Use Custom Resources

• 1: Extend the Kubernetes API with CustomResourceDefinitions
• 2: Versions in CustomResourceDefinitions

1 - 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:

• Killercoda
• Play with Kubernetes

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:

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: 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:

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: 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 ratcheting

You need to enable the CRDValidationRatcheting feature gate to use this behavior, which then applies to all CustomResourceDefinitions in your cluster.

Provided you enabled the feature gate, Kubernetes implements validation racheting for CustomResourceDefinitions. The API server is willing to accept updates to resources that are not valid after the update, provided that each part of the resource that failed to validate was not changed by the update operation. In other words, any invalid part of the resource that 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 the OpenAPIV3 schema under certain conditions. Users can update to the new schema safely without bumping the version of the object or breaking workflows.

While most validations placed in the OpenAPIV3 schema of a CRD support ratcheting, there are a few exceptions. The following OpenAPIV3 schema validations are not supported by ratcheting under the implementation in Kubernetes 1.28 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-validations For Kubernetes 1.28, CRD validation rules](#validation-rules) are ignored by ratcheting. This may change in later Kubernetes releases.
• x-kubernetes-list-type Errors arising from changing the list type of a subschema will not be ratcheted. For example adding set onto a list with duplicates will always result in an error.
• x-kubernetes-map-keys Errors arising from changing the map keys of a list schema will not be ratcheted.
• required Errors arising from changing the list of required fields will not be ratcheted.
• properties Adding/removing/modifying the names of properties is not ratcheted, but changes to validations in each properties' schemas and subschemas may be ratcheted if the name of the property stays the same.
• additionalProperties To remove a previously specified additionalProperties validation will not be ratcheted.

Validation rules

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:

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:

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:

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 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:

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.

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 returned whenever 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 set fieldPath 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.

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:

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

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.

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 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

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 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/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.

Categories

Categories is a list of grouped resources the custom resource belongs to (eg. all). You can use kubectl get <category-name> to list the resources belonging to the category.

The following example adds all in the list of categories in the CustomResourceDefinition and illustrates how to output the custom resource using kubectl get all.

Save the following CustomResourceDefinition to resourcedefinition.yaml:

apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: crontabs.stable.example.com
spec:
  group: stable.example.com
  versions:
    - name: v1
      served: true
      storage: true
      schema:
        openAPIV3Schema:
          type: object
          properties:
            spec:
              type: object
              properties:
                cronSpec:
                  type: string
                image:
                  type: string
                replicas:
                  type: integer
  scope: Namespaced
  names:
    plural: crontabs
    singular: crontab
    kind: CronTab
    shortNames:
    - ct
    # categories is a list of grouped resources the custom resource belongs to.
    categories:
    - all

and create it:

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: 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 specify the category when using kubectl get:

kubectl get all

and it will include the custom resources of kind CronTab:

NAME                          AGE
crontabs/my-new-cron-object   3s

What's next

• Read about custom resources.

• See CustomResourceDefinition.

• Serve multiple versions of a CustomResourceDefinition.

2 - Versions in CustomResourceDefinitions

This page explains how to add versioning information to CustomResourceDefinitions, to indicate the stability level of your CustomResourceDefinitions or advance your API to a new version with conversion between API representations. It also describes how to upgrade an object from one version to another.

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:

• Killercoda
• Play with Kubernetes

You should have an initial understanding of custom resources.

Overview

The CustomResourceDefinition API provides a workflow for introducing and upgrading to new versions of a CustomResourceDefinition.

When a CustomResourceDefinition is created, the first version is set in the CustomResourceDefinition spec.versions list to an appropriate stability level and a version number. For example v1beta1 would indicate that the first version is not yet stable. All custom resource objects will initially be stored at this version.

Once the CustomResourceDefinition is created, clients may begin using the v1beta1 API.

Later it might be necessary to add new version such as v1.

