You may have a requirement that you need to connect to a Kubernetes cluster from your Go application. Thankfully the Kubernetes team provides a way to do that with client-go. But how can you use this to connect to the cluster?
There is a slight difference if your code will be running external or internal to the cluster. Let’s take a look at these different approaches.
Connect from outside of the cluster
To connect to your Kubernetes cluster, you can do something similar:
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package main
import (
"context"
"fmt"
"os"
"path/filepath"
v1 "k8s.io/apimachinery/pkg/apis/meta/v1"
"k8s.io/client-go/kubernetes"
"k8s.io/client-go/tools/clientcmd"
)
func main() {
fmt.Println("Get Kubernetes pods")
userHomeDir, err := os.UserHomeDir()
if err != nil {
fmt.Printf("error getting user home dir: %v\n", err)
os.Exit(1)
}
kubeConfigPath := filepath.Join(userHomeDir, ".kube", "config")
fmt.Printf("Using kubeconfig: %s\n", kubeConfigPath)
kubeConfig, err := clientcmd.BuildConfigFromFlags("", kubeConfigPath)
if err != nil {
fmt.Printf("error getting Kubernetes config: %v\n", err)
os.Exit(1)
}
clientset, err := kubernetes.NewForConfig(kubeConfig)
if err != nil {
fmt.Printf("error getting Kubernetes clientset: %v\n", err)
os.Exit(1)
}
pods, err := clientset.CoreV1().Pods("kube-system").List(context.Background(), v1.ListOptions{})
if err != nil {
fmt.Printf("error getting pods: %v\n", err)
os.Exit(1)
}
for _, pod := range pods.Items {
fmt.Printf("Pod name: %s\n", pod.Name)
}
}
Walking through this, we first need to get our Kubernetes config:
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userHomeDir, err := os.UserHomeDir()
if err != nil {
fmt.Printf("error getting user home dir: %v\n", err)
os.Exit(1)
}
kubeConfigPath := filepath.Join(userHomeDir, ".kube", "config")
In my case here I just assume that it is located in ~/.kube/config. That is the default, but if you want to have some flexibility to your config location you will likely use a flag to override the default.
Now we can create the config object:
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kubeConfig, err := clientcmd.BuildConfigFromFlags("", kubeConfigPath)
With the config, we can create the clientset:
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clientset, err := kubernetes.NewForConfig(kubeConfig)
if err != nil {
fmt.Printf("error getting Kubernetes clientset: %v\n", err)
os.Exit(1)
}
Finally, once you have the clientset you can access different API groups and resource types. In my case for this example I just list the pods in the kube-system namespace:
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pods, err := clientset.CoreV1().Pods("kube-system").List(context.Background(), v1.ListOptions{})
if err != nil {
fmt.Printf("error getting pods: %v\n", err)
os.Exit(1)
}
for _, pod := range pods.Items {
fmt.Printf("Pod name: %s\n", pod.Name)
}
The output is as expected:
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Get Kubernetes pods
Using kubeconfig: /home/trstringer/.kube/config
Pod name: coredns-558bd4d5db-77b5b
Pod name: coredns-558bd4d5db-sskcx
Pod name: etcd-kind-control-plane
Pod name: kindnet-xg859
Pod name: kube-apiserver-kind-control-plane
Pod name: kube-controller-manager-kind-control-plane
Pod name: kube-proxy-vpp5d
Pod name: kube-scheduler-kind-control-plane
Connect from inside the cluster
The only different when connecting from within the cluster is how the config is retrieved. You will use k8s.io/client-go/rest and make a call to rest.InClusterConfig(). After error checking, pass that config output to kubernetes.NewForConfig() and the rest of the code is the same.
It’s common to have some logic to first try to connect as if the application is inside the cluster and if that fails attempt to connect as an external process.
Summary
With client-go you can easily connect your Go code to Kubernetes. Next time we’ll look at how to test this type of code!