schema:
type: object
properties:
spec:
type: object
properties:
cronSpec:
type: string
pattern: '^(\d+|\*)(/\d+)?(\s+(\d+|\*)(/\d+)?){4}$'
default: "5 0 * * *"
image:
type: string
replicas:
type: integer
minimum: 1
maximum: 10
default: 1
scope: Namespaced
names:
plural: crontabs
singular: crontab
kind: CronTab
shortNames:
- ct
```
With this both `cronSpec` and `replicas` are defaulted:
```yaml
apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
name: my-new-cron-object
spec:
image: my-awesome-cron-image
```
leads to
```yaml
apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
name: my-new-cron-object
spec:
cronSpec: "5 0 * * *"
image: my-awesome-cron-image
replicas: 1
```
Defaulting happens on the object
* in the request to the API server using the request version defaults,
* when reading from etcd using the storage version defaults,
* after mutating admission plugins with non-empty patches using the admission webhook object version defaults.
Defaults applied when reading data from etcd are not automatically written back to etcd. An update request via the API is required to persist those defaults back into etcd.
Default values must be pruned (with the exception of defaults for `metadata` fields) and must validate against a provided schema.
Default values for `metadata` fields of `x-kubernetes-embedded-resources: true` nodes (or parts of a default value covering `metadata`) are not pruned during CustomResourceDefinition creation, but through the pruning step during handling of requests.
#### Defaulting and Nullable
**New in 1.20:** null values for fields that either don't specify the nullable flag, or give it a `false` value, will be pruned before defaulting happens. If a default is present, it will be applied. When nullable is `true`, null values will be conserved and won't be defaulted.
For example, given the OpenAPI schema below:
```yaml
type: object
properties:
spec:
type: object
properties:
foo:
type: string
nullable: false
default: "default"
bar:
type: string
nullable: true
baz:
type: string
```
creating an object with null values for `foo` and `bar` and `baz`
```yaml
spec:
foo: null
bar: null
baz: null
```
leads to
```yaml
spec:
foo: "default"
bar: null
```
with `foo` pruned and defaulted because the field is non-nullable, `bar` maintaining the null value due to `nullable: true`, and `baz` pruned because the field is non-nullable and has no default.
### Publish Validation Schema in OpenAPI v2
CustomResourceDefinition [OpenAPI v3 validation schemas](#validation) which are [structural](#specifying-a-structural-schema) and [enable pruning](#field-pruning) are published as part of the [OpenAPI v2 spec](/docs/concepts/overview/kubernetes-api/#openapi-and-swagger-definitions) from Kubernetes API server.
The [kubectl](/docs/reference/kubectl/) command-line tool consumes the published schema to perform client-side validation (`kubectl create` and `kubectl apply`), schema explanation (`kubectl explain`) on custom resources. The published schema can be consumed for other purposes as well, like client generation or documentation.
The OpenAPI v3 validation schema is converted to OpenAPI v2 schema, and
show up in `definitions` and `paths` fields in the [OpenAPI v2 spec](/docs/concepts/overview/kubernetes-api/#openapi-and-swagger-definitions).
The following modifications are applied during the conversion to keep backwards compatibility with
kubectl in previous 1.13 version. These modifications prevent kubectl from being over-strict and rejecting
valid OpenAPI schemas that it doesn't understand. The conversion won't modify the validation schema defined in CRD,
and therefore won't affect [validation](#validation) in the API server.
1. The following fields are removed as they aren't supported by OpenAPI v2 (in future versions OpenAPI v3 will be used without these restrictions)
- The fields `allOf`, `anyOf`, `oneOf` and `not` are removed
2. If `nullable: true` is set, we drop `type`, `nullable`, `items` and `properties` because OpenAPI v2 is not able to express nullable. To avoid kubectl to reject good objects, this is necessary.
### Additional printer columns
The kubectl tool relies on server-side output formatting. Your cluster's API server decides which
columns are shown by the `kubectl get` command. You can customize these columns for a
CustomResourceDefinition. The following example adds the `Spec`, `Replicas`, and `Age`
columns.
