docs/content/v1.20/concepts/composite-resource-definitions.md

title	weight	description
Composite Resource Definitions	40	Composite Resource Definitions or XRDs define custom API schemas

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

{{< hint "note" >}}

Read the [composite resources]({{<ref "./composite-resources">}}) page for more information about composite resources.

Read the [Claims]({{<ref "./claims">}}) page for more information about Claims. {{}}

{{<expand "Confused about Compositions, XRDs, XRs and Claims?" >}} Crossplane has four core components that users commonly mix up:

• [Compositions]({{<ref "./compositions" >}}) - A template to define how to create resources.
• Composite Resource Definition (XRD) - This page. A custom API specification.
• [Composite Resource]({{<ref "./composite-resources">}}) (XR) - Created by using the custom API defined in a Composite Resource Definition. XRs use the Composition template to create new managed resources.
• [Claims]({{<ref "./claims" >}}) (XRC) - Like a Composite Resource, but with namespace scoping. {{}}

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

• Offering a Claim.
• Defining connection secrets.
• 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: xmydatabases.example.org
spec:
  group: example.org
  names:
    kind: XMyDatabase
    plural: xmydatabases
  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 {{}}xmydatabases.example.org{{}} creates a custom resource definition {{}}xmydatabases.example.org{{}}.

kubectl api-resources
NAME                              SHORTNAMES   APIVERSION          NAMESPACED   KIND
xmydatabases.example.org                       v1alpha1            false        xmydatabases
# Removed for brevity

{{<hint "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.

{{<hint "tip" >}} Group names are cluster scoped. Choose group names that don't conflict with Providers.
Avoid Provider names in the group. {{< /hint >}}

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.

{{<hint "important" >}} The XRD {{}}metadata.name{{}} must be {{}}plural{{}} name, . (dot character), {{}}group{{}}.

For example, {{}}xmydatabases.example.org{{}} matches the {{}}plural{{}} name {{}}xmydatabases{{}}, . {{}}group{{}} name, {{}}example.org{{}}.

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

{{<hint "note" >}} All schemas follow the Kubernetes custom resource definition OpenAPIv3 structural schema. {{< /hint >}}

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"

{{<hint "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. {{< /hint >}}

{{<hint "important" >}}

Changing or expanding the XRD schema requires restarting the [Crossplane pod]({{<ref "./pods#crossplane-pod">}}) to take effect. {{< /hint >}}

Required fields

By default all fields in a schema are optional. Define a parameter as required with the {{< hover label="required" line="25">}}required{{}} attribute.

In this example the XRD requires {{< hover label="required" line="19">}}region{{}} and {{< hover label="required" line="21">}}size{{}} but {{< hover label="required" line="23">}}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:

• spec.resourceRef
• spec.resourceRefs
• spec.claimRef
• spec.writeConnectionSecretToRef
• status.conditions
• status.connectionDetails

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.

{{< hint "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. {{< /hint >}}

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

{{< hint "note" >}} Changing which version is referenceable:true requires [updating the compositeTypeRef.apiVersion]({{<ref "./compositions#enable-composite-resources" >}}) of any Compositions referencing that XRD. {{< /hint >}}

Multiple schema versions

{{<hint "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. {{< /hint >}}

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            

{{<hint "important" >}}

Changing or expanding the XRD schema requires restarting the [Crossplane pod]({{<ref "./pods#crossplane-pod">}}) to take effect. {{< /hint >}}

Enable Claims

Optionally, XRDs can allow Claims to use the XRD API.

{{<hint "note" >}}

Read the [Claims]({{<ref "./claims">}}) page for more information about Claims. {{}}

XRDs offer Claims with a {{}}claimNames{{}} object.

The {{}}claimNames{{}} defines a {{}}kind{{}} and {{}}plural{{}} like the XRD {{}}names{{}} object.
Also like XRD {{}}names{{}}, use UpperCamelCase for the {{}}kind{{}} and lowercase for the {{}}plural{{}}.

The Claim {{}}kind{{}} and {{}}plural{{}} must be unique. They can't match any other Claim or other XRD {{}}kind{{}}.

