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Composite Resource Definitions

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This document is for a preview version of Crossplane.

This document applies to Crossplane v2.0-preview and not to the latest release v1.19.

Don't use Crossplane v2.0-preview in production.

Composite resource definitions (XRDs) define the schema for a custom API.
Users create composite resources (XRs) using the API schema defined by an XRD.

Note
Read the composite resources page for more information about composite resources.

A composite resource or XR is a custom API.

You use two Crossplane types to create a new custom API:

  • A Composite Resource Definition (XRD) - This page. Defines the XR’s schema.
  • A Composition - Configures how the XR creates other resources.

Crossplane XRDs are like Kubernetes custom resource definitions. XRDs require fewer fields and add options related to Crossplane, like connection secrets.

Creating a CompositeResourceDefinition

Creating a CompositeResourceDefinition consists of:

Optionally, CompositeResourceDefinitions also support:

Composite resource definitions (XRDs) create new API endpoints inside a Kubernetes cluster.

Creating a new API requires defining an API group, name and version.

 1apiVersion: apiextensions.crossplane.io/v1
 2kind: CompositeResourceDefinition
 3metadata: 
 4  name: mydatabases.example.org
 5spec:
 6  group: example.org
 7  names:
 8    kind: XMyDatabase
 9    plural: mydatabases
10  versions:
11  - name: v1alpha1
12  # Removed for brevity

After applying an XRD, Crossplane creates a new Kubernetes custom resource definition matching the defined API.

For example, the XRD mydatabases.example.org creates a custom resource definition mydatabases.example.org.

1kubectl api-resources
2NAME                              SHORTNAMES   APIVERSION          NAMESPACED   KIND
3mydatabases.example.org                        v1alpha1            true         mydatabases
4# Removed for brevity
Warning
You can’t change the XRD group or names.
You must delete and recreate the XRD to change the group or names.

XRD groups

Groups define a collection of related API endpoints. The group can be any value, but common convention is to map to a fully qualified domain name.

Many XRDs may use the same group to create a logical collection of APIs.

For example a database group may have a relational and nosql kinds.

XRD names

The names field defines how to refer to this specific XRD.
The required name fields are:

  • kind - the kind value to use when calling this API. The kind is UpperCamelCased. Crossplane recommends starting XRD kinds with an X to show it’s a custom Crossplane API definition.
  • plural - the plural name used for the API URL. The plural name must be lowercase.
Important

The XRD metadata.name must be plural name, . (dot character), group.

For example, mydatabases.example.org matches the plural name mydatabases, . group name, example.org.

 1apiVersion: apiextensions.crossplane.io/v1
 2kind: CompositeResourceDefinition
 3metadata: 
 4  name: mydatabases.example.org
 5spec:
 6  group: example.org
 7  names:
 8    kind: XMyDatabase
 9    plural: mydatabases
10    # Removed for brevity

XRD versions

The XRD version is like the API versioning used by Kubernetes. The version shows how mature or stable the API is and increments when changing, adding or removing fields in the API.

Crossplane doesn’t require specific versions or a specific version naming convention, but following Kubernetes API versioning guidelines is strongly recommended.

  • v1alpha1 - A new API that may change at any time.
  • v1beta1 - An existing API that’s considered stable. Breaking changes are strongly discouraged.
  • v1 - A stable API that doesn’t have breaking changes.

Define a schema

The schema defines the names of the parameters, the data types of the parameters and which parameters are required or optional.

Note
All schemas follow the Kubernetes custom resource definition OpenAPIv3 structural schema.

Each version of the API has a unique schema.

All XRD schemas validate against the openAPIV3Schema. The schema is an OpenAPI object with the properties of a spec object.

Inside the spec.properties is the custom API definition.

In this example, the key region is a string.

 1apiVersion: apiextensions.crossplane.io/v1
 2kind: CompositeResourceDefinition
 3metadata:
 4  name: xdatabases.custom-api.example.org
 5spec:
 6  group: custom-api.example.org
 7  names:
 8    kind: xDatabase
 9    plural: xdatabases
10  versions:
11  - name: v1alpha1
12    schema:
13      openAPIV3Schema:
14        type: object
15        properties:
16          spec:
17            type: object
18            properties:
19              region:
20                type: string
21    # Removed for brevity

A composite resource using this API references the group/version and kind. The spec has the region key with a string value.

