CRD

🔍 What is a CRD in Kubernetes?

A CRD (Custom Resource Definition) allows you to extend Kubernetes with your own custom resource types—like MyDatabase, BackupJob, or AppConfig—just like built-in types such as Pod, Service, or Deployment.

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❌ Problem

Kubernetes only knows how to manage built-in resources (e.g., Pods, Services, ConfigMaps). But real-world use cases often need custom domain-specific objects, like:

• Defining a KafkaCluster

• Creating a PostgreSQL database with parameters

• Representing a Backup or Restore job

If you want Kubernetes to manage such non-native resources, you’re out of luck… unless you can teach Kubernetes about them.

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✅ Solution: Custom Resource Definition (CRD)

A CRD lets you define a new resource type in Kubernetes.

After defining it, you can use kubectl to create, read, update, and delete your custom resources, just like you do with native resources.

What It Enables:

• Kubernetes-native API for your custom objects

• GitOps-style config management for app/domain logic

• Combine with a controller or Operator for automation

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🌍 Real-World Example

🐘 Example: Define a PostgreSQL Cluster as a Custom Resource

CRD YAML (Postgresql CRD definition):

yamlCopyEditapiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: postgresqls.acid.zalan.do
spec:
  group: acid.zalan.do
  names:
    kind: postgresql
    listKind: postgresqlList
    plural: postgresqls
    singular: postgresql
  scope: Namespaced
  versions:
    - name: v1
      served: true
      storage: true
      schema:
        openAPIV3Schema:
          type: object
          properties:
            spec:
              type: object

Custom Resource (an instance of Postgresql):

yamlCopyEditapiVersion: acid.zalan.do/v1
kind: postgresql
metadata:
  name: my-database
spec:
  teamId: "myteam"
  numberOfInstances: 2
  postgresql:
    version: "15"
  volume:
    size: 5Gi

Now you can run: kubectl apply -f my-postgres.yaml And Kubernetes treats it like a native resource!

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🧠 Summary

Concept	Description
CRD	Schema definition for a new custom resource (e.g., KafkaCluster)
Custom Resource (CR)	An actual object created from that schema (e.g., my-kafka-cluster)
Why use CRD?	To represent domain-specific objects that Kubernetes doesn’t support natively
Bonus	Combine with a controller/operator to automate behavior (e.g., scaling DB)

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🛠 Common Real-Life CRDs You’ll See

CRD	What it Does
Prometheus (via Prometheus Operator)	Defines monitoring targets, rules, alerts
Certificate (via Cert-Manager)	Automatically manages SSL certificates
ArgoCDApplication	Defines GitOps-based app deployments
KafkaCluster	Represents a Kafka cluster in Kubernetes
Backup / Restore	Used by Velero or other backup tools

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