Creating a Sample Kubernetes Controller

Step 1: Define the API Package

Create the API definition files under the directory structure:

pkg/api/<groupname>/<version>/

Inside this folder, create the following files:

• types.go — Define your Custom Resource (CR) Go structs here.
• doc.go — Package documentation and API group metadata.
• register.go — Register your types with the scheme.

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Step 2: Generate Boilerplate Code

Use Kubernetes code generators to generate necessary deepcopy functions, clientsets, listers, and informers.

• Ensure you have the boilerplate files such as boilerplate.go.txt, tools.go, and the script update-codegen.sh.
• Run the code generator script:

bash hack/update-codegen.sh

This generates all the required client and deepcopy code based on your API definitions.

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Step 3: Generate the CustomResourceDefinition (CRD)

Use the controller-gen tool to generate CRD YAML manifests from your API types.

1. Install controller-gen if you haven't:

go install sigs.k8s.io/controller-tools/cmd/controller-gen@latest

2. Generate CRDs and output them to the manifests directory:

go run sigs.k8s.io/controller-tools/cmd/controller-gen crd paths=./pkg/api/... output:crd:dir=./manifests

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Step 4: Implement the Controller Logic

• Implement the reconciliation logic for your controller.
• Define event handlers and business logic to react to changes in your custom resources.

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Step 5: Register the Controller with the Manager

In your main.go:

• Configure the Kubernetes client and clientset.
• Create shared informer factories.
• Initialize your controller.
• Register the controller with the controller-runtime manager.