Adding a new version:

1. Pick a conversion strategy. Since custom resource objects need the ability to be served at both versions, that means they will sometimes be served in a different version than the one stored. To make this possible, the custom resource objects must sometimes be converted between the version they are stored at and the version they are served at. If the conversion involves schema changes and requires custom logic, a conversion webhook should be used. If there are no schema changes, the default None conversion strategy may be used and only the apiVersion field will be modified when serving different versions.
2. If using conversion webhooks, create and deploy the conversion webhook. See the Webhook conversion for more details.
3. Update the CustomResourceDefinition to include the new version in the spec.versions list with served:true. Also, set spec.conversion field to the selected conversion strategy. If using a conversion webhook, configure spec.conversion.webhookClientConfig field to call the webhook.

Once the new version is added, clients may incrementally migrate to the new version. It is perfectly safe for some clients to use the old version while others use the new version.

Migrate stored objects to the new version:

1. See the upgrade existing objects to a new stored version section.

It is safe for clients to use both the old and new version before, during and after upgrading the objects to a new stored version.

Removing an old version:

1. Ensure all clients are fully migrated to the new version. The kube-apiserver logs can be reviewed to help identify any clients that are still accessing via the old version.
2. Set served to false for the old version in the spec.versions list. If any clients are still unexpectedly using the old version they may begin reporting errors attempting to access the custom resource objects at the old version. If this occurs, switch back to using served:true on the old version, migrate the remaining clients to the new version and repeat this step.
3. Ensure the upgrade of existing objects to the new stored version step has been completed.
   1. Verify that the storage is set to true for the new version in the spec.versions list in the CustomResourceDefinition.
   2. Verify that the old version is no longer listed in the CustomResourceDefinition status.storedVersions.
4. Remove the old version from the CustomResourceDefinition spec.versions list.
5. Drop conversion support for the old version in conversion webhooks.

Specify multiple versions

The CustomResourceDefinition API versions field can be used to support multiple versions of custom resources that you have developed. Versions can have different schemas, and conversion webhooks can convert custom resources between versions. Webhook conversions should follow the Kubernetes API conventions wherever applicable. Specifically, See the API change documentation for a set of useful gotchas and suggestions.

This example shows a CustomResourceDefinition with two versions. For the first example, the assumption is all versions share the same schema with no conversion between them. The comments in the YAML provide more context.

• apiextensions.k8s.io/v1
• apiextensions.k8s.io/v1beta1

apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  # name must match the spec fields below, and be in the form: <plural>.<group>
  name: crontabs.example.com
spec:
  # group name to use for REST API: /apis/<group>/<version>
  group: example.com
  # list of versions supported by this CustomResourceDefinition
  versions:
  - name: v1beta1
    # 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
    # A schema is required
    schema:
      openAPIV3Schema:
        type: object
        properties:
          host:
            type: string
          port:
            type: string
  - name: v1
    served: true
    storage: false
    schema:
      openAPIV3Schema:
        type: object
        properties:
          host:
            type: string
          port:
            type: string
  # The conversion section is introduced in Kubernetes 1.13+ with a default value of
  # None conversion (strategy sub-field set to None).
  conversion:
    # None conversion assumes the same schema for all versions and only sets the apiVersion
    # field of custom resources to the proper value
    strategy: None
  # 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

# Deprecated in v1.16 in favor of apiextensions.k8s.io/v1
apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
metadata:
  # name must match the spec fields below, and be in the form: <plural>.<group>
  name: crontabs.example.com
spec:
  # group name to use for REST API: /apis/<group>/<version>
  group: example.com
  # list of versions supported by this CustomResourceDefinition
  versions:
  - name: v1beta1
    # 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
  - name: v1
    served: true
    storage: false
  validation:
    openAPIV3Schema:
      type: object
      properties:
        host:
          type: string
        port:
          type: string
  # The conversion section is introduced in Kubernetes 1.13+ with a default value of
  # None conversion (strategy sub-field set to None).
  conversion:
    # None conversion assumes the same schema for all versions and only sets the apiVersion
    # field of custom resources to the proper value
    strategy: None
  # 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 PascalCased singular type. Your resource manifests use this.
    kind: CronTab
    # shortNames allow shorter string to match your resource on the CLI
    shortNames:
    - ct

You can save the CustomResourceDefinition in a YAML file, then use kubectl apply to create it.

kubectl apply -f my-versioned-crontab.yaml

After creation, the API server starts to serve each enabled version at an HTTP REST endpoint. In the above example, the API versions are available at /apis/example.com/v1beta1 and /apis/example.com/v1.