{{<hint "tip" >}} Common Crossplane convention is to use {{}}claimNames{{}} that match the XRD {{}}names{{}}, but without the beginning "x." {{}}

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
  claimNames:
    kind: Database
    plural: databases
  versions:
  # Removed for brevity

{{<hint "important" >}} You can't change the {{}}claimNames{{}} after they're defined. You must delete and recreate the XRD to change the {{}}claimNames{{}}. {{}}

Manage connection secrets

When a composite resource creates managed resources, Crossplane provides any [connection secrets]({{<ref "./managed-resources#writeconnectionsecrettoref">}}) to the composite resource or Claim. This requires the creators of composite resources and Claims to know the secrets provided by a managed resource. In other cases, Crossplane administrators may not want to expose some or all the generated connection secrets.

XRDs can define a list of {{}}connectionSecretKeys{{}} to limit what's provided to a composite resource or Claim.

Crossplane only provides the keys listed in the {{}}connectionSecretKeys{{}} to the composite resource or Claim using this XRD. Any other connection secrets aren't passed to the composite resource or Claim.

{{<hint "important" >}} The keys listed in the {{}}connectionSecretKeys{{}} must match the key names listed in the Composition's connectionDetails.

An XRD ignores any keys listed that aren't created by a managed resource.

For more information read the [Composition documentation]({{<ref "./compositions#store-connection-details">}}). {{< /hint >}}

For example, an XRD passes the keys {{}}username{{}}, {{}}password{{}} and {{}}address{{}}.

Composite resources or Claims save these in the secret defined by their writeConnectionSecretToRef field.

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
  connectionSecretKeys:
    - username
    - password
    - address
  versions:
  # Removed for brevity

{{<hint "warning">}} You can't change the connectionSecretKeys of an XRD. You must delete and recreate the XRD to change the connectionSecretKeys. {{}}

For more information on connection secrets read the [Connection Secrets knowledge base article]({{<ref "connection-details">}}).

Set composite resource defaults

XRDs can set default parameters for composite resources and Claims.

defaultCompositeDeletePolicy

The defaultCompositeDeletePolicy defines the default value for the claim's compositeDeletePolicy property if the user doesn't specify a value when creating the claim. The claim controller uses the compositeDeletePolicy property to specify the propagation policy when deleting the associated composite. The compositeDeletePolicy doesn't apply to standalone composites that don't have associated claims.

Using a defaultCompositeDeletePolicy: Background policy causes the CRD for the claim to have the default value Background for the compositeDeletePolicy property. When a deleted claim has the compositeDeletePolicy property set to Background the claim controller deletes the composite resource using the propagation policy background and returns, relying on Kubernetes to delete the remaining child objects, like managed resources, nested composites and secrets.

Using defaultCompositeDeletePolicy: Foreground causes the CRD for the claim to have the compositeDeletePolicy default value Foreground. When a deleted claim has the compositeDeletePolicy property set to Foreground the controller deletes the associated composite using the propagation policy foreground. This causes Kubernetes to use foreground cascading deletion which deletes all child resources before deleting the parent resource. The claim controller waits for the composite deletion to finish before returning.

When creating a claim the user can override the defaultCompositeDeletePolicy by including the spec.compositeDeletePolicy property with either the Background or Foreground value.

The default value is defaultCompositeDeletePolicy: Background.

Set {{}}defaultCompositeDeletePolicy: Foreground{{}} to change the XRD deletion policy.

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

defaultCompositionRef

It's possible for multiple [Compositions]({{<ref "./compositions">}}) to reference the same XRD. If more than one Composition references the same XRD, the composite resource or Claim 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 and Claims use the updated Composition revision.

Set the XRD defaultCompositionUpdatePolicy to Manual to prevent composite resources and Claims 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 Claims 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 or Claims to use a specific Composition use the enforcedCompositionRef setting in the XRD.

For example, to require all composite resources and Claims 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.

The OFFERED field indicates this XRD offers a Claim and Crossplane installed the Kubernetes custom resource definitions for the Claim.

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
    Reason:                WatchingCompositeResourceClaim
    Status:                True
    Type:                  Offered
# 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

WatchingCompositeResourceClaim

Reason: WatchingCompositeResourceClaim indicates Crossplane defined the new Kubernetes custom resource definitions related to the offered Claims and is watching for the creation of new Claims.

Type: Offered
Status: True
Reason: WatchingCompositeResourceClaim

TerminatingCompositeResourceClaim

Reason: TerminatingCompositeResourceClaim indicates Crossplane is deleting the custom resource definitions related to the offered Claims and is terminating the Claims controller.

Type: Offered
Status: False
Reason: TerminatingCompositeResourceClaim