Informer In Go Watch Resource Changes In Custom Resource

MIster jashon

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Oct 04, 2024

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Keeping an Eye on Your Custom Resources: Informer in Go

In the dynamic world of Kubernetes, managing and responding to changes in your custom resources is crucial. This is where the informer comes into play, a powerful tool within the Go language that allows you to watch for changes in your custom resources and react accordingly.

What is an Informer?

An informer is a Go library component that simplifies the process of watching for changes in Kubernetes resources. It provides a convenient and efficient way to monitor your custom resources and handle events like additions, deletions, and updates. Think of it as a vigilant guard keeping a close watch over your custom resources, alerting you to any changes.

How does it work?

The informer uses a combination of the Kubernetes API and a local cache to monitor your custom resources. Here's a breakdown of how it operates:

1. List and Watch: The informer initially lists all existing instances of your custom resource from the Kubernetes API server. It then starts watching for any changes to these resources.
2. Local Cache: The informer maintains a local cache of the resources it is watching. This cache acts as a snapshot of the current state of your resources.
3. Event Handling: Whenever a change occurs in your custom resources, the Kubernetes API server sends an event to the informer.
4. Cache Updates: The informer updates its local cache based on the received event. This ensures that the cache remains synchronized with the actual state of the resources.
5. Triggering Actions: You can define callback functions or handlers to be executed whenever the informer detects a change in the cache. This allows you to respond to events like new resource creation, resource deletion, or resource updates.

Why use Informer?

Using an informer for monitoring your custom resources offers several advantages:

• Efficiency: The informer's local cache prevents unnecessary API calls, resulting in improved performance and reduced resource consumption.
• Simplicity: The informer provides a streamlined API for handling resource changes, making it easier to write your custom logic.
• Reliability: The informer ensures consistency between your local view of resources and the actual state in the Kubernetes cluster.

Implementing Informer

Let's illustrate how to implement an informer in your Go application:

1. Dependencies:

Make sure you have the necessary Kubernetes client libraries installed:

go get k8s.io/client-go/informers
go get k8s.io/client-go/kubernetes

2. Create Informer Factory:

Instantiate an Informer Factory to create informers for your custom resources.

import (
    "k8s.io/client-go/informers"
    metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
    "k8s.io/client-go/kubernetes"
    "k8s.io/client-go/rest"
)

// Create a new Kubernetes clientset
config, err := rest.InClusterConfig() 
if err != nil {
    // Handle error
}
clientset, err := kubernetes.NewForConfig(config)
if err != nil {
    // Handle error
}

// Create an informer factory
factory := informers.NewSharedInformerFactory(clientset, 0)

3. Define Informer:

Create an informer for your custom resource:

informer := factory.CustomResources().V1().Resource("my-resource").Informer()

4. Set Up Event Handler:

Define a handler function to be executed when the informer detects a change in the cache:

informer.AddEventHandler(func(obj interface{}) {
    // Handle the event here
    switch t := obj.(type) {
    case *v1.MyResource:
        // Process the newly added or updated MyResource object
        // ...
    case *v1.MyResource:
        // Process the deleted MyResource object
        // ...
    }
})

5. Start Informer:

Start the informer to begin monitoring your custom resources.

factory.Start(make(chan struct{}))

Example:

Let's assume you have a custom resource called MyResource with the following definition:

apiVersion: mygroup.mydomain.com/v1
kind: MyResource
metadata:
  name: my-resource
spec:
  # ... resource spec ...

You can use the informer to watch for changes in this resource and perform actions based on those changes. Here's a simplified example:

package main

import (
	"context"
	"fmt"
	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
	"k8s.io/client-go/informers"
	"k8s.io/client-go/kubernetes"
	"k8s.io/client-go/rest"
	"k8s.io/client-go/tools/cache"
)

// MyResource is a placeholder for your custom resource definition
type MyResource struct {
	metav1.TypeMeta   `json:",inline"`
	metav1.ObjectMeta `json:"metadata,omitempty"`
	Spec              MyResourceSpec `json:"spec,omitempty"`
}

type MyResourceSpec struct {
	// ... your resource spec definition ...
}

func main() {
	config, err := rest.InClusterConfig()
	if err != nil {
		panic(err)
	}
	clientset, err := kubernetes.NewForConfig(config)
	if err != nil {
		panic(err)
	}

	// Create an informer factory
	factory := informers.NewSharedInformerFactory(clientset, 0)

	// Define informer for MyResource
	informer := factory.CustomResources().V1().Resource("my-resource").Informer()

	// Add event handler
	informer.AddEventHandler(cache.ResourceEventHandlerFuncs{
		AddFunc: func(obj interface{}) {
			myResource := obj.(*MyResource)
			fmt.Printf("New MyResource created: %s\n", myResource.GetName())
		},
		DeleteFunc: func(obj interface{}) {
			myResource := obj.(*MyResource)
			fmt.Printf("MyResource deleted: %s\n", myResource.GetName())
		},
		UpdateFunc: func(oldObj, newObj interface{}) {
			oldMyResource := oldObj.(*MyResource)
			newMyResource := newObj.(*MyResource)
			fmt.Printf("MyResource updated: %s\n", newMyResource.GetName())
			// Compare oldMyResource and newMyResource for changes
		},
	})

	// Start informer
	factory.Start(make(chan struct{}))

	// Wait for the informer to sync
	if !cache.WaitForCacheSync(context.Background(), informer.HasSynced) {
		panic("Timed out waiting for caches to sync")
	}

	// Continue with your application logic here
	// ...
}

Conclusion

The informer is a powerful tool in your Go toolkit when dealing with custom resources in Kubernetes. Its ability to efficiently watch for changes and provide a consistent view of your resources makes it an essential component for building robust and responsive Kubernetes applications.

By leveraging the informer, you can monitor your custom resources, react to events, and keep your applications synchronized with the dynamic nature of the Kubernetes environment. This ensures your applications are always aware of changes and can adapt accordingly.