Version priority

Regardless of the order in which versions are defined in a CustomResourceDefinition, the version with the highest priority is used by kubectl as the default version to access objects. The priority is determined by parsing the name field to determine the version number, the stability (GA, Beta, or Alpha), and the sequence within that stability level.

The algorithm used for sorting the versions is designed to sort versions in the same way that the Kubernetes project sorts Kubernetes versions. Versions start with a v followed by a number, an optional beta or alpha designation, and optional additional numeric versioning information. Broadly, a version string might look like v2 or v2beta1. Versions are sorted using the following algorithm:

• Entries that follow Kubernetes version patterns are sorted before those that do not.
• For entries that follow Kubernetes version patterns, the numeric portions of the version string is sorted largest to smallest.
• If the strings beta or alpha follow the first numeric portion, they sorted in that order, after the equivalent string without the beta or alpha suffix (which is presumed to be the GA version).
• If another number follows the beta, or alpha, those numbers are also sorted from largest to smallest.
• Strings that don't fit the above format are sorted alphabetically and the numeric portions are not treated specially. Notice that in the example below, foo1 is sorted above foo10. This is different from the sorting of the numeric portion of entries that do follow the Kubernetes version patterns.

This might make sense if you look at the following sorted version list:

- v10
- v2
- v1
- v11beta2
- v10beta3
- v3beta1
- v12alpha1
- v11alpha2
- foo1
- foo10

For the example in Specify multiple versions, the version sort order is v1, followed by v1beta1. This causes the kubectl command to use v1 as the default version unless the provided object specifies the version.

Version deprecation

Starting in v1.19, a CustomResourceDefinition can indicate a particular version of the resource it defines is deprecated. When API requests to a deprecated version of that resource are made, a warning message is returned in the API response as a header. The warning message for each deprecated version of the resource can be customized if desired.

A customized warning message should indicate the deprecated API group, version, and kind, and should indicate what API group, version, and kind should be used instead, if applicable.

• apiextensions.k8s.io/v1
• apiextensions.k8s.io/v1beta1

apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
  name: crontabs.example.com
spec:
  group: example.com
  names:
    plural: crontabs
    singular: crontab
    kind: CronTab
  scope: Namespaced
  versions:
  - name: v1alpha1
    served: true
    storage: false
    # This indicates the v1alpha1 version of the custom resource is deprecated.
    # API requests to this version receive a warning header in the server response.
    deprecated: true
    # This overrides the default warning returned to API clients making v1alpha1 API requests.
    deprecationWarning: "example.com/v1alpha1 CronTab is deprecated; see http://example.com/v1alpha1-v1 for instructions to migrate to example.com/v1 CronTab"
    
    schema: ...
  - name: v1beta1
    served: true
    # This indicates the v1beta1 version of the custom resource is deprecated.
    # API requests to this version receive a warning header in the server response.
    # A default warning message is returned for this version.
    deprecated: true
    schema: ...
  - name: v1
    served: true
    storage: true
    schema: ...

