Developing Kubernetes Operators and Controllers

Tutorial 2 of 5

Developing Kubernetes Operators and Controllers

1. Introduction

Goal of the Tutorial

This tutorial aims to provide a comprehensive introduction to Kubernetes Operators and Controllers, demonstrating how to develop and manage them effectively.

What You Will Learn

You'll learn about the following topics:
- The role and importance of Operators and Controllers in Kubernetes
- How to create and manage Custom Controllers and Operators
- Writing code for Operators and Custom Controllers
- Best practices for developing and managing Kubernetes Operators and Controllers

Prerequisites

Basic understanding of Kubernetes and its architecture
Programming experience, particularly in Go, as it is the primary language for writing Kubernetes Operators and Controllers

2. Step-by-Step Guide

Concepts

Controllers: In Kubernetes, a Controller is a control loop that watches the shared state of the cluster through the apiserver and makes changes attempting to move the current state towards the desired state.
Operators: An Operator in Kubernetes is a method of packaging, deploying, and managing a Kubernetes application.

Best Practices and Tips

It's recommended to use existing Operators and Controllers if they fit your use case, before opting to develop custom ones.
When developing custom Operators and Controllers, ensure they are designed to handle failures and are scalable.
Make use of client-go library for creating Controllers, as it provides tools to interact with Kubernetes API.

3. Code Examples

Example: Creating a Custom Controller

package main

import (
    "fmt"
    "time"
    "k8s.io/client-go/informers"
    "k8s.io/client-go/tools/cache"
)

func main() {
    // Set up informer factory
    factory := informers.NewSharedInformerFactory(client, time.Hour*24)

    // Create a new informer for Pod objects
    informer := factory.Core().V1().Pods().Informer()

    // Set up an event handler for when Pod resources change
    informer.AddEventHandler(cache.ResourceEventHandlerFuncs{
        AddFunc: func(obj interface{}) {
            fmt.Println("Pod created: %s", obj)
        },
        DeleteFunc: func(obj interface{}) {
            fmt.Println("Pod deleted: %s", obj)
        },
    })

    // Start the informer
    informer.Run(stopCh)
}

This code creates a simple Kubernetes Controller that watches for changes to Pod resources and logs when a Pod is created or deleted.

4. Summary

This tutorial covered the following topics:
- Understanding of Kubernetes Operators and Controllers
- Creation and management of Custom Controllers and Operators
- Coding for Operators and Custom Controllers

Next Steps for Learning

Explore more about built-in Kubernetes Controllers (ReplicationController, DaemonSet, etc.)
Learn more about Operator SDK for creating Operators

Additional Resources

5. Practice Exercises

Exercise 1: Create a Controller that watches for changes to Service resources and logs when a Service is created or deleted.

Exercise 2: Develop an Operator that manages a simple stateful application.

Tips for Further Practice

Try altering the code for different kinds of resources like Deployments, ConfigMaps etc.
Experiment with complex stateful applications for your Operators.