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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. It also describes how to upgrade an object from one version to another.

Note: All specified versions must use the same schema. There is no schema conversion between versions.

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

The CustomResourceDefinition API supports a versions field that you can use to support multiple versions of custom resources that you have developed, and indicate the stability of a given custom resource. All versions must currently use the same schema, so if you need to add a field, you must add it to all versions.

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. 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
  # 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 create to create it.

kubectl create -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.

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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