How to Monitor Kubernetes Service Events Using the Go Client?

Watching Kubernetes Service Events with Go Client

Problem:

How to monitor and receive notifications when a service is created, deleted, or updated in Kubernetes using the client-go library?

Solution:

Informer and ListWatch

To watch service events, we can use watchlist and informer from the client-go library. A watchlist allows us to create a watcher to monitor specified resources, while an informer provides a higher-level interface that handles the watch process.

Here's how to watch service events using client-go:

package main

import (
    "fmt"
    "flag"
    "time"

    k8sclient "k8s.io/client-go/kubernetes"
    corev1 "k8s.io/client-go/pkg/api/v1"
    "k8s.io/client-go/tools/clientcmd"
    "k8s.io/client-go/tools/cache"
    "k8s.io/client-go/pkg/fields"
)

func main() {
    var kubeconfig = flag.String("kubeconfig", "./config", "path to the kubeconfig file")
    flag.Parse()

    config, err := clientcmd.BuildConfigFromFlags("", *kubeconfig)
    if err != nil {
        panic(err.Error())
    }

    clientset, err := k8sclient.NewForConfig(config)
    if err != nil {
        panic(err.Error())
    }

    watchlist := cache.NewListWatchFromClient(clientset.Core().RESTClient(), "services", corev1.NamespaceDefault, fields.Everything())

    _, controller := cache.NewInformer(
        watchlist,
        &corev1.Service{},
        time.Second * 0,
        cache.ResourceEventHandlerFuncs{
            AddFunc: func(obj interface{}) {
                fmt.Printf("Service added: %v\n", obj)
            },
            DeleteFunc: func(obj interface{}) {
                fmt.Printf("Service deleted: %v\n", obj)
            },
            UpdateFunc: func(oldObj, newObj interface{}) {
                fmt.Printf("Service updated: %v\n", oldObj)
            },
        },
    )

    stop := make(chan struct{})
    go controller.Run(stop)
    for {
        time.Sleep(time.Second)
    }
}

Explanation:

We first create a watchlist using NewListWatchFromClient, specifying the resource type (services), namespace (default), and field selector (everything). Then, we create an informer using NewInformer and provide a list of event handlers to handle the different events (add, delete, update). Within the event handlers, we print the appropriate message when a service is added, deleted, or updated. Finally, we start the watch by running the controller in a goroutine. This code will continuously monitor service events and print the details of any changes.

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