Watch CRDs Using Dynamic Informers in Go

Introduction

Custom Resource Definitions (CRDs) are a powerful mechanism in Kubernetes that allows users to define their own custom resources. Dynamic informers provide a way to watch for changes to CRDs in a dynamic and flexible manner. This article will guide you through the process of using dynamic informers to watch CRDs in Go.

Why Use Dynamic Informers?

Dynamic informers offer several advantages over traditional informers:

• Improved flexibility: They can be used to watch any CRD, regardless of its definition.
• Simplified setup: The informer can be initialized with just a few lines of code, eliminating the need for complex registration processes.
• Reduced boilerplate code: Dynamic informers handle the serialization and deserialization of CRD data automatically.

How to Use Dynamic Informers

1. Import Necessary Packages

To get started, import the following packages:

import (
    "context"

    "github.com/kubernetes-sigs/controller-runtime/pkg/client"
    "github.com/kubernetes-sigs/controller-runtime/pkg/dynamicclient"
    "github.com/kubernetes-sigs/controller-runtime/pkg/informers"
)

2. Create a Dynamic Client

Next, create a dynamic client to access the Kubernetes API server:

client, err := client.New(client.Options{})
if err != nil {
    // Handle error
}

3. Create an Informer Factory

An informer factory is used to create dynamic informers. Create one using the dynamic client:

informerFactory := informers.NewDynamicSharedInformerFactory(client, 0)

4. Create a Dynamic Informer

Finally, create a dynamic informer for the desired CRD:

informer := informerFactory.ForResource(schema.GroupVersionResource{
    Group:    "example.com",
    Version:  "v1",
    Resource: "foos",
})

5. Watch for Changes

Once the informer is created, you can watch for changes to the CRD using a channel:

ch := informer.Informer().Runs(context.Background())

The channel will receive notifications whenever an object in the CRD changes. You can handle these notifications in your controller logic.

Common Mistakes to Avoid

When using dynamic informers, it is important to avoid these common mistakes:

• Using the wrong group/version/resource: Ensure that these parameters match the CRD you want to watch.
• Not handling informer errors: The informer can produce errors, so it is crucial to handle them gracefully.
• Creating multiple informers for the same CRD: This can lead to duplicate notifications and unnecessary performance overhead.

Step-by-Step Approach

To summarize the process:

1. Import necessary packages.
2. Create a dynamic client.
3. Create an informer factory.
4. Create a dynamic informer for the desired CRD.
5. Watch for changes using a channel.

Benefits of Using Dynamic Informers

Dynamic informers provide numerous benefits:

• Simplified CRD monitoring: Enables easy and efficient monitoring of changes to custom resources.
• Enhanced flexibility: Supports watching any CRD without requiring specific registration or boilerplate code.
• Improved performance: Avoids the overhead of maintaining multiple traditional informers for different CRDs.

Stories and Lessons

Story 1:

A large-scale enterprise was managing dozens of custom resources using traditional informers. This became increasingly complex and error-prone as the number of CRDs grew. By switching to dynamic informers, they were able to simplify their codebase and reduce maintenance costs.

Story 2:

A startup was developing a new controller that needed to watch multiple CRDs. Using dynamic informers allowed them to quickly get started and reduce the time spent on setting up the monitoring infrastructure.

Story 3:

A team was migrating their controller to a new platform that did not support traditional informers. By using dynamic informers, they were able to maintain the same functionality without major code changes.

Lesson: Dynamic informers offer a versatile and efficient way to watch CRDs, providing significant benefits in terms of flexibility, performance, and ease of use.

Conclusion

Dynamic informers are a powerful tool for monitoring Custom Resource Definitions in Kubernetes. They provide flexibility, simplified setup, and reduced boilerplate code, making them an essential part of any controller that interacts with CRDs. By embracing dynamic informers, you can streamline your application development and enhance the reliability of your custom resources.