Extend the Kubernetes API with CustomResourceDefinitions (2024)

This page shows how to install acustom resourceinto the Kubernetes API by creating aCustomResourceDefinition.

Before you begin

You need to have a Kubernetes cluster, and the kubectl command-line tool mustbe 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 acluster, you can create one by usingminikubeor you can use one of these Kubernetes playgrounds:

• Killercoda
• Play with Kubernetes

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 tothe 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 Servercreates a new RESTful resource path for each version you specify. The customresource 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-inobjects, deleting a namespace deletes all custom objects in that namespace.CustomResourceDefinitions themselves are non-namespaced and are available toall namespaces.

For example, if you save the following CustomResourceDefinition to resourcedefinition.yaml:

apiVersion: apiextensions.k8s.io/v1kind: CustomResourceDefinitionmetadata: # name must match the spec fields below, and be in the form: <plural>.<group> name: crontabs.stable.example.comspec: # 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 theCustomResourceDefinition object you created above.

It might take a few seconds for the endpoint to be created.You can watch the Established condition of your CustomResourceDefinitionto be true or watch the discovery information of the API server for yourresource to show up.

Create custom objects

After the CustomResourceDefinition object has been created, you can createcustom objects. Custom objects can contain custom fields. These fields cancontain arbitrary JSON.In the following example, the cronSpec and image custom fields are set in acustom object of kind CronTab. The kind CronTab comes from the spec of theCustomResourceDefinition object you created above.

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

apiVersion: "stable.example.com/v1"kind: CronTabmetadata: name: my-new-cron-objectspec: 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 AGEmy-new-cron-object 6s

Resource names are not case-sensitive when using kubectl, and you can use eitherthe 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 fieldsfrom the YAML you used to create it:

apiVersion: v1items:- 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-imagekind: Listmetadata: resourceVersion: "" selfLink: ""

Delete a CustomResourceDefinition

When you delete a CustomResourceDefinition, the server will uninstall the RESTful API endpointand delete all custom objects stored in it.

kubectl delete -f resourcedefinition.yamlkubectl get crontabs

Error from server (NotFound): Unable to list {"stable.example.com" "v1" "crontabs"}: the server could notfind 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-infields apiVersion, kind and metadata, which the API server validatesimplicitly). With OpenAPI v3.0 validation a schema can bespecified, which is validated during creation and updates, compare below fordetails and limits of such a schema.

With apiextensions.k8s.io/v1 the definition of a structural schema ismandatory for CustomResourceDefinitions. In the beta version ofCustomResourceDefinition, the structural schema was optional.

A structural schema is an OpenAPI v3.0 validation schema 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:

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: objectdescription: "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 theCustomResourceDefinition.

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 lackstructural schemas, and spec.preserveUnknownFields might be true.

For legacy CustomResourceDefinition objects created asapiextensions.k8s.io/v1beta1 with spec.preserveUnknownFields set totrue, the following is also true:

• Pruning is not enabled.
• You can store arbitrary data.

For compatibility with apiextensions.k8s.io/v1, update your customresource definitions to:

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

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

apiVersion: "stable.example.com/v1"kind: CronTabmetadata: name: my-new-cron-objectspec: 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/v1kind: CronTabmetadata: creationTimestamp: 2017-05-31T12:56:35Z generation: 1 name: my-new-cron-object namespace: default resourceVersion: "285" uid: 9423255b-4600-11e7-af6a-28d2447dc82bspec: 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 addingthe --validate=false command line option.Because the OpenAPI validation schemas are also publishedto clients, kubectl also checks for unknown fields and rejects those objects well before theywould 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 toopt-out of that for specific sub-trees of fields by adding x-kubernetes-preserve-unknown-fields: true in thestructural OpenAPI v3 validation schema.

