Understanding Kubernetes Custom Resources: Extending Kubernetes Capabilities

Introduction

Kubernetes provides a powerful mechanism to extend its core functionality through Custom Resources, Custom Resource Definitions (CRDs), and Custom Controllers. This article breaks down these concepts and explains how they allow you to enhance Kubernetes beyond its native resources.

What are Custom Resources?

Custom Resources are extensions of the Kubernetes API that allow you to define and use new types of resources in your Kubernetes cluster. Unlike built-in resources like Deployments or Services, custom resources let you:

• Add specialized functionality

• Implement domain-specific features

• Extend Kubernetes capabilities for specific use cases

Key Components of Custom Resources

1. Custom Resource Definition (CRD)

• A way to define a new type of resource in Kubernetes

• Specifies the structure and validation rules for the custom resource

• Similar to a schema or blueprint for your new resource type

2. Custom Resource (CR)

• An actual instance of the custom resource

• Created according to the rules defined in the CRD

• Used by users to leverage the new functionality

3. Custom Controller

• Watches and manages the custom resources

• Implements the actual logic and behavior for the custom resource

• Typically written in Go, using client-go and controller-runtime libraries

Real-world Example: Istio Service Mesh

The speaker used Istio as a practical example of custom resources:

• Istio introduces resources like VirtualService

• CRD defines the structure of VirtualService

• Istio's custom controller manages these resources

• Users can create VirtualService resources to configure service mesh behavior

Deployment Process

Implementing custom resources involves three main steps:

1. Deploy Custom Resource Definition (CRD)

   • DevOps engineers deploy the CRD to extend Kubernetes API

   • Can be done via Kubernetes manifests, Helm charts, or operators

2. Deploy Custom Controller

   • Implements the logic for managing the custom resource

   • Watches for changes and takes appropriate actions

   • Often uses frameworks like Kubernetes controller-runtime

3. Create Custom Resources

   • Users or developers create instances of the custom resource

   • Validated against the CRD

   • Processed by the custom controller

Writing Custom Controllers

Preferred Technologies

• Go is the most popular language for writing Kubernetes controllers

• Reasons include:

  • Kubernetes is written in Go

  • Strong concurrency support

  • Robust client-go library

  • Well-supported in the CNCF ecosystem

High-level Controller Development

• Use client-go to interact with Kubernetes API

• Set up watchers and listeners

• Create worker queues

• Process resources and implement desired state

Popular Custom Resource Projects

Many Cloud Native Computing Foundation (CNCF) projects implement custom resources:

• Argo CD

• Istio

• Prometheus

• CoreDNS

• Crossplane

Best Practices for DevOps Engineers

• Understand the documentation of custom resource systems

• Know how to deploy CRDs and controllers

• Be able to debug and troubleshoot custom resource issues

• Continuously learn about new custom resource implementations

Conclusion

Custom Resources, CRDs, and Custom Controllers provide a powerful extension mechanism for Kubernetes. They allow organizations to create more specialized, domain-specific Kubernetes experiences while maintaining the core Kubernetes workflow.

Code Example Resources

• Kubernetes sample controller: Recommended for learning

• Official Kubernetes documentation

• GitHub repositories of CNCF projects

Learning Paths

• Start with understanding the concepts

• Study sample controllers

• Practice deploying and managing custom resources

• Explore CNCF project implementations

Call to Action

• Experiment with custom resources in your Kubernetes clusters

• Contribute to open-source custom resource projects

• Keep learning and expanding your Kubernetes skills