Concepts

Crossplane extends Kubernetes allowing it to create and manage resources external to the Kubernetes cluster. Crossplane enables platform engineers to create custom APIs and abstractions combining both native Kubernetes resources and cloud resources under a single control plane.

With custom APIs, the platform users, like developers, don’t need to know any details about the underlying resources or requirements.

The platform users only need to know the details exposed by the platform, like big or small or US or EU. Platform users don’t need to know any details about the underlying provider like instance type or region names.

Crossplane uses multiple core components to manage the various elements of building and managing external resources through Kubernetes.

• The Crossplane pods include the core Crossplane pod and Crossplane RBAC manager pod. Together these pods manage all Crossplane components and resources.

• Providers connect Kubernetes to any external provider, like AWS, Azure or GCP. Providers translate Kubernetes native manifests and API calls into external API calls. Providers are responsible for creating, deleting and managing the lifecycle of their resources.

• Managed resources are Kubernetes objects representing things the Provider created outside of Kubernetes. Creating a managed resource in Kubernetes requires a Provider to create a resource. Deleting a managed resource requires a Provider to delete the associated external resource.

• Compositions are a template of managed resources. Compositions describe more complex deployments, combining multiple managed resources and any resource customizations, like the size of a database or the cloud provider region.

• Composite Resource Definitions represent a custom API, created by platform engineers and consumed by developers or end users. Composite resource definitions use an OpenAPIv3 schema to further extend Kubernetes with custom API endpoints, revisions and more.

• Composite Resources represent all the objects created by a user calling the custom API. Every time a user access the custom API Crossplane creates a single Composite Resource and links all the related managed resources to it.

• Claims are like Composite Resources, but exist in a Kubernetes namespace. Every Claim links to a single cluster scoped Composite Resource. Platform users create Claims in their unique namespace, isolating their resources from other teams in other namespaces.

• Composition Functions are custom programs, written your programming language of choice, to apply logic and loops before or after Crossplane creates resources.

• Patches and Transforms allow platform engineers to use user inputs to their custom API and change how Crossplane creates resources. Patches and transforms allow for flexible and abstract inputs like big or encrypted to have specific meanings when creating the actual managed resources.

• EnvironmentConfigs are an in-memory data store, like a Kubernetes ConfigMap. EnvironmentConfigs are useful for custom resource mapping or storing and retrieving data across Claims and Composite Resources.

• Usages defining critical resources or custom dependency mappings. Usages can prevent Crossplane from deleting or can ensure that a parent resource waits for Crossplane to delete all child resources first.

• Packages are a convenient way to package up an entire custom platform and define any other Crossplane related requirements. Packages define how to install Providers, custom APIs or composition functions.

• ImageConfigs are for centralized control of the configuration of Crossplane package images.