Composite Resource Definitions

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.

What are XRs, XRDs and Compositions?

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:

• Defining a custom API group.
• Defining a custom API name.
• Defining a custom API schema and version.

Optionally, CompositeResourceDefinitions also support:

• Setting composite resource defaults.

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

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

apiVersion: apiextensions.crossplane.io/v1
kind: CompositeResourceDefinition
metadata: 
  name: mydatabases.example.org
spec:
  group: example.org
  names:
    kind: XMyDatabase
    plural: mydatabases
  versions:
  - name: v1alpha1
  # 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.

kubectl api-resources
NAME                              SHORTNAMES   APIVERSION          NAMESPACED   KIND
mydatabases.example.org                        v1alpha1            true         mydatabases
# 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.

Multiple 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.

tip

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.

apiVersion: apiextensions.crossplane.io/v1
kind: CompositeResourceDefinition
metadata: 
  name: mydatabases.example.org
spec:
  group: example.org
  names:
    kind: XMyDatabase
    plural: mydatabases
    # 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.

apiVersion: apiextensions.crossplane.io/v1
kind: CompositeResourceDefinition
metadata:
  name: xdatabases.custom-api.example.org
spec:
  group: custom-api.example.org
  names:
    kind: xDatabase
    plural: xdatabases
  versions:
  - name: v1alpha1
    schema:
      openAPIV3Schema:
        type: object
        properties:
          spec:
            type: object
            properties:
              region:
                type: string
    # 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.

apiVersion: custom-api.example.org/v1alpha1
kind: xDatabase
metadata:
  name: my-composite-resource
spec: 
  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.

tip

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.

apiVersion: apiextensions.crossplane.io/v1
kind: CompositeResourceDefinition
metadata:
  name: xdatabases.custom-api.example.org
spec:
  group: custom-api.example.org
  names:
    kind: xDatabase
    plural: xdatabases
  versions:
  - name: v1alpha1
    schema:
      openAPIV3Schema:
        type: object
        properties:
          spec:
            type: object
            properties:
              region:
                type: string 
              size:
                type: string  
              name:
                type: string  
            required: 
              - region 
              - size
    # 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.

# Removed for brevity
- name: v1alpha1
    schema:
      openAPIV3Schema:
        type: object
        properties:
          spec:
            type: object
            properties:
              size:
                type: string  
              name:
                type: string 
              location:
                type: object
                properties:
                  country: 
                    type: string 
                  zone:
                    type: string
                required:
                  - country
            required:  
              - size
              - 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.

apiVersion: apiextensions.crossplane.io/v1
kind: CompositeResourceDefinition
metadata:
  name: xdatabases.custom-api.example.org
spec:
  group: custom-api.example.org
  names:
    kind: xDatabase
    plural: xdatabases
  versions:
  - name: v1alpha1
    served: true
    referenceable: true
    schema:
      openAPIV3Schema:
        type: object
        properties:
          spec:
            type: object
            properties:
              region:
                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.

apiVersion: apiextensions.crossplane.io/v1
kind: CompositeResourceDefinition
metadata:
  name: xdatabases.custom-api.example.org
spec:
  group: custom-api.example.org
  names:
    kind: xDatabase
    plural: xdatabases
  versions:
  - name: v1alpha1
    schema:
      openAPIV3Schema:
        type: object
        properties:
          spec:
            type: object
            properties:
              region:
                type: string  
  - name: v1
    schema:
      openAPIV3Schema:
        type: object
        properties:
          spec:
            type: object
            properties:
              region:
                type: string 
              size:
                type: string            

tip

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.

apiVersion: apiextensions.crossplane.io/v1
kind: CompositeResourceDefinition
metadata:
  name: xdatabases.custom-api.example.org
spec:
  defaultCompositionRef: 
    name: myComposition
  group: custom-api.example.org
  names:
  # Removed for brevity
  versions:
  # 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.

apiVersion: apiextensions.crossplane.io/v1
kind: CompositeResourceDefinition
metadata:
  name: xdatabases.custom-api.example.org
spec:
  defaultCompositionUpdatePolicy: Manual
  group: custom-api.example.org
  names:
  # Removed for brevity
  versions:
  # 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.

apiVersion: apiextensions.crossplane.io/v1
kind: CompositeResourceDefinition
metadata:
  name: xdatabases.custom-api.example.org
spec:
  enforcedCompositionRef: 
    name: myComposition
  group: custom-api.example.org
  names:
  # Removed for brevity
  versions:
  # Removed for brevity

Verify a CompositeResourceDefinition

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

kubectl get xrd                                
NAME                                ESTABLISHED   OFFERED   AGE
xdatabases.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.

kubectl describe xrd
Name:         xpostgresqlinstances.database.starter.org
API Version:  apiextensions.crossplane.io/v1
Kind:         CompositeResourceDefinition
# Removed for brevity
Status:
  Conditions:
    Reason:                WatchingCompositeResource
    Status:                True
    Type:                  Established
# 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.

Type: Established
Status: True
Reason: WatchingCompositeResource

TerminatingCompositeResource

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

Type: Established
Status: False
Reason: TerminatingCompositeResource