# Deprecated in v1.16 in favor of apiextensions.k8s.io/v1
apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
metadata:
  name: crontabs.example.com
spec:
  group: example.com
  names:
    plural: crontabs
    singular: crontab
    kind: CronTab
  scope: Namespaced
  validation: ...
  versions:
  - name: v1alpha1
    served: true
    storage: false
    # This indicates the v1alpha1 version of the custom resource is deprecated.
    # API requests to this version receive a warning header in the server response.
    deprecated: true
    # This overrides the default warning returned to API clients making v1alpha1 API requests.
    deprecationWarning: "example.com/v1alpha1 CronTab is deprecated; see http://example.com/v1alpha1-v1 for instructions to migrate to example.com/v1 CronTab"
  - name: v1beta1
    served: true
    # This indicates the v1beta1 version of the custom resource is deprecated.
    # API requests to this version receive a warning header in the server response.
    # A default warning message is returned for this version.
    deprecated: true
  - name: v1
    served: true
    storage: true

Version removal

An older API version cannot be dropped from a CustomResourceDefinition manifest until existing stored data has been migrated to the newer API version for all clusters that served the older version of the custom resource, and the old version is removed from the status.storedVersions of the CustomResourceDefinition.

apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
  name: crontabs.example.com
spec:
  group: example.com
  names:
    plural: crontabs
    singular: crontab
    kind: CronTab
  scope: Namespaced
  versions:
  - name: v1beta1
    # This indicates the v1beta1 version of the custom resource is no longer served.
    # API requests to this version receive a not found error in the server response.
    served: false
    schema: ...
  - name: v1
    served: true
    # The new served version should be set as the storage version
    storage: true
    schema: ...

Webhook conversion

The above example has a None conversion between versions which only sets the apiVersion field on conversion and does not change the rest of the object. The API server also supports webhook conversions that call an external service in case a conversion is required. For example when:

• custom resource is requested in a different version than stored version.
• Watch is created in one version but the changed object is stored in another version.
• custom resource PUT request is in a different version than storage version.

To cover all of these cases and to optimize conversion by the API server, the conversion requests may contain multiple objects in order to minimize the external calls. The webhook should perform these conversions independently.

Write a conversion webhook server

Please refer to the implementation of the custom resource conversion webhook server that is validated in a Kubernetes e2e test. The webhook handles the ConversionReview requests sent by the API servers, and sends back conversion results wrapped in ConversionResponse. Note that the request contains a list of custom resources that need to be converted independently without changing the order of objects. The example server is organized in a way to be reused for other conversions. Most of the common code are located in the framework file that leaves only one function to be implemented for different conversions.

Permissible mutations

A conversion webhook must not mutate anything inside of metadata of the converted object other than labels and annotations. Attempted changes to name, UID and namespace are rejected and fail the request which caused the conversion. All other changes are ignored.

Deploy the conversion webhook service

Documentation for deploying the conversion webhook is the same as for the admission webhook example service. The assumption for next sections is that the conversion webhook server is deployed to a service named example-conversion-webhook-server in default namespace and serving traffic on path /crdconvert.

Configure CustomResourceDefinition to use conversion webhooks

The None conversion example can be extended to use the conversion webhook by modifying conversion section of the spec:

• apiextensions.k8s.io/v1
• apiextensions.k8s.io/v1beta1

apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  # name must match the spec fields below, and be in the form: <plural>.<group>
  name: crontabs.example.com
spec:
  # group name to use for REST API: /apis/<group>/<version>
  group: example.com
  # list of versions supported by this CustomResourceDefinition
  versions:
  - name: v1beta1
    # 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
    # Each version can define its own schema when there is no top-level
    # schema is defined.
    schema:
      openAPIV3Schema:
        type: object
        properties:
          hostPort:
            type: string
  - name: v1
    served: true
    storage: false
    schema:
      openAPIV3Schema:
        type: object
        properties:
          host:
            type: string
          port:
            type: string
  conversion:
    # a Webhook strategy instruct API server to call an external webhook for any conversion between custom resources.
    strategy: Webhook
    # webhook is required when strategy is `Webhook` and it configures the webhook endpoint to be called by API server.
    webhook:
      # conversionReviewVersions indicates what ConversionReview versions are understood/preferred by the webhook.
      # The first version in the list understood by the API server is sent to the webhook.
      # The webhook must respond with a ConversionReview object in the same version it received.
      conversionReviewVersions: ["v1","v1beta1"]
      clientConfig:
        service:
          namespace: default
          name: example-conversion-webhook-server
          path: /crdconvert
        caBundle: "Ci0tLS0tQk...<base64-encoded PEM bundle>...tLS0K"
  # 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

