CRDs & Operators

Custom Resource Definitions

What is a CRD?

• Extends the Kubernetes API with new resource types
• Once a CRD is created, you can create/read/update/delete custom resources (CRs) using kubectl
• No need to modify or recompile the API server

CRD Manifest

apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: backups.stable.example.com    # must be <plural>.<group>
spec:
  group: stable.example.com
  versions:
    - name: v1
      served: true
      storage: true
      schema:
        openAPIV3Schema:
          type: object
          properties:
            spec:
              type: object
              properties:
                schedule:
                  type: string
                retentionDays:
                  type: integer
  scope: Namespaced                     # or Cluster
  names:
    plural: backups
    singular: backup
    kind: Backup
    shortNames:
      - bk

Creating a CRD

kubectl apply -f backup-crd.yaml

# verify the CRD is registered
kubectl get crd backups.stable.example.com

Creating a Custom Resource

apiVersion: stable.example.com/v1
kind: Backup
metadata:
  name: daily-backup
spec:
  schedule: "0 2 * * *"
  retentionDays: 30

kubectl apply -f daily-backup.yaml

# list custom resources
kubectl get backups
kubectl get bk          # using shortName

# describe a custom resource
kubectl describe backup daily-backup

# delete a custom resource
kubectl delete backup daily-backup

Operators

What is an Operator?

• A pattern that combines a CRD with a custom controller
• The controller watches for changes to custom resources and takes action (reconciliation loop)
• Encodes operational knowledge (install, upgrade, backup, scaling) into software

How Operators Work

1. You define a CRD (e.g., PostgresCluster)
2. The operator's controller watches for PostgresCluster resources
3. When you create/update/delete a PostgresCluster, the controller reacts:
   • Creates Deployments, Services, ConfigMaps, PVCs as needed
   • Handles upgrades, failover, backups automatically

Common Operators

• cert-manager — automates TLS certificate management
• Prometheus Operator — manages Prometheus monitoring stack
• Strimzi — manages Apache Kafka clusters

Useful Commands

# list all CRDs in the cluster
kubectl get crd

# describe a CRD
kubectl describe crd <crd-name>

# list custom resources of a type
kubectl get <resource-name>

# delete a CRD (also deletes all its custom resources!)
kubectl delete crd <crd-name>

# check API resources (includes CRDs)
kubectl api-resources | grep <group>

Notes

1. Deleting a CRD deletes all custom resources of that type — be careful.
2. CRDs alone don't do anything — you need a controller (operator) to act on the custom resources.