Save the CustomResourceDefinition to `resourcedefinition.yaml`:
```yaml
apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
name: crontabs.stable.example.com
spec:
group: stable.example.com
scope: Namespaced
names:
plural: crontabs
singular: crontab
kind: CronTab
shortNames:
- ct
versions:
- name: v1
served: true
storage: true
schema:
openAPIV3Schema:
type: object
properties:
spec:
type: object
properties:
cronSpec:
type: string
image:
type: string
replicas:
type: integer
additionalPrinterColumns:
- name: Spec
type: string
description: The cron spec defining the interval a CronJob is run
jsonPath: .spec.cronSpec
- name: Replicas
type: integer
description: The number of jobs launched by the CronJob
jsonPath: .spec.replicas
- name: Age
type: date
jsonPath: .metadata.creationTimestamp
```
Create the CustomResourceDefinition:
```shell
kubectl apply -f resourcedefinition.yaml
```
Create an instance using the `my-crontab.yaml` from the previous section.
Invoke the server-side printing:
```shell
kubectl get crontab my-new-cron-object
```
Notice the `NAME`, `SPEC`, `REPLICAS`, and `AGE` columns in the output:
```
NAME SPEC REPLICAS AGE
my-new-cron-object * * * * * 1 7s
```
{{< note >}}
The `NAME` column is implicit and does not need to be defined in the CustomResourceDefinition.
{{< /note >}}
#### Priority
Each column includes a `priority` field. Currently, the priority
differentiates between columns shown in standard view or wide view (using the `-o wide` flag).
- Columns with priority `0` are shown in standard view.
- Columns with priority greater than `0` are shown only in wide view.
#### Type
A column's `type` field can be any of the following (compare [OpenAPI v3 data types](https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.0.md#dataTypes)):
- `integer` – non-floating-point numbers
- `number` – floating point numbers
- `string` – strings
- `boolean` – `true` or `false`
- `date` – rendered differentially as time since this timestamp.
If the value inside a CustomResource does not match the type specified for the column,
the value is omitted. Use CustomResource validation to ensure that the value
types are correct.
#### Format
A column's `format` field can be any of the following:
- `int32`
- `int64`
- `float`
- `double`
- `byte`
- `date`
- `date-time`
- `password`
The column's `format` controls the style used when `kubectl` prints the value.
### Subresources
Custom resources support `/status` and `/scale` subresources.
The status and scale subresources can be optionally enabled by
defining them in the CustomResourceDefinition.
#### Status subresource
When the status subresource is enabled, the `/status` subresource for the custom resource is exposed.
- The status and the spec stanzas are represented by the `.status` and `.spec` JSONPaths respectively inside of a custom resource.
- `PUT` requests to the `/status` subresource take a custom resource object and ignore changes to anything except the status stanza.
- `PUT` requests to the `/status` subresource only validate the status stanza of the custom resource.
- `PUT`/`POST`/`PATCH` requests to the custom resource ignore changes to the status stanza.
- The `.metadata.generation` value is incremented for all changes, except for changes to `.metadata` or `.status`.
- Only the following constructs are allowed at the root of the CRD OpenAPI validation schema:
- `description`
- `example`
- `exclusiveMaximum`
- `exclusiveMinimum`
- `externalDocs`
- `format`
- `items`
- `maximum`
- `maxItems`
- `maxLength`
- `minimum`
- `minItems`
- `minLength`
- `multipleOf`
- `pattern`
- `properties`
- `required`
- `title`
- `type`
- `uniqueItems`
#### Scale subresource
When the scale subresource is enabled, the `/scale` subresource for the custom resource is exposed.
The `autoscaling/v1.Scale` object is sent as the payload for `/scale`.
To enable the scale subresource, the following fields are defined in the CustomResourceDefinition.
- `specReplicasPath` defines the JSONPath inside of a custom resource that corresponds to `scale.spec.replicas`.
- It is a required value.
- Only JSONPaths under `.spec` and with the dot notation are allowed.
- If there is no value under the `specReplicasPath` in the custom resource,
the `/scale` subresource will return an error on GET.
- `statusReplicasPath` defines the JSONPath inside of a custom resource that corresponds to `scale.status.replicas`.