# Deprecated in v1.16 in favor of apiextensions.k8s.io/v1
apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
metadata:
  # name must match the spec fields below, and be in the form: <plural>.<group>
  name: crontabs.example.com
spec:
  # group name to use for REST API: /apis/<group>/<version>
  group: example.com
  # prunes object fields that are not specified in OpenAPI schemas below.
  preserveUnknownFields: false
  # list of versions supported by this CustomResourceDefinition
  versions:
  - name: v1beta1
    # 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
    # Each version can define its own schema when there is no top-level
    # schema is defined.
    schema:
      openAPIV3Schema:
        type: object
        properties:
          hostPort:
            type: string
  - name: v1
    served: true
    storage: false
    schema:
      openAPIV3Schema:
        type: object
        properties:
          host:
            type: string
          port:
            type: string
  conversion:
    # a Webhook strategy instruct API server to call an external webhook for any conversion between custom resources.
    strategy: Webhook
    # webhookClientConfig is required when strategy is `Webhook` and it configures the webhook endpoint to be called by API server.
    webhookClientConfig:
      service:
        namespace: default
        name: example-conversion-webhook-server
        path: /crdconvert
      caBundle: "Ci0tLS0tQk...<base64-encoded PEM bundle>...tLS0K"
  # 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

You can save the CustomResourceDefinition in a YAML file, then use kubectl apply to apply it.

kubectl apply -f my-versioned-crontab-with-conversion.yaml

Make sure the conversion service is up and running before applying new changes.

Contacting the webhook

Once the API server has determined a request should be sent to a conversion webhook, it needs to know how to contact the webhook. This is specified in the webhookClientConfig stanza of the webhook configuration.

Conversion webhooks can either be called via a URL or a service reference, and can optionally include a custom CA bundle to use to verify the TLS connection.

URL

url gives the location of the webhook, in standard URL form (scheme://host:port/path).

The host should not refer to a service running in the cluster; use a service reference by specifying the service field instead. The host might be resolved via external DNS in some apiservers (i.e., kube-apiserver cannot resolve in-cluster DNS as that would be a layering violation). host may also be an IP address.

Please note that using localhost or 127.0.0.1 as a host is risky unless you take great care to run this webhook on all hosts which run an apiserver which might need to make calls to this webhook. Such installations are likely to be non-portable or not readily run in a new cluster.

The scheme must be "https"; the URL must begin with "https://".

Attempting to use a user or basic auth (for example "user:password@") is not allowed. Fragments ("#...") and query parameters ("?...") are also not allowed.

Here is an example of a conversion webhook configured to call a URL (and expects the TLS certificate to be verified using system trust roots, so does not specify a caBundle):

• apiextensions.k8s.io/v1
• apiextensions.k8s.io/v1beta1

apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
...
spec:
  ...
  conversion:
    strategy: Webhook
    webhook:
      clientConfig:
        url: "https://my-webhook.example.com:9443/my-webhook-path"
...

# Deprecated in v1.16 in favor of apiextensions.k8s.io/v1
apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
...
spec:
  ...
  conversion:
    strategy: Webhook
    webhookClientConfig:
      url: "https://my-webhook.example.com:9443/my-webhook-path"
...

Service Reference

The service stanza inside webhookClientConfig is a reference to the service for a conversion webhook. If the webhook is running within the cluster, then you should use service instead of url. The service namespace and name are required. The port is optional and defaults to 443. The path is optional and defaults to "/".

Here is an example of a webhook that is configured to call a service on port "1234" at the subpath "/my-path", and to verify the TLS connection against the ServerName my-service-name.my-service-namespace.svc using a custom CA bundle.

