Crossplane

Overview

Crossplane is an open-source Kubernetes add-on that extends the Kubernetes API to provision and manage cloud infrastructure. Instead of using separate IaC tools, you define cloud resources as Kubernetes custom resources and manage them with kubectl, GitOps, and the Kubernetes reconciliation loop.

Core Concepts

Providers

Install cloud providers to manage external resources:

yaml

apiVersion: pkg.crossplane.io/v1
kind: Provider
metadata:
  name: provider-aws
spec:
  package: xpkg.upbound.io/upbound/provider-aws-s3:v1.7.0

Managed Resources

Cloud resources represented as Kubernetes objects:

yaml

apiVersion: s3.aws.upbound.io/v1beta2
kind: Bucket
metadata:
  name: my-app-assets
spec:
  forProvider:
    region: us-east-1
    tags:
      Environment: prod
  providerConfigRef:
    name: aws-config

Composite Resources (XRDs + Compositions)

Define custom APIs that abstract cloud details:

CompositeResourceDefinition (XRD)

yaml

apiVersion: apiextensions.crossplane.io/v1
kind: CompositeResourceDefinition
metadata:
  name: xdatabases.platform.example.com
spec:
  group: platform.example.com
  names:
    kind: XDatabase
    plural: xdatabases
  claimNames:
    kind: Database
    plural: databases
  versions:
    - name: v1alpha1
      served: true
      referenceable: true
      schema:
        openAPIV3Schema:
          type: object
          properties:
            spec:
              type: object
              properties:
                engine:
                  type: string
                  enum: [postgres, mysql]
                size:
                  type: string
                  enum: [small, medium, large]
              required: [engine, size]

Composition

yaml

apiVersion: apiextensions.crossplane.io/v1
kind: Composition
metadata:
  name: xdatabases.aws.platform.example.com
spec:
  compositeTypeRef:
    apiVersion: platform.example.com/v1alpha1
    kind: XDatabase
  resources:
    - name: rds-instance
      base:
        apiVersion: rds.aws.upbound.io/v1beta2
        kind: Instance
        spec:
          forProvider:
            engine: postgres
            instanceClass: db.t3.micro
            allocatedStorage: 20
      patches:
        - fromFieldPath: spec.engine
          toFieldPath: spec.forProvider.engine
        - type: FromCompositeFieldPath
          fromFieldPath: spec.size
          toFieldPath: spec.forProvider.instanceClass
          transforms:
            - type: map
              map:
                small: db.t3.micro
                medium: db.t3.medium
                large: db.t3.large

Claims

Application teams request infrastructure using the custom API:

yaml

apiVersion: platform.example.com/v1alpha1
kind: Database
metadata:
  name: orders-db
  namespace: orders
spec:
  engine: postgres
  size: medium

Installation

bash

# Install Crossplane via Helm
helm repo add crossplane-stable https://charts.crossplane.io/stable
helm install crossplane crossplane-stable/crossplane \
  --namespace crossplane-system --create-namespace

Best Practices

•Use Compositions + Claims to create platform APIs that abstract cloud-specific details from application teams.
•Use GitOps (Argo CD, Flux) to manage Crossplane resources declaratively.
•Pin provider versions for reproducible infrastructure.
•Use providerConfigRef to separate credentials per environment.
•Use patches and transforms in Compositions to map simple claim inputs to complex cloud configurations.
•Monitor resource health with kubectl get managed — Crossplane continuously reconciles desired vs actual state.