Dynamically add label to Pods in Kubernetes with Client-go - kubernetes

Question

I want to create a controller and listen to the pod events when new pod is created (by a deployment) then add all labels belong to deployment to the created pod, is this possible at scale with client-go?

Answer 1

In order to observe pod events, you need to use informers. Informers have built-in optimizations to avoid overloading API servers.
There is a patch method available in the PodInterface that allows you to add a label to a pod.
Here is a sample code for your reference. In the main function, the informer code is added, and the LabelPod function implements the label logic.
package main
import (
"context"
"encoding/json"
"fmt"
"time"
v1 "k8s.io/api/core/v1"
metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
"k8s.io/apimachinery/pkg/labels"
"k8s.io/apimachinery/pkg/types"
"k8s.io/apimachinery/pkg/util/wait"
"k8s.io/client-go/informers"
"k8s.io/client-go/kubernetes"
"k8s.io/client-go/rest"
"k8s.io/client-go/tools/cache"
)
type patchStringValue struct {
Op string `json:"op"`
Path string `json:"path"`
Value string `json:"value"`
}
func main() {
clientSet := GetK8sClient()
labelOptions := informers.WithTweakListOptions(func(opts *metav1.ListOptions) {
opts.LabelSelector = GetLabelSelectorForDeployment("deployment-name", "namespace-name")
})
informers := informers.NewSharedInformerFactoryWithOptions(clientSet, 10*time.Second, informers.WithNamespace("namespace-name"), labelOptions)
podInformer := informers.Core().V1().Pods()
podInformer.Informer().AddEventHandler(
cache.ResourceEventHandlerFuncs{
AddFunc: handleAdd,
},
)
informers.Start(wait.NeverStop)
informers.WaitForCacheSync(wait.NeverStop)
}
func GetLabelSelectorForDeployment(Name string, Namespace string) string {
clientSet := GetK8sClient()
k8sClient := clientSet.AppsV1()
deployment, _ := k8sClient.Deployments(Namespace).Get(context.Background(), Name, metav1.GetOptions{})
labelSet := labels.Set(deployment.Spec.Selector.MatchLabels)
return string(labelSet.AsSelector().String())
}
func handleAdd(obj interface{}) {
k8sClient := GetK8sClient().CoreV1()
pod := obj.(*v1.Pod)
fmt.Println("Pod", pod.GetName(), pod.Spec.NodeName, pod.Spec.Containers)
payload := []patchStringValue{{
Op: "replace",
Path: "/metadata/labels/testLabel",
Value: "testValue",
}}
payloadBytes, _ := json.Marshal(payload)
_, updateErr := k8sClient.Pods(pod.GetNamespace()).Patch(context.Background(), pod.GetName(), types.JSONPatchType, payloadBytes, metav1.PatchOptions{})
if updateErr == nil {
fmt.Println(fmt.Sprintf("Pod %s labelled successfully.", pod.GetName()))
} else {
fmt.Println(updateErr)
}
}
func GetK8sClient() *kubernetes.Clientset {
config, err := rest.InClusterConfig()
if err != nil {
panic(err.Error())
}
// creates the clientset
clientset, err := kubernetes.NewForConfig(config)
if err != nil {
panic(err.Error())
}
return clientset
}

Answer 2

What about this??
Here are the pods before they are labeled manually:
❯ kubectl get po --show-labels
NAME READY STATUS RESTARTS AGE LABELS
nginx-deploy-6bdc4445fd-5qlhg 1/1 Running 0 13h app=nginx,pod-template-hash=6bdc4445fd
nginx-deploy-6bdc4445fd-pgkhb 1/1 Running 0 13h app=nginx,pod-template-hash=6bdc4445fd
nginx-deploy-6bdc4445fd-xdz59 1/1 Running 0 13h app=nginx,pod-template-hash=6bdc4445fd
I, here, label the pod "nginx-deploy-6bdc4445fd-5qlhg" with the label "test=2":
❯ kubectl label pod nginx-deploy-6bdc4445fd-5qlhg test=2
pod/nginx-deploy-6bdc4445fd-5qlhg labeled
Here are the pods after they are labeled manually:
❯ kubectl get po --show-labels
NAME READY STATUS RESTARTS AGE LABELS
nginx-deploy-6bdc4445fd-5qlhg 1/1 Running 0 13h app=nginx,pod-template-hash=6bdc4445fd,test=2
nginx-deploy-6bdc4445fd-pgkhb 1/1 Running 0 13h app=nginx,pod-template-hash=6bdc4445fd
nginx-deploy-6bdc4445fd-xdz59 1/1 Running 0 13h app=nginx,pod-template-hash=6bdc4445fd