• apiextensions.k8s.io/v1
• apiextensions.k8s.io/v1beta1

apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
...
spec:
  ...
  conversion:
    strategy: Webhook
    webhook:
      clientConfig:
        service:
          namespace: my-service-namespace
          name: my-service-name
          path: /my-path
          port: 1234
        caBundle: "Ci0tLS0tQk...<base64-encoded PEM bundle>...tLS0K"
...

# Deprecated in v1.16 in favor of apiextensions.k8s.io/v1
apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
...
spec:
  ...
  conversion:
    strategy: Webhook
    webhookClientConfig:
      service:
        namespace: my-service-namespace
        name: my-service-name
        path: /my-path
        port: 1234
      caBundle: "Ci0tLS0tQk...<base64-encoded PEM bundle>...tLS0K"
...

Webhook request and response

Request

Webhooks are sent a POST request, with Content-Type: application/json, with a ConversionReview API object in the apiextensions.k8s.io API group serialized to JSON as the body.

Webhooks can specify what versions of ConversionReview objects they accept with the conversionReviewVersions field in their CustomResourceDefinition:

• apiextensions.k8s.io/v1
• apiextensions.k8s.io/v1beta1

apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
...
spec:
  ...
  conversion:
    strategy: Webhook
    webhook:
      conversionReviewVersions: ["v1", "v1beta1"]
      ...

# Deprecated in v1.16 in favor of apiextensions.k8s.io/v1
apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
...
spec:
  ...
  conversion:
    strategy: Webhook
    conversionReviewVersions: ["v1", "v1beta1"]
    ...

API servers send the first ConversionReview version in the conversionReviewVersions list they support. If none of the versions in the list are supported by the API server, the custom resource definition will not be allowed to be created. If an API server encounters a conversion webhook configuration that was previously created and does not support any of the ConversionReview versions the API server knows how to send, attempts to call to the webhook will fail.

This example shows the data contained in an ConversionReview object for a request to convert CronTab objects to example.com/v1:

• apiextensions.k8s.io/v1
• apiextensions.k8s.io/v1beta1

{
  "apiVersion": "apiextensions.k8s.io/v1",
  "kind": "ConversionReview",
  "request": {
    # Random uid uniquely identifying this conversion call
    "uid": "705ab4f5-6393-11e8-b7cc-42010a800002",
    
    # The API group and version the objects should be converted to
    "desiredAPIVersion": "example.com/v1",
    
    # The list of objects to convert.
    # May contain one or more objects, in one or more versions.
    "objects": [
      {
        "kind": "CronTab",
        "apiVersion": "example.com/v1beta1",
        "metadata": {
          "creationTimestamp": "2019-09-04T14:03:02Z",
          "name": "local-crontab",
          "namespace": "default",
          "resourceVersion": "143",
          "uid": "3415a7fc-162b-4300-b5da-fd6083580d66"
        },
        "hostPort": "localhost:1234"
      },
      {
        "kind": "CronTab",
        "apiVersion": "example.com/v1beta1",
        "metadata": {
          "creationTimestamp": "2019-09-03T13:02:01Z",
          "name": "remote-crontab",
          "resourceVersion": "12893",
          "uid": "359a83ec-b575-460d-b553-d859cedde8a0"
        },
        "hostPort": "example.com:2345"
      }
    ]
  }
}

{
  # Deprecated in v1.16 in favor of apiextensions.k8s.io/v1
  "apiVersion": "apiextensions.k8s.io/v1beta1",
  "kind": "ConversionReview",
  "request": {
    # Random uid uniquely identifying this conversion call
    "uid": "705ab4f5-6393-11e8-b7cc-42010a800002",
    
