Code:play 

package main

import (
	"fmt"

	"k8s.