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.
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
Make sure your Kubernetes cluster has a master version of 1.11.0 or higher.
Read about custom resources.
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
versionfield instead of
versionfield is deprecated and optional, but if it is not empty, it must match the first item in the
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
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
alpha designation, and
optional additional numeric versioning information. Broadly, a version string might look
v2beta1. Versions are sorted using the following algorithm:
alphafollow the first numeric portion, they sorted in that order, after the equivalent string without the
alphasuffix (which is presumed to be the GA version).
alpha, those numbers are also sorted from largest to smallest.
foo1is 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
Note: Webhook conversion is available as beta since 1.15, and as alpha since Kubernetes 1.13. The
CustomResourceWebhookConversionfeature 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
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.
Please refer to the implementation of the custom resource conversion webhook
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
ClientAuthfield 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.
A conversion webhook must not mutate anything inside of
metadata of the converted object other than
annotations. Attempted changes to
namespace are rejected and fail the request which caused the conversion. All other changes are just ignored.
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
default namespace and serving traffic on path
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.
None conversion example can be extended to use the conversion webhook by modifying
section of the
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 # Each version can define it's own schema when there is no top-level # schema is defined. schema: openAPIV3Schema: properties: hostPort: type: string - name: v1 served: true storage: false schema: openAPIV3Schema: 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.
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
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 gives the location of the webhook, in standard URL form
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
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
127.0.0.1 as a
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 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
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" ...
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:
v1beta1. You create an object. It is persisted in storage at version
v1to your CustomResourceDefinition and designate it as the storage version.
v1beta1, then you read the object again at version
v1. Both returned objects are identical except for the apiVersion field.
v1. You now have two objects, one of which is at
v1beta1, and the other of which is at
v1since that is the current storage version.
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.
When deprecating versions and dropping support, devise a storage upgrade
procedure. The following is an example procedure to upgrade from
v1as the storage in the CustomResourceDefinition file and apply it using kubectl. The
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