    # The API group and version the objects should be converted to
    "desiredAPIVersion": "example.com/v1",
    
    # The list of objects to convert.
    # May contain one or more objects, in one or more versions.
    "objects": [
      {
        "kind": "CronTab",
        "apiVersion": "example.com/v1beta1",
        "metadata": {
          "creationTimestamp": "2019-09-04T14:03:02Z",
          "name": "local-crontab",
          "namespace": "default",
          "resourceVersion": "143",
          "uid": "3415a7fc-162b-4300-b5da-fd6083580d66"
        },
        "hostPort": "localhost:1234"
      },
      {
        "kind": "CronTab",
        "apiVersion": "example.com/v1beta1",
        "metadata": {
          "creationTimestamp": "2019-09-03T13:02:01Z",
          "name": "remote-crontab",
          "resourceVersion": "12893",
          "uid": "359a83ec-b575-460d-b553-d859cedde8a0"
        },
        "hostPort": "example.com:2345"
      }
    ]
  }
}

Response

Webhooks respond with a 200 HTTP status code, Content-Type: application/json, and a body containing a ConversionReview object (in the same version they were sent), with the response stanza populated, serialized to JSON.

If conversion succeeds, a webhook should return a response stanza containing the following fields:

• uid, copied from the request.uid sent to the webhook
• result, set to {"status":"Success"}
• convertedObjects, containing all of the objects from request.objects, converted to request.desiredVersion

Example of a minimal successful response from a webhook:

• apiextensions.k8s.io/v1
• apiextensions.k8s.io/v1beta1

{
  "apiVersion": "apiextensions.k8s.io/v1",
  "kind": "ConversionReview",
  "response": {
    # must match <request.uid>
    "uid": "705ab4f5-6393-11e8-b7cc-42010a800002",
    "result": {
      "status": "Success"
    },
    # Objects must match the order of request.objects, and have apiVersion set to <request.desiredAPIVersion>.
    # kind, metadata.uid, metadata.name, and metadata.namespace fields must not be changed by the webhook.
    # metadata.labels and metadata.annotations fields may be changed by the webhook.
    # All other changes to metadata fields by the webhook are ignored.
    "convertedObjects": [
      {
        "kind": "CronTab",
        "apiVersion": "example.com/v1",
        "metadata": {
          "creationTimestamp": "2019-09-04T14:03:02Z",
          "name": "local-crontab",
          "namespace": "default",
          "resourceVersion": "143",
          "uid": "3415a7fc-162b-4300-b5da-fd6083580d66"
        },
        "host": "localhost",
        "port": "1234"
      },
      {
        "kind": "CronTab",
        "apiVersion": "example.com/v1",
        "metadata": {
          "creationTimestamp": "2019-09-03T13:02:01Z",
          "name": "remote-crontab",
          "resourceVersion": "12893",
          "uid": "359a83ec-b575-460d-b553-d859cedde8a0"
        },
        "host": "example.com",
        "port": "2345"
      }
    ]
  }
}

{
  # Deprecated in v1.16 in favor of apiextensions.k8s.io/v1
  "apiVersion": "apiextensions.k8s.io/v1beta1",
  "kind": "ConversionReview",
  "response": {
    # must match <request.uid>
    "uid": "705ab4f5-6393-11e8-b7cc-42010a800002",
    "result": {
      "status": "Failed"
    },
    # Objects must match the order of request.objects, and have apiVersion set to <request.desiredAPIVersion>.
    # kind, metadata.uid, metadata.name, and metadata.namespace fields must not be changed by the webhook.
    # metadata.labels and metadata.annotations fields may be changed by the webhook.
    # All other changes to metadata fields by the webhook are ignored.
    "convertedObjects": [
      {
        "kind": "CronTab",
        "apiVersion": "example.com/v1",
        "metadata": {
          "creationTimestamp": "2019-09-04T14:03:02Z",
          "name": "local-crontab",
          "namespace": "default",
          "resourceVersion": "143",
          "uid": "3415a7fc-162b-4300-b5da-fd6083580d66"
        },
        "host": "localhost",
        "port": "1234"
      },
      {
        "kind": "CronTab",
        "apiVersion": "example.com/v1",
        "metadata": {
          "creationTimestamp": "2019-09-03T13:02:01Z",
          "name": "remote-crontab",
          "resourceVersion": "12893",
          "uid": "359a83ec-b575-460d-b553-d859cedde8a0"
        },
        "host": "example.com",
        "port": "2345"
      }
    ]
  }
}

