Crossplane extends Kubernetes to manage cloud infrastructure via CRDs, enabling platform teams to build self-service infrastructure APIs. Understanding Compositions, XRDs, Claims, and providers is essential for Kubernetes-native infrastructure management.
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What is the role of a CompositeResourceDefinition (XRD) in Crossplane?
A CompositeResourceDefinition (XRD) is a CRD that defines a new custom API type for your platform. For example, an XRD might define a PostgreSQLInstance type with fields like storageGB and version. Platform teams design these APIs; application teams create instances (Claims) of them without needing to know the underlying cloud resource details.
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What is the relationship between a Crossplane Composition and a CompositeResource (XR)?
A Composition is the implementation logic — it defines which managed resources to create and how to map XR fields to their properties. When an application team creates an XR (or Claim), Crossplane uses the matching Composition to provision the actual cloud resources (e.g., RDS instance, VPC, security group) on behalf of the requester.
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What is the difference between a Crossplane Claim and a CompositeResource (XR)?
A Claim is a namespace-scoped proxy that maps to a cluster-scoped XR. Application teams create Claims in their namespace (e.g., a PostgreSQL claim in the app-team namespace). Crossplane creates a corresponding cluster-scoped XR and binds them. This separation allows platform teams to control cluster-level resources while app teams work in their namespace.
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A Crossplane Composition uses transforms with type: string and fmt: 'db-%s'. What does this accomplish?
Crossplane Composition transforms manipulate field values before setting them on managed resources. A string transform with fmt: 'db-%s' formats the input value into the pattern (e.g., input 'prod' → 'db-prod'). Transforms enable deriving resource names, labels, and configs from XR fields without requiring changes to managed resource providers.
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What does the Crossplane provider component do in a Crossplane installation?
A Crossplane provider (e.g., provider-aws, provider-gcp) is a package containing controllers for cloud-specific managed resources. Each controller reconciles a managed resource CRD (e.g., RDSInstance) by making API calls to the target cloud. Providers are installed as packages, extending Crossplane's capabilities for each cloud platform.