- It is a required value.
- Only JSONPaths under `.status` and with the dot notation are allowed.
- If there is no value under the `statusReplicasPath` in the custom resource,
the status replica value in the `/scale` subresource will default to 0.
- `labelSelectorPath` defines the JSONPath inside of a custom resource that corresponds to `Scale.Status.Selector`.
- It is an optional value.
- It must be set to work with HPA.
- Only JSONPaths under `.status` or `.spec` and with the dot notation are allowed.
- If there is no value under the `labelSelectorPath` in the custom resource,
the status selector value in the `/scale` subresource will default to the empty string.
- The field pointed by this JSON path must be a string field (not a complex selector struct) which contains a serialized label selector in string form.
In the following example, both status and scale subresources are enabled.
Save the CustomResourceDefinition to `resourcedefinition.yaml`:
```yaml
apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
name: crontabs.stable.example.com
spec:
group: stable.example.com
versions:
- name: v1
served: true
storage: true
schema:
openAPIV3Schema:
type: object
properties:
spec:
type: object
properties:
cronSpec:
type: string
image:
type: string
replicas:
type: integer
status:
type: object
properties:
replicas:
type: integer
labelSelector:
type: string
# 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
scope: Namespaced
names:
plural: crontabs
singular: crontab
kind: CronTab
shortNames:
- ct
```
And create it:
```shell
kubectl apply -f resourcedefinition.yaml
```
After the CustomResourceDefinition object has been created, you can create custom objects.
If you save the following YAML to `my-crontab.yaml`:
```yaml
apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
name: my-new-cron-object
spec:
cronSpec: "* * * * */5"
image: my-awesome-cron-image
replicas: 3
```
and create it:
```shell
kubectl apply -f my-crontab.yaml
```
Then new namespaced RESTful API endpoints are created at:
```
/apis/stable.example.com/v1/namespaces/*/crontabs/status
```
and
```
/apis/stable.example.com/v1/namespaces/*/crontabs/scale
```
A custom resource can be scaled using the `kubectl scale` command.
For example, the following command sets `.spec.replicas` of the
custom resource created above to 5:
```shell
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
```
You can use a [PodDisruptionBudget](/docs/tasks/run-application/configure-pdb/) to protect custom
resources that have the scale subresource enabled.
### Categories
Categories is a list of grouped resources the custom resource belongs to (eg. `all`).
You can use `kubectl get <category-name>` to list the resources belonging to the category.
The following example adds `all` in the list of categories in the CustomResourceDefinition
and illustrates how to output the custom resource using `kubectl get all`.
Save the following CustomResourceDefinition to `resourcedefinition.yaml`:
```yaml
apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
name: crontabs.stable.example.com
spec:
group: stable.example.com
versions:
- name: v1
served: true
storage: true
schema:
openAPIV3Schema:
type: object
properties:
spec:
type: object
properties:
cronSpec:
type: string
image:
type: string
replicas:
type: integer
scope: Namespaced
names:
plural: crontabs
singular: crontab
kind: CronTab
shortNames:
- ct
# categories is a list of grouped resources the custom resource belongs to.
categories:
- all
```
and create it:
```shell
kubectl apply -f resourcedefinition.yaml
```
After the CustomResourceDefinition object has been created, you can create custom objects.
Save the following YAML to `my-crontab.yaml`:
```yaml
apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
name: my-new-cron-object
spec:
cronSpec: "* * * * */5"
image: my-awesome-cron-image
```
and create it:
```shell
kubectl apply -f my-crontab.yaml
```
You can specify the category when using `kubectl get`:
```
kubectl get all
```
and it will include the custom resources of kind `CronTab`:
```console
NAME AGE
crontabs/my-new-cron-object 3s
```
## {{% heading "whatsnext" %}}
* Read about [custom resources](/docs/concepts/extend-kubernetes/api-extension/custom-resources/).
* See [CustomResourceDefinition](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#customresourcedefinition-v1-apiextensions-k8s-io).
* Serve [multiple versions](/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definition-versioning/) of a
CustomResourceDefinition.
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