If conversion fails, a webhook should return a response stanza containing the following fields:

• uid, copied from the request.uid sent to the webhook
• result, set to {"status":"Failed"}

Example of a response from a webhook indicating a conversion request failed, with an optional message:

• apiextensions.k8s.io/v1
• apiextensions.k8s.io/v1beta1

{
  "apiVersion": "apiextensions.k8s.io/v1",
  "kind": "ConversionReview",
  "response": {
    "uid": "<value from request.uid>",
    "result": {
      "status": "Failed",
      "message": "hostPort could not be parsed into a separate host and port"
    }
  }
}

{
  # Deprecated in v1.16 in favor of apiextensions.k8s.io/v1
  "apiVersion": "apiextensions.k8s.io/v1beta1",
  "kind": "ConversionReview",
  "response": {
    "uid": "<value from request.uid>",
    "result": {
      "status": "Failed",
      "message": "hostPort could not be parsed into a separate host and port"
    }
  }
}

Writing, reading, and updating versioned CustomResourceDefinition objects

When an object is written, it is stored at the version designated as the storage version at the time of the write. If the storage version changes, existing objects are never converted automatically. However, newly-created or updated objects are written at the new storage version. It is possible for an object to have been written at a version that is no longer served.

When you read an object, you specify the version as part of the path. You can request an object at any version that is currently served. If you specify a version that is different from the object's stored version, Kubernetes returns the object to you at the version you requested, but the stored object is not changed on disk.

What happens to the object that is being returned while serving the read request depends on what is specified in the CRD's spec.conversion:

• if the default strategy value None is specified, the only modifications to the object are changing the apiVersion string and perhaps pruning unknown fields (depending on the configuration). Note that this is unlikely to lead to good results if the schemas differ between the storage and requested version. In particular, you should not use this strategy if the same data is represented in different fields between versions.
• if webhook conversion is specified, then this mechanism controls the conversion.

If you update an existing object, it is rewritten at the version that is currently the storage version. This is the only way that objects can change from one version to another.

To illustrate this, consider the following hypothetical series of events:

1. The storage version is v1beta1. You create an object. It is stored at version v1beta1
2. You add version v1 to your CustomResourceDefinition and designate it as the storage version. Here the schemas for v1 and v1beta1 are identical, which is typically the case when promoting an API to stable in the Kubernetes ecosystem.
3. You read your object at version v1beta1, then you read the object again at version v1. Both returned objects are identical except for the apiVersion field.
4. You create a new object. It is stored at version v1. You now have two objects, one of which is at v1beta1, and the other of which is at v1.
5. You update the first object. It is now stored at version v1 since that is the current storage version.

Previous storage versions

The API server records each version which has ever been marked as the storage version in the status field storedVersions. Objects may have been stored at any version that has ever been designated as a storage version. No objects can exist in storage at a version that has never been a storage version.

Upgrade existing objects to a new stored version

When deprecating versions and dropping support, select a storage upgrade procedure.

Option 1: Use the Storage Version Migrator

1. Run the storage Version migrator
2. Remove the old version from the CustomResourceDefinition status.storedVersions field.

Option 2: Manually upgrade the existing objects to a new stored version

The following is an example procedure to upgrade from v1beta1 to v1.

1. Set v1 as the storage in the CustomResourceDefinition file and apply it using kubectl. The storedVersions is now v1beta1, v1.
2. Write an upgrade procedure to list all existing objects and write them with the same content. This forces the backend to write objects in the current storage version, which is v1.
3. Remove v1beta1 from the CustomResourceDefinition status.storedVersions field.

kubectl patch customresourcedefinitions <CRD_Name> --subresource='status' --type='merge' -p '{"status":{"storedVersions":["v1"]}}'