1apiVersion: custom-api.example.org/v1alpha1
2kind: xDatabase
3metadata:
4  name: my-composite-resource
5spec: 
6  region: "US"
Tip

The custom API defined inside the spec.properties is an OpenAPIv3 specification. The data models page of the Swagger documentation provides a list of examples using data types and input restrictions.

The Kubernetes documentation lists the set of special restrictions on what your OpenAPIv3 custom API can use.

Important
Changing or expanding the XRD schema requires restarting the Crossplane pod to take effect.
Required fields

By default all fields in a schema are optional. Define a parameter as required with the required attribute.

In this example the XRD requires region and size but name is optional.

 1apiVersion: apiextensions.crossplane.io/v1
 2kind: CompositeResourceDefinition
 3metadata:
 4  name: xdatabases.custom-api.example.org
 5spec:
 6  group: custom-api.example.org
 7  names:
 8    kind: xDatabase
 9    plural: xdatabases
10  versions:
11  - name: v1alpha1
12    schema:
13      openAPIV3Schema:
14        type: object
15        properties:
16          spec:
17            type: object
18            properties:
19              region:
20                type: string 
21              size:
22                type: string  
23              name:
24                type: string  
25            required: 
26              - region 
27              - size
28    # Removed for brevity

According to the OpenAPIv3 specification, the required field is per-object. If a schema contains multiple objects the schema may need multiple required fields.

This XRD defines two objects:

  1. the top-level spec object
  2. a second location object

The spec object requires the size and location but name is optional.

Inside the required location object, country is required and zone is optional.

 1# Removed for brevity
 2- name: v1alpha1
 3    schema:
 4      openAPIV3Schema:
 5        type: object
 6        properties:
 7          spec:
 8            type: object
 9            properties:
10              size:
11                type: string  
12              name:
13                type: string 
14              location:
15                type: object
16                properties:
17                  country: 
18                    type: string 
19                  zone:
20                    type: string
21                required:
22                  - country
23            required:  
24              - size
25              - location

The Swagger “Describing Parameters” documentation has more examples.

Crossplane reserved fields

Crossplane doesn’t allow the following fields in a schema:

  • Any field under the object spec.crossplane
  • Any field under the object status.crossplane
  • status.conditions

Crossplane ignores any fields matching the reserved fields.

Serve and reference a schema

To use a schema it must be served: true and referenceable: true.

 1apiVersion: apiextensions.crossplane.io/v1
 2kind: CompositeResourceDefinition
 3metadata:
 4  name: xdatabases.custom-api.example.org
 5spec:
 6  group: custom-api.example.org
 7  names:
 8    kind: xDatabase
 9    plural: xdatabases
10  versions:
11  - name: v1alpha1
12    served: true
13    referenceable: true
14    schema:
15      openAPIV3Schema:
16        type: object
17        properties:
18          spec:
19            type: object
20            properties:
21              region:
22                type: string

Composite resources can use any schema version set as served: true.
Kubernetes rejects any composite resource using a schema version set as served: false.

Tip
Setting a schema version as served:false causes errors for users using an older schema. This can be an effective way to identify and upgrade users before deleting the older schema version.

The referenceable: true field indicates which version of the schema Compositions use. Only one version can be referenceable.

Note
Changing which version is referenceable:true requires updating the compositeTypeRef.apiVersion of any Compositions referencing that XRD.

Multiple schema versions

Warning

Crossplane supports defining multiple versions, but the schema of each version can’t change any existing fields, also called “making a breaking change.”

Breaking schema changes between versions requires the use of conversion webhooks.

New versions may define new optional parameters, but new required fields are a “breaking change.”

Crossplane XRDs use Kubernetes custom resource definitions for versioning. Read the Kubernetes documentation on versions in CustomResourceDefinitions for more background on versions and breaking changes.

Crossplane recommends implementing breaking schema changes as brand new XRDs.

For XRDs, to create a new version of an API add a new name in the versions list.

For example, this XRD version v1alpha1 only has the field region.

A second version, v1 expands the API to have both region and size.

