Extend the Kubernetes API with CustomResourceDefinitions
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Extend the Kubernetes API with CustomResourceDefinitions

This page shows how to install a custom resource into the Kubernetes API by creating a CustomResourceDefinition.

Before you begin

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. It is recommended to run this tutorial on a cluster with at least two nodes that are not acting as control plane hosts. If you do not already have a cluster, you can create one by using minikube or you can use one of these Kubernetes playgrounds:

Your Kubernetes server must be at or later than version 1.16. To check the version, enter kubectl version. 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.

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 custom resource created from a CRD object can be either namespaced or cluster-scoped, as specified in the CRD’s spec.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:

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:

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:

apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
  name: my-new-cron-object
spec:
  cronSpec: "* * * * */5"
  image: my-awesome-cron-image

and create it:

kubectl apply -f my-crontab.yaml

You can then manage your CronTab objects using kubectl. For example:

kubectl get crontab

Should print a list like this:

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:

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:

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.

kubectl delete -f resourcedefinition.yaml
kubectl get crontabs

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 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 which:

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
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).
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).
if metadata is specified, then only restrictions on metadata.name and metadata.generateName are allowed.

Non-structural example 1:

allOf:
- properties:
    foo:
      ...

conflicts with rule 2. The following would be correct:

properties:
  foo:
    ...
allOf:
- properties:
    foo:
      ...

Non-structural example 2:

allOf:
- items:
    properties:
      foo:
        ...

conflicts with rule 2. The following would be correct:

items:
  properties:
    foo:
      ...
allOf:
- items:
    properties:
      foo:
        ...

Non-structural example 3:

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:

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, etcd. As with native Kubernetes resources such as ConfigMap, if you specify a field that the API server does not recognize, the unknown field is pruned (removed) before being persisted.

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:

Use a structural OpenAPI schema.
Set spec.preserveUnknownFields to false.

If you save the following YAML to my-crontab.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:

kubectl create --validate=false -f my-crontab.yaml -o yaml

Your output is similar to:

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 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.

For example:

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:

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):

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:

json:
  spec:
    foo: abc
    bar: def
    something: x
  status:
    something: x

is pruned to:

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:

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):

x-kubernetes-int-or-string: true
anyOf:
  - type: integer
  - type: string
...

and

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, x-kubernetes-int-or-string: true is unfolded to one of the two patterns shown above.

RawExtension

RawExtensions (as in runtime.RawExtension) 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:

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.:

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 for more information about serving multiple versions of your CustomResourceDefinition and migrating your objects from one version to another.

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:

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, by x-kubernetes-validations when the Validation Rules feature is enabled, and you can add additional validation using admission webhooks.

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) expressions when the Validation rules feature is enabled and the CustomResourceDefinition schema is a structural schema.

Refer to the structural schemas 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:

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:

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:

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:

kubectl apply -f my-crontab.yaml

then you get an 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", "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:

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:

kubectl apply -f my-crontab.yaml
crontab "my-new-cron-object" created

Validation rules

FEATURE STATE: Kubernetes v1.25 [beta]

Validation rules are in beta since 1.25 and the CustomResourceValidationExpressions feature gate is enabled by default to validate custom resource based on validation rules. You can disable this feature by explicitly setting the CustomResourceValidationExpressions feature gate to false, for the kube-apiserver component. This feature is only available if the schema is a structural schema.

Validation rules use the Common Expression Language (CEL) 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:

  ...
  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:

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.

For example, a 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.

For example, a rule like self.nonExistingField > 0 against a non-existing field will return the following 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.

For example, a 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
`!(‘MY_KEY’ in self.map1) \|\| self[‘MY_KEY’].matches(‘^[a-zA-Z]$’)`	Validate the value of a map for a specific key, if it is in the map
`self.envars.filter(e, e.name == ‘MY_ENV’).all(e, e.value.matches(‘^[a-zA-Z]$’)`	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 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
`size(self.clusters.filter(c, c.name == self.primary)) == 1`	Validate that the ‘primary’ property has one and only one occurrence in the ‘clusters’ listMap

Xref: Supported evaluation on CEL

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:

  ...
  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.

