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Versions of 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. If you do not already have a cluster, you can create one by using Minikube, or you can use one of these Kubernetes playgrounds:

To check the version, enter kubectl version.

Overview

FEATURE STATE: Kubernetes v1.15 beta
This feature is currently in a beta state, meaning:

  • The version names contain beta (e.g. v2beta3).
  • Code is well tested. Enabling the feature is considered safe. Enabled by default.
  • Support for the overall feature will not be dropped, though details may change.
  • The schema and/or semantics of objects may change in incompatible ways in a subsequent beta or stable release. When this happens, we will provide instructions for migrating to the next version. This may require deleting, editing, and re-creating API objects. The editing process may require some thought. This may require downtime for applications that rely on the feature.
  • Recommended for only non-business-critical uses because of potential for incompatible changes in subsequent releases. If you have multiple clusters that can be upgraded independently, you may be able to relax this restriction.
  • Please do try our beta features and give feedback on them! After they exit beta, it may not be practical for us to make more changes.

The CustomResourceDefinition API supports a versions field that you can use to support multiple versions of custom resources that you have developed. Versions can have different schemas with a conversion webhook to 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.

Note: Earlier iterations included a version field instead of versions. The version field is deprecated and optional, but if it is not empty, it must match the first item in the versions field.

Specify multiple versions

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.

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
  # 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

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:

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.

Webhook conversion

FEATURE STATE: Kubernetes v1.15 beta
This feature is currently in a beta state, meaning:

  • The version names contain beta (e.g. v2beta3).
  • Code is well tested. Enabling the feature is considered safe. Enabled by default.
  • Support for the overall feature will not be dropped, though details may change.
  • The schema and/or semantics of objects may change in incompatible ways in a subsequent beta or stable release. When this happens, we will provide instructions for migrating to the next version. This may require deleting, editing, and re-creating API objects. The editing process may require some thought. This may require downtime for applications that rely on the feature.
  • Recommended for only non-business-critical uses because of potential for incompatible changes in subsequent releases. If you have multiple clusters that can be upgraded independently, you may be able to relax this restriction.
  • Please do try our beta features and give feedback on them! After they exit beta, it may not be practical for us to make more changes.
Note: Webhook conversion is available as beta since 1.15, and as alpha since Kubernetes 1.13. The CustomResourceWebhookConversion feature must be enabled, which is the case automatically for many clusters for beta features. Please refer to the feature gate documentation for more information.

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:

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.

Note: The example conversion webhook server leaves the ClientAuth field empty, which defaults to NoClientCert. This means that the webhook server does not authenticate the identity of the clients, supposedly API servers. If you need mutual TLS or other ways to authenticate the clients, see how to authenticate API servers.

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 just 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.

Note: When the webhook server is deployed into the Kubernetes cluster as a service, it has to be exposed via a service on port 443 (The server itself can have an arbitrary port but the service object should map it to port 443). The communication between the API server and the webhook service may fail if a different port is used for the service.

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:

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 it's 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 configure the webhook endpoint to be
    # called by API server.
    webhookClientConfig:
      service:
        namespace: default
        name: example-conversion-webhook-server
        path: /crdconvert
      caBundle: <pem encoded ca cert that signs the server cert used by the webhook>
  # 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 installs are likely to be non-portable, i.e., not easy to turn up in a new cluster.

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

Attempting to use a user or basic auth e.g. “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):

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.

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"
...

Writing, reading, and updating versioned CustomResourceDefinition objects

When an object is written, it is persisted 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. If you specify a version that is different from the object’s persisted version, Kubernetes returns the object to you at the version you requested, but the persisted object is neither changed on disk, nor converted in any way (other than changing the apiVersion string) while serving the request. You can request an object at any version that is currently served.

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 persisted in storage at version v1beta1
  2. You add version v1 to your CustomResourceDefinition and designate it as the storage version.
  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 persisted in storage 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 persisted 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 persisted 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, devise a storage upgrade procedure. 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. Update the CustomResourceDefinition Status by removing v1beta1 from storedVersions field.

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