 1apiVersion: apiextensions.crossplane.io/v1
 2kind: CompositeResourceDefinition
 3metadata:
 4  name: xdatabases.custom-api.example.org
 5spec:
 6  group: custom-api.example.org
 7  names:
 8    kind: xDatabase
 9    plural: xdatabases
10  versions:
11  - name: v1alpha1
12    schema:
13      openAPIV3Schema:
14        type: object
15        properties:
16          spec:
17            type: object
18            properties:
19              region:
20                type: string  
21  - name: v1
22    schema:
23      openAPIV3Schema:
24        type: object
25        properties:
26          spec:
27            type: object
28            properties:
29              region:
30                type: string 
31              size:
32                type: string
Important
Changing or expanding the XRD schema requires restarting the Crossplane pod to take effect.

Set composite resource defaults

XRDs can set default parameters for composite resources.

defaultCompositionRef

It’s possible for multiple Compositions to reference the same XRD. If more than one Composition references the same XRD, the composite resource must select which Composition to use.

An XRD can define the default Composition to use with the defaultCompositionRef value.

Set a defaultCompositionRef to set the default Composition.

 1apiVersion: apiextensions.crossplane.io/v1
 2kind: CompositeResourceDefinition
 3metadata:
 4  name: xdatabases.custom-api.example.org
 5spec:
 6  defaultCompositionRef: 
 7    name: myComposition
 8  group: custom-api.example.org
 9  names:
10  # Removed for brevity
11  versions:
12  # Removed for brevity

defaultCompositionUpdatePolicy

Changes to a Composition generate a new Composition revision. By default all composite resources use the updated Composition revision.

Set the XRD defaultCompositionUpdatePolicy to Manual to prevent composite resources from automatically using the new revision.

The default value is defaultCompositionUpdatePolicy: Automatic.

Set defaultCompositionUpdatePolicy: Manual to set the default Composition update policy for composite resources and using this XRD.

 1apiVersion: apiextensions.crossplane.io/v1
 2kind: CompositeResourceDefinition
 3metadata:
 4  name: xdatabases.custom-api.example.org
 5spec:
 6  defaultCompositionUpdatePolicy: Manual
 7  group: custom-api.example.org
 8  names:
 9  # Removed for brevity
10  versions:
11  # Removed for brevity

enforcedCompositionRef

To require all composite resources to use a specific Composition use the enforcedCompositionRef setting in the XRD.

For example, to require all composite resources using this XRD to use the Composition myComposition set enforcedCompositionRef.name: myComposition.

 1apiVersion: apiextensions.crossplane.io/v1
 2kind: CompositeResourceDefinition
 3metadata:
 4  name: xdatabases.custom-api.example.org
 5spec:
 6  enforcedCompositionRef: 
 7    name: myComposition
 8  group: custom-api.example.org
 9  names:
10  # Removed for brevity
11  versions:
12  # Removed for brevity

Verify a CompositeResourceDefinition

Verify an XRD with kubectl get compositeresourcedefinition or the short form, kubectl get xrd.

1kubectl get xrd                                
2NAME                                ESTABLISHED   OFFERED   AGE
3xdatabases.custom-api.example.org   True          True      22m

The ESTABLISHED field indicates Crossplane installed the Kubernetes custom resource definition for this XRD.

XRD conditions

Crossplane uses a standard set of Conditions for XRDs.
View the conditions of a XRD under their Status with kubectl describe xrd.

 1kubectl describe xrd
 2Name:         xpostgresqlinstances.database.starter.org
 3API Version:  apiextensions.crossplane.io/v1
 4Kind:         CompositeResourceDefinition
 5# Removed for brevity
 6Status:
 7  Conditions:
 8    Reason:                WatchingCompositeResource
 9    Status:                True
10    Type:                  Established
11# Removed for brevity

WatchingCompositeResource

Reason: WatchingCompositeResource indicates Crossplane defined the new Kubernetes custom resource definitions related to the composite resource and is watching for the creation of new composite resources.

1Type: Established
2Status: True
3Reason: WatchingCompositeResource

TerminatingCompositeResource

Reason: TerminatingCompositeResource indicates Crossplane is deleting the custom resource definitions related to the composite resource and is terminating the composite resource controller.

1Type: Established
2Status: False
3Reason: TerminatingCompositeResource