  ...
  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(...).

  ...
  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.

  ...
  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.

  ...
  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

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.xdashprop > 0`
redactd	`self.redactunderscores__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, OpenAPI types, Kubernetes Structural Schemas.

The messageExpression field

Similar to the message field, which defines the string reported for a validation rule failure, messageExpression allows you to use a CEL expression to construct the message string. This allows you to insert more descriptive information into the validation failure message. messageExpression must evaluate a string and may use the same variables that are available to the rule field. For example:

x-kubernetes-validations:
- rule: "self.x <= self.maxLimit"
  messageExpression: '"x exceeded max limit of " + string(self.maxLimit)'

Keep in mind that CEL string concatenation (+ operator) does not auto-cast to string. If you have a non-string scalar, use the string(<value>) function to cast the scalar to a string like shown in the above example.

messageExpression must evaluate to a string, and this is checked while the CRD is being written. Note that it is possible to set message and messageExpression on the same rule, and if both are present, messageExpression will be used. However, if messageExpression evaluates to an error, the string defined in message will be used instead, and the messageExpression error will be logged. This fallback will also occur if the CEL expression defined in messageExpression generates an empty string, or a string containing line breaks.

If one of the above conditions are met and no message has been set, then the default validation failure message will be used instead.

messageExpression is a CEL expression, so the restrictions listed in Resource use by validation functions apply. If evaluation halts due to resource constraints during messageExpression execution, then no further validation rules will be executed.

Validation functions

Functions available include:

CEL standard functions, defined in the list of standard definitions
CEL standard macros
CEL extended string function library
Kubernetes CEL extension library

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:

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 setor atomicarray parent schemas make it impossible to unambiguously correlate a self with oldSelf.

Here are some examples for transition rules:

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`

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:

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:

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):

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:

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:

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.

Defaulting allows to specify default values in the OpenAPI v3 validation schema:

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:

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

leads to

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

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:

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

spec:
  foo: null
  bar: null
  baz: null

leads to

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

CustomResourceDefinition OpenAPI v3 validation schemas which are structural and enable pruning are published as OpenAPI v3 and OpenAPI v2 from Kubernetes API server. It is recommended to use the OpenAPI v3 document as it is a lossless representation of the CustomResourceDefinition OpenAPI v3 validation schema while OpenAPI v2 represents a lossy conversion.

The 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.

Compatibility with OpenAPI V2

For compatibility with OpenAPI V2, the OpenAPI v3 validation schema performs a lossy conversion to the OpenAPI v2 schema. The schema show up in definitions and paths fields in the OpenAPI v2 spec.

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 in the API server.

The following fields are removed as they aren’t supported by OpenAPI v2.
- The fields allOf, anyOf, oneOf and not are removed
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:

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:

kubectl apply -f resourcedefinition.yaml

Create an instance using the my-crontab.yaml from the previous section.

Invoke the server-side printing:

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.

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):

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:

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:

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:

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:

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:

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 to protect custom resources that have the scale subresource enabled.

What’s next

Read about custom resources.
See CustomResourceDefinition.
Serve multiple versions of a CustomResourceDefinition.

Extend the Kubernetes API with CustomResourceDefinitions

Extend the Kubernetes API with CustomResourceDefinitions

Before you begin

Create a CustomResourceDefinition

Create custom objects

Delete a CustomResourceDefinition

Specifying a structural schema

Field pruning

Controlling pruning

IntOrString

RawExtension

Serving multiple versions of a CRD

Advanced topics

Finalizers

Validation

Validation rules

The messageExpression field

Validation functions

Transition rules

Resource use by validation functions

Defaulting

Defaulting and Nullable

Publish Validation Schema in OpenAPI

Compatibility with OpenAPI V2

Additional printer columns

Priority

Type

Format

Subresources

Status subresource

Scale subresource

Categories

What’s next