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Crossplane lets platform teams offer developers a Kubernetes-native API for requesting capabilities while keeping infrastructure choices and policy behind that interface. A developer submits a custom resource; Crossplane reconciles it through a Composition and its function pipeline into the resources the platform defines.
How Crossplane turns a request into platform resources
Crossplane describes itself as a control plane framework for platform engineering. It extends Kubernetes with custom resources, letting a platform team define an API that developers can use without making them manage every implementation detail. See Crossplane documentation for the version deployed in your environment; the documentation surfaced for this topic is v2.4, and APIs or CLI behavior may change between versions.
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The main pieces have distinct roles:
- Composite Resource Definition (XRD): defines the schema and scope of a composite API. It establishes the fields a user can provide and the kind of resource Kubernetes accepts.
- Composite resource (XR): an instance of that API. It represents a set of resources through one Kubernetes object.
- Composition: describes the resources to create for an XR. Its function pipeline determines the composed resources as users create or update XRs.
For example, the Crossplane CLI guide uses a WebApp request that produces a Kubernetes Deployment and Service. That is an illustrative composition, not a fixed Crossplane behavior: a platform team chooses what its API means and what resources it creates. See Crossplane’s Composition documentation.
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Start with the capability a developer needs, not with a list of infrastructure fields to expose. A useful platform API makes the supported request understandable and stable while allowing the platform team to control implementation and policy in the Composition.
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Decide which inputs belong in the API and which decisions should remain platform-owned. Exposing more options can help specialized users, but it also expands the interface the platform team must support. The right balance depends on your tenancy model, governance requirements, and users’ expertise.
Choose namespaced or cluster scope
An XRD can define a namespaced or cluster-scoped API. Namespaced resources can fit a model where requests belong to a namespace; cluster-scoped resources may fit other tenancy and access arrangements. Neither choice is universally correct: align scope with how your organization separates tenants and grants access. The XRD documentation covers the generated Kubernetes API and its scope: Composite Resource Definitions.
Connect the XRD to a Composition pipeline
After defining the API schema, create a Composition that explains how an XR maps to underlying resources. Composition functions are pipeline extensions that produce those resources from the XR. The interface developers use can therefore remain focused on the requested outcome while the Composition handles the implementation.
Function approaches can include YAML patch-and-transform, templates, CUE, KCL, Python, or custom code. Their exact compatibility and configuration depend on the Crossplane version and installed functions, so check the documentation for the version you run before choosing an implementation. Avoid exposing implementation-specific details as API guarantees unless you intend to support them as part of the developer contract.
Keep the project reviewable and render it locally
A Crossplane project can keep API definitions, Compositions, embedded functions, examples, and metadata together. This makes the platform API and its implementation easier to review as one unit. The CLI project workflow supports rendering a Composition locally so you can preview its output before applying it to a Kubernetes environment.
- Define the XRD schema and include example XR requests that show valid inputs.
- Write the Composition and function pipeline that translates those requests into composed resources.
- Use the Crossplane CLI project workflow to render the Composition locally and inspect the generated resources. Follow the commands and requirements in the Crossplane CLI guide for your installed version.
- Install and exercise the API and Composition in a suitable Kubernetes environment, then inspect reconciliation, readiness, access permissions, and resource lifecycle behavior.
Local rendering is a preview of composition output; it does not by itself establish that a full deployment will reconcile successfully in every cluster. Treat environment testing and operational checks as platform-team responsibilities rather than as guarantees of the rendering workflow.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Grant only the permissions the pipeline needs
Crossplane’s service account can create, update, and delete resources installed by providers or defined by an XRD. Other resource kinds that a Composition pipeline manages need an additional RBAC grant. This is not unrestricted authority over every object in the cluster, but it does make the service account’s effective permissions an important part of the platform design. See the Composition and permissions documentation.
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Before enabling a new composed kind, identify which account performs the operation and grant the necessary permissions deliberately. Review those grants alongside the Composition so changes to the resources a pipeline creates do not silently outgrow the access model.
Expose scaling only when the API maps it
An XRD can expose Kubernetes’ scale subresource. When its scale mapping connects the appropriate XR fields, standard consumers such as kubectl scale, the Horizontal Pod Autoscaler, and KEDA can control the composite resource. This depends on configuring the mapping; merely defining an XRD does not make every XR scalable. Crossplane’s guide explains the configuration: Scaling composite resources.
Make the API a supported contract
A self-service API works best when developers can understand what they are requesting and operators can evolve how it is delivered without surprising callers. Keep the schema intentional, document the meaning of fields, and test both the rendered resources and the behavior in the target environment. Crossplane supplies the Kubernetes API and composition framework; the platform team remains responsible for scope, policy, permissions, and the contract it chooses to support.
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