How to switch kubernetes contexts dynamically with client-go? - kubernetes

Question

I'm building a CLI application that would allow me to run an arbitrary command in my shell against any kube cluster in my kubeconfig that matches a given regex. I want to use the official client-go package to accomplish this, but for some reason, switching kube contexts is less than intuitive. So I'm starting by modifying the example out-of-cluster program in the repo, and I'm struggling with just switching the context to the one I specify. Here is the code I started with, which gets the number of pods in the cluster loaded in the kubeconfig:
package main
import (
"context"
"flag"
"fmt"
"path/filepath"
metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
"k8s.io/client-go/kubernetes"
"k8s.io/client-go/tools/clientcmd"
"k8s.io/client-go/util/homedir"
)
func main() {
var kubeconfig *string
if home := homedir.HomeDir(); home != "" {
kubeconfig = flag.String("kubeconfig", filepath.Join(home, ".kube", "config"), "(optional) absolute path to the kubeconfig file")
} else {
kubeconfig = flag.String("kubeconfig", "", "absolute path to the kubeconfig file")
}
flag.Parse()
// use the current context in kubeconfig
config, err := clientcmd.BuildConfigFromFlags("", *kubeconfig)
if err != nil {
panic(err.Error())
}
// create the clientset
clientset, err := kubernetes.NewForConfig(config)
if err != nil {
panic(err.Error())
}
pods, err := clientset.CoreV1().Pods("").List(context.TODO(), metav1.ListOptions{})
if err != nil {
panic(err.Error())
}
fmt.Printf("There are %d pods in the test cluster\n", len(pods.Items))
}
Unfortunately, I cannot figure out how to load a specific cluster with a name as defined in my kubeconfig. I would love to have a sort of SwitchContext("cluster-name") function, but the number of Configs, ClientConfigs, RawConfigs, and restclient.Configs are confusing me. Any help would be appreciated!
System: Ubuntu 22.04, Intel, kube server version 1.23.8-gke.1900, client version 1.25.3

Answer 1

You can override the current context via NewNonInteractiveDeferredLoadingClientConfig method from clientcmd package.
package main
import (
"context"
"flag"
"fmt"
"k8s.io/client-go/rest"
"path/filepath"
metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
"k8s.io/client-go/kubernetes"
"k8s.io/client-go/tools/clientcmd"
"k8s.io/client-go/util/homedir"
)
func main() {
var kubeconfig *string
if home := homedir.HomeDir(); home != "" {
kubeconfig = flag.String("kubeconfig", filepath.Join(home, ".kube", "config"), "(optional) absolute path to the kubeconfig file")
} else {
kubeconfig = flag.String("kubeconfig", "", "absolute path to the kubeconfig file")
}
flag.Parse()
// use the current context in kubeconfig
config, err := clientcmd.BuildConfigFromFlags("", *kubeconfig)
if err != nil {
panic(err.Error())
}
// using `contextName` context in kubeConfig
contextName := "gce"
config, err = buildConfigWithContextFromFlags(contextName, *kubeconfig)
if err != nil {
panic(err)
}
// create the clientset
clientset, err := kubernetes.NewForConfig(config)
if err != nil {
panic(err.Error())
}
pods, err := clientset.CoreV1().Pods("").List(context.TODO(), metav1.ListOptions{})
if err != nil {
panic(err.Error())
}
fmt.Printf("There are %d pods in the test cluster\n", len(pods.Items))
}
func buildConfigWithContextFromFlags(context string, kubeconfigPath string) (*rest.Config, error) {
return clientcmd.NewNonInteractiveDeferredLoadingClientConfig(
&clientcmd.ClientConfigLoadingRules{ExplicitPath: kubeconfigPath},
&clientcmd.ConfigOverrides{
CurrentContext: context,
}).ClientConfig()
}