For example:

type: objectproperties: 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: objectproperties: 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: objectproperties: 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 thefollowing is structural:

type: objectproperties: 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: trueanyOf: - type: integer - type: string...

and

x-kubernetes-int-or-string: trueallOf: - 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: objectproperties: 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 versioningfor more information about serving multiple versions of yourCustomResourceDefinition 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: CronTabmetadata: finalizers: - stable.example.com/finalizer

Identifiers of custom finalizers consist of a domain name, a forward slash and the name ofthe 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 themetadata.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 alsoimpossible to force the deletion of an object.

When the metadata.deletionTimestamp field is set, controllers watching the object execute anyfinalizers they handle and remove the finalizer from the list after they are done. It is theresponsibility 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 isdeleted by Kubernetes.

Validation

Custom resources are validated viaOpenAPI v3 schemas,by x-kubernetes-validations when the Validation Rules feature is enabled, and youcan add additional validation usingadmission 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 usingCommon Expression Language (CEL) expressions when theValidation rules feature is enabled and the CustomResourceDefinition schema is astructural schema.

Refer to the structural schemas section for otherrestrictions and CustomResourceDefinition features.

The schema is defined in the CustomResourceDefinition. In the following example, theCustomResourceDefinition 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/v1kind: CustomResourceDefinitionmetadata: name: crontabs.stable.example.comspec: 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: CronTabmetadata: name: my-new-cron-objectspec: 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: CronTabmetadata: name: my-new-cron-objectspec: cronSpec: "* * * * */5" image: my-awesome-cron-image replicas: 5

And create it:

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

Validation ratcheting

FEATURE STATE: Kubernetes v1.30 [beta]

If you are using a version of Kubernetes older than v1.30, you need to explicitlyenable the CRDValidationRatchetingfeature gate touse this behavior, which then applies to all CustomResourceDefinitions in yourcluster.

Provided you enabled the feature gate, Kubernetes implements validation rachetingfor CustomResourceDefinitions. The API server is willing to accept updates to resources thatare not valid after the update, provided that each part of the resource that failed to validatewas not changed by the update operation. In other words, any invalid part of the resourcethat remains invalid must have already been wrong.You cannot use this mechanism to update a valid resource so that it becomes invalid.

This feature allows authors of CRDs to confidently add new validations to theOpenAPIV3 schema under certain conditions. Users can update to the new schemasafely without bumping the version of the object or breaking workflows.

While most validations placed in the OpenAPIV3 schema of a CRD supportratcheting, there are a few exceptions. The following OpenAPIV3 schemavalidations are not supported by ratcheting under the implementation in Kubernetes1.30 and if violated will continue to throw an error as normally:

• Quantors

  • allOf
  • oneOf
  • anyOf
  • not
  • any validations in a descendent of one of these fields

• x-kubernetes-validationsFor Kubernetes 1.28, CRD validation rules](#validation-rules) are ignored byratcheting. Starting with Alpha 2 in Kubernetes 1.29, x-kubernetes-validationsare ratcheted only if they do not refer to oldSelf.

  Transition Rules are never ratcheted: only errors raised by rules that do notuse oldSelf will be automatically ratcheted if their values are unchanged.

  To write custom ratcheting logic for CEL expressions, check out optionalOldSelf.

• x-kubernetes-list-typeErrors arising from changing the list type of a subschema will not beratcheted. For example adding set onto a list with duplicates will alwaysresult in an error.

• x-kubernetes-map-keysErrors arising from changing the map keys of a list schema will not beratcheted.

• requiredErrors arising from changing the list of required fields will not be ratcheted.

• propertiesAdding/removing/modifying the names of properties is not ratcheted, butchanges to validations in each properties' schemas and subschemas may be ratchetedif the name of the property stays the same.

• additionalPropertiesTo remove a previously specified additionalProperties validation will not beratcheted.

• metadataErrors that come from Kubernetes' built-in validation of an object's metadataare not ratcheted (such as object name, or characters in a label value).If you specify your own additional rules for the metadata of a custom resource,that additional validation will be ratcheted.

