PLEASE NOTE: This document applies to v1.4 version and not to the latest release v1.6Documentation for other releases can be found by using the version selector in the top right of any doc page.
Crossplane Composite Resources are opinionated Kubernetes Custom Resources that are composed of Managed Resources. We often call them XRs for short.
Composite Resources are designed to let you build your own platform with your own opinionated concepts and APIs without needing to write a Kubernetes controller from scratch. Instead, you define the schema of your XR and teach Crossplane which Managed Resources it should compose (i.e. create) when someone creates the XR you defined.
If you’re already familiar with Composite Resources and looking for a detailed configuration reference or some tips, tricks, and troubleshooting information, try the Composition Reference.
Below is an example of a Composite Resource:
apiVersion: database.example.org/v1alpha1 kind: XPostgreSQLInstance metadata: name: my-db spec: parameters: storageGB: 20 compositionRef: name: production writeConnectionSecretToRef: namespace: crossplane-system name: my-db-connection-details
You define your own XRs, so they can be of whatever API version and kind you like, and contain whatever spec and status fields you need.
The first step towards using Composite Resources is configuring Crossplane so
that it knows what XRs you’d like to exist, and what to do when someone creates
one of those XRs. This is done using a
resource and one or more
Once you’ve configured Crossplane with the details of your new XR you can either create one directly, or use a claim. Typically only the folks responsible for configuring Crossplane (often a platform or SRE team) have permission to create XRs directly. Everyone else manages XRs via a lightweight proxy resource called a Composite Resource Claim (or claim for short). More on that later.
If you’re coming from the Terraform world you can think of an XRD as similar to the
variableblocks of a Terraform module, while the
Compositionis the rest of the module’s HCL code that describes how to use those variables to create a bunch of resources. In this analogy the XR or claim is a little like a
tfvarsfile providing inputs to the module.
CompositeResourceDefinition (or XRD) defines the type and schema of your XR.
It lets Crossplane know that you want a particular kind of XR to exist, and what
fields that XR should have. An XRD is a little like a
(CRD), but slightly more opinionated. Writing an XRD is mostly a matter of
specifying an OpenAPI “structural schema”.
The XRD that defines the
XPostgreSQLInstance XR above would look like this:
apiVersion: apiextensions.crossplane.io/v1 kind: CompositeResourceDefinition metadata: name: xpostgresqlinstances.database.example.org spec: group: database.example.org names: kind: XPostgreSQLInstance plural: xpostgresqlinstances claimNames: kind: PostgreSQLInstance plural: postgresqlinstances versions: - name: v1alpha1 served: true referenceable: true schema: openAPIV3Schema: type: object properties: spec: type: object properties: parameters: type: object properties: storageGB: type: integer required: - storageGB required: - parameters
You might notice that the
XPostgreSQLInstance example above has some fields
that don’t appear in the XRD, like the
compositionRef fields. This is because Crossplane automatically injects some
standard Crossplane Resource Model (XRM) fields into all XRs.
Composition lets Crossplane know what to do when someone creates a Composite
Composition creates a link between an XR and a set of one or
more Managed Resources - when the XR is created, updated, or deleted the set of
Managed Resources are created, updated or deleted accordingly.
You can add multiple Compositions for each XRD, and choose which should be used when XRs are created. This allows a Composition to act like a class of service - for example you could configure one Composition for each environment you support, such as production, staging, and development.
Composition for the above
XPostgreSQLInstance might look like this:
apiVersion: apiextensions.crossplane.io/v1 kind: Composition metadata: name: example labels: crossplane.io/xrd: xpostgresqlinstances.database.example.org provider: gcp spec: writeConnectionSecretsToNamespace: crossplane-system compositeTypeRef: apiVersion: database.example.org/v1alpha1 kind: XPostgreSQLInstance resources: - name: cloudsqlinstance base: apiVersion: database.gcp.crossplane.io/v1beta1 kind: CloudSQLInstance spec: forProvider: databaseVersion: POSTGRES_9_6 region: us-central1 settings: tier: db-custom-1-3840 dataDiskType: PD_SSD ipConfiguration: ipv4Enabled: true authorizedNetworks: - value: "0.0.0.0/0" patches: - type: FromCompositeFieldPath fromFieldPath: spec.parameters.storageGB toFieldPath: spec.forProvider.settings.dataDiskSizeGb
Composition tells Crossplane that when someone creates an
XPostgreSQLInstance XR Crossplane should create a
storageGB field of the
XPostgreSQLInstance should be used to
dataDiskSizeGb field of the
CloudSQLInstance. This is only a
small subset of the functionality a
Composition enables - take a look at the
reference page to learn more.
We almost always talk about XRs composing Managed Resources, but actually an XR can also compose other XRs to allow nested layers of abstraction. XRs don’t support composing arbitrary Kubernetes resources (e.g. Deployments, operators, etc) directly but you can do so using our Kubernetes and Helm providers.
Crossplane uses Composite Resource Claims (or just claims, for short) to allow application operators to provision and manage XRs. When we talk about using XRs it’s typically implied that the XR is being used via a claim. Claims are almost identical to their corresponding XRs. It helps to think of a claim as an application team’s interface to an XR. You could also think of claims as the public (app team) facing part of the opinionated platform API, while XRs are the private (platform team) facing part.
A claim for the
XPostgreSQLInstance XR above would look like this:
apiVersion: database.example.org/v1alpha1 kind: PostgreSQLInstance metadata: namespace: default name: my-db spec: parameters: storageGB: 20 compositionRef: name: production writeConnectionSecretToRef: name: my-db-connection-details
There are three key differences between an XR and a claim:
kindthan the XR - by convention the XR’s
kindwithout the proceeding
X. For example a
Not all XRs offer a claim - doing so is optional. See the XRD section of the Composition reference to learn how to offer a claim.
Claims may seem a little superfluous at first, but they enable some handy scenarios, including:
Private XRs. Sometimes a platform team might not want a type of XR to be
directly consumed by their application teams. For example because the XR
represents ‘supporting’ infrastructure - consider the above VPC
XNetwork XR. App
teams might create
PostgreSQLInstance claims that reference (i.e. consume)
XNetwork, but they shouldn’t be creating their own. Similarly, some
kinds of XR might be intended only for ‘nested’ use - intended only to be
composed by other XRs.
Global XRs. Not all infrastructure is conceptually namespaced. Say your
organisation uses team scoped namespaces. A
PostgreSQLInstance that belongs
to Team A should probably be part of the
team-a namespace - you’d represent
this by creating a
PostgreSQLInstance claim in that namespace. On the other
XNetwork XR we mentioned previously could be referenced (i.e. used)
by XRs from many different namespaces - it doesn’t exist to serve a particular
Pre-provisioned XRs. Finally, separating claims from XRs allows a platform team to pre-provision certain kinds of XR. Typically an XR is created on-demand in response to the creation of a claim, but it’s also possible for a claim to instead request an existing XR. This can allow application teams to instantly claim infrastructure like database instances that would otherwise take minutes to provision on-demand.