How to Watch CustomResourceDefinitions (CRDs) with Client-Go?

Watch CustomResourceDefinitions (CRDs) with Client-Go

CustomResourceDefinitions (CRDs) extend Kubernetes' core API to allow users to create and manage their own resource types. To watch for changes to CRDs, you can utilize client-go, a Kubernetes client library.

Client-Go for Standard Resources

Client-go provides a straightforward mechanism to watch for changes in standard resources like Services. The following example demonstrates how to watch for new or modified Services:

<code class="go">import (
    "k8s.io/api/core/v1"
    metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
    "k8s.io/client-go/kubernetes"
    "k8s.io/client-go/tools/clientcmd"
)

func handleNewServices(clientset *kubernetes.Clientset) {
    for {
        serviceStreamWatcher, err := clientset.CoreV1().Services("").Watch(metav1.ListOptions{})
        if err != nil {
            panic(err.Error())
        }
        
        // Process events
        // ...
    }
}</code>

Extending Client-Go for CRDs

Client-go does not natively recognize CRDs. To support them, you need to generate a client for your custom resources. Kubernetes provides code-generation tools for this purpose.

For instance, to create a client for the ApiGateway CRD defined in the provided snippet, follow the steps outlined in [this blog post](link to blog post).

Code Generation

1. Install code-generation tools: go install k8s.io/code-generator/cmd/client-gen

2. Generate the client: Run the following command, replacing your-group with the group of your CRD:

   client-gen --input-base "" --input your-group/v1 --output-base ./pkg --output-package pkg/clientset/versioned --clientset-name versioned

This will generate the necessary API and client structs in the pkg directory.

Controller Example

Refer to [this sample controller](link to sample controller) for an example of how to watch for your CRD using the generated client. The example_controller package contains the code to handle watch events.

Kubebuilder

To simplify the process of generating client configs and controllers for CRDs, you can use [kubebuilder](link to kubebuilder). This tool automates many of the steps described above.

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