Validation rules

FEATURE STATE: Kubernetes v1.29 [stable]

Validation rules use the Common Expression Language (CEL)to validate custom resource values. Validation rules are included inCustomResourceDefinition 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 validationrules 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: CronTabmetadata: name: my-new-cron-objectspec: 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, theabove 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 returnthe 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:

Xref: Supported evaluation on CEL

• If the Rule is scoped to the root of a resource, it may make field selection into any fieldsdeclared in the OpenAPIv3 schema of the CRD as well as apiVersion, kind, metadata.name andmetadata.generateName. This includes selection of fields in both the spec and status in thesame 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 selectablevia self.field and field presence can be checked via has(self.field). Null valued fields are treated asabsent 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 mapare accessible via self[mapKey], map containment can be checked via mapKey in self and allentries 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] andalso 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:

The apiVersion, kind, metadata.name and metadata.generateName are always accessible fromthe root of the object and from any x-kubernetes-embedded-resource annotated objects. No othermetadata properties are accessible.

Unknown data preserved in custom resources via x-kubernetes-preserve-unknown-fields is notaccessible 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" isrecursively 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:

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:

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 ofthe list type:

• set: X + Y performs a union where the array positions of all elements in X are preservedand 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 butthe values are overwritten by values in Y when the key sets of X and Y intersect. Elementsin Y with non-intersecting keys are appended, retaining their partial order.

Here is the declarations type mapping between OpenAPIv3 and CEL type:

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 rulefield. 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. Ifyou have a non-string scalar, use the string(<value>) function to cast the scalar to a stringlike 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 possibleto set message and messageExpression on the same rule, and if both are present, messageExpressionwill be used. However, if messageExpression evaluates to an error, the string defined in messagewill be used instead, and the messageExpression error will be logged. This fallback will also occur ifthe CEL expression defined in messageExpression generates an empty string, or a string containing linebreaks.

If one of the above conditions are met and no message has been set, then the default validation failuremessage 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 constraintsduring messageExpression execution, then no further validation rules will be executed.

Setting messageExpression is optional.

The message field

If you want to set a static message, you can supply message rather than messageExpression.The value of message is used as an opaque error string if validation fails.

Setting message is optional.

The reason field

You can add a machine-readable validation failure reason within a validation, to be returnedwhenever a request fails this validation rule.

For example:

x-kubernetes-validations:- rule: "self.x <= self.maxLimit" reason: "FieldValueInvalid"

The HTTP status code returned to the caller will match the reason of the first failed validation rule.The currently supported reasons are: "FieldValueInvalid", "FieldValueForbidden", "FieldValueRequired", "FieldValueDuplicate".If not set or unknown reasons, default to use "FieldValueInvalid".

Setting reason is optional.

The fieldPath field

You can specify the field path returned when the validation fails.

For example:

x-kubernetes-validations:- rule: "self.foo.test.x <= self.maxLimit" fieldPath: ".foo.test.x"

In the example above, the validation checks the value of field x should be less than the value of maxLimit.If no fieldPath specified, when validation fails, the fieldPath would be default to wherever self scoped.With fieldPath specified, the returned error will have fieldPath properly refer to the location of field x.

The fieldPath value must be a relative JSON path that is scoped to the location of this x-kubernetes-validations extension in the schema.Additionally, it should refer to an existing field within the schema.For example when validation checks if a specific attribute foo under a map testMap, you could setfieldPath to ".testMap.foo" or .testMap['foo']'.If the validation requires checking for unique attributes in two lists, the fieldPath can be set to either of the lists.For example, it can be set to .testList1 or .testList2.It supports child operation to refer to an existing field currently.Refer to JSONPath support in Kubernetes for more info.The fieldPath field does not support indexing arrays numerically.

Setting fieldPath is optional.

The optionalOldSelf field

FEATURE STATE: Kubernetes v1.30 [beta]

If your cluster does not have CRD validation ratcheting enabled,the CustomResourceDefinition API doesn't include this field, and trying to set it may resultin an error.

The optionalOldSelf field is a boolean field that alters the behavior of Transition Rules describedbelow. Normally, a transition rule will not evaluate if oldSelf cannot be determined:during object creation or when a new value is introduced in an update.

If optionalOldSelf is set to true, then transition rules will always beevaluated and the type of oldSelf be changed to a CEL Optional type.

optionalOldSelf is useful in cases where schema authors would like a morecontrol tool than provided by the default equality based behavior ofto introduce newer, usually stricter constraints on new values, while stillallowing old values to be "grandfathered" or ratcheted using the older validation.

Example Usage:

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 atransition rule. Transition rules allow schema authors to prevent certain transitions between twootherwise valid states. For example:

type: stringenum: ["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 orremoved by placing a transition rule on the parent node. Transition rules are never applied tocustom resource creation. When placed on an optional field, a transition rule will not apply toupdate 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 iscomparable between the old object and the new object. For example, list items and theirdescendants (spec.foo[10].bar) can't necessarily be correlated between an existing object and alater update to the same object.

Errors will be generated on CRD writes if a schema node contains a transition rule that can never beapplied, e.g. "path: update rule rule cannot be set on schema because the schema or its parentschema 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:

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 isestimated to be prohibitively expensive to execute, the API server rejects the createor update operation, and returns an error message.A similar system is used at runtime that observes the actions the interpreter takes. If the interpreter executestoo 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 ofits 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 onlyspecify 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 anyrisk 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 takeslonger 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 notgiven, 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, andmaxLength for anything that will be processed in a validation rule in order to prevent validationerrors 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, andmaxLength 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, thisis a very expensive rule.

If you do not specify any validation limit, the estimated cost of this rule will exceed theper-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 theestimated cost of the rule itself. For instance, the following rule will have the same estimated cost as theprevious 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 expensivethan a non-nested list with the same rule. A rule that would have been allowed on a non-nested list might needlower 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

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

apiVersion: apiextensions.k8s.io/v1kind: CustomResourceDefinitionmetadata: name: crontabs.stable.example.comspec: 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: CronTabmetadata: name: my-new-cron-objectspec: image: my-awesome-cron-image

leads to

apiVersion: "stable.example.com/v1"kind: CronTabmetadata: name: my-new-cron-objectspec: 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 mustvalidate against a provided schema.

Default values for metadata fields of x-kubernetes-embedded-resources: true nodes (or parts ofa default value covering metadata) are not pruned during CustomResourceDefinition creation, butthrough 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 afalse value, will be pruned before defaulting happens. If a default is present, it will beapplied. When nullable is true, null values will be conserved and won't be defaulted.

For example, given the OpenAPI schema below:

type: objectproperties: 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 nullvalue due to nullable: true, and baz pruned because the field is non-nullable and has nodefault.

Publish Validation Schema in OpenAPI

CustomResourceDefinition OpenAPI v3 validation schemas which arestructural and enable pruning are publishedas OpenAPI v3 andOpenAPI v2 from Kubernetes API server. It is recommended to use the OpenAPI v3 documentas it is a lossless representation of the CustomResourceDefinition OpenAPI v3 validation schemawhile OpenAPI v2 represents a lossy conversion.

The kubectl command-line tool consumes the published schema to performclient-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 conversionto the OpenAPI v2 schema. The schema show up in definitions and paths fields in theOpenAPI v2 spec.

The following modifications are applied during the conversion to keep backwards compatibility withkubectl in previous 1.13 version. These modifications prevent kubectl from being over-strict and rejectingvalid 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.

1. The following fields are removed as they aren't supported by OpenAPI v2.

   • 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 isnot 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 whichcolumns are shown by the kubectl get command. You can customize these columns for aCustomResourceDefinition. The following example adds the Spec, Replicas, and Agecolumns.

Save the CustomResourceDefinition to resourcedefinition.yaml:

apiVersion: apiextensions.k8s.io/v1kind: CustomResourceDefinitionmetadata: name: crontabs.stable.example.comspec: 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 AGEmy-new-cron-object * * * * * 1 7s

Field selectors

Field Selectorslet clients select custom resources based on the value of one or more resourcefields.

All custom resources support the metadata.name and metadata.namespace fieldselectors.

Fields declared in a CustomResourceDefinitionmay also be used with field selectors when included in the spec.versions[*].selectableFields field of theCustomResourceDefinition.

Selectable fields for custom resources

FEATURE STATE: Kubernetes v1.30 [alpha]

FEATURE STATE: Kubernetes v1.30 [alpha]

You need to enable the CustomResourceFieldSelectorsfeature gate touse this behavior, which then applies to all CustomResourceDefinitions in yourcluster.

The spec.versions[*].selectableFields field of a CustomResourceDefinition may be used todeclare which other fields in a custom resource may be used in field selectors.The following example adds the .spec.color and .spec.size fields asselectable fields.

Save the CustomResourceDefinition to shirt-resource-definition.yaml:

apiVersion: apiextensions.k8s.io/v1kind: CustomResourceDefinitionmetadata: name: shirts.stable.example.comspec: group: stable.example.com scope: Namespaced names: plural: shirts singular: shirt kind: Shirt versions: - name: v1 served: true storage: true schema: openAPIV3Schema: type: object properties: spec: type: object properties: color: type: string size: type: string selectableFields: - jsonPath: .spec.color - jsonPath: .spec.size additionalPrinterColumns: - jsonPath: .spec.color name: Color type: string - jsonPath: .spec.size name: Size type: string

Create the CustomResourceDefinition:

kubectl apply -f https://k8s.io/examples/customresourcedefinition/shirt-resource-definition.yaml

Define some Shirts by editing shirt-resources.yaml; for example:

---apiVersion: stable.example.com/v1kind: Shirtmetadata: name: example1spec: color: blue size: S---apiVersion: stable.example.com/v1kind: Shirtmetadata: name: example2spec: color: blue size: M---apiVersion: stable.example.com/v1kind: Shirtmetadata: name: example3spec: color: green size: M

Create the custom resources:

kubectl apply -f https://k8s.io/examples/customresourcedefinition/shirt-resources.yaml

Get all the resources:

kubectl get shirts.stable.example.com

The output is:

NAME COLOR SIZEexample1 blue Sexample2 blue Mexample3 green M

Fetch blue shirts (retrieve Shirts with a color of blue):

kubectl get shirts.stable.example.com --field-selector spec.color=blue

Should output:

NAME COLOR SIZEexample1 blue Sexample2 blue M

Get only resources with a color of green and a size of M:

kubectl get shirts.stable.example.com --field-selector spec.color=green,spec.size=M

Should output:

NAME COLOR SIZEexample2 blue M

Priority

Each column includes a priority field. Currently, the prioritydifferentiates 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 (compareOpenAPI 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 valuetypes 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 bydefining 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 JSONPathsrespectively inside of a custom resource.

• PUT requests to the /status subresource take a custom resource object and ignore changes toanything except the status stanza.

• PUT requests to the /status subresource only validate the status stanza of the customresource.

• 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 toScale.Status.Selector.

  • It is an optional value.
  • It must be set to work with HPA and VPA.
  • 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/v1kind: CustomResourceDefinitionmetadata: name: crontabs.stable.example.comspec: 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: CronTabmetadata: name: my-new-cron-objectspec: 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 thecustom resource created above to 5:

kubectl scale --replicas=5 crontabs/my-new-cron-objectcrontabs "my-new-cron-object" scaledkubectl get crontabs my-new-cron-object -o jsonpath='{.spec.replicas}'5

You can use a PodDisruptionBudget to protect customresources 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 CustomResourceDefinitionand illustrates how to output the custom resource using kubectl get all.

Save the following CustomResourceDefinition to resourcedefinition.yaml:

apiVersion: apiextensions.k8s.io/v1kind: CustomResourceDefinitionmetadata: name: crontabs.stable.example.comspec: 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:

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:

apiVersion: "stable.example.com/v1"kind: CronTabmetadata: name: my-new-cron-objectspec: cronSpec: "* * * * */5" image: my-awesome-cron-image

and create it:

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:

NAME AGEcrontabs/my-new-cron-object 3s

What's next

• Read about custom resources.

• See CustomResourceDefinition.

• Serve multiple versions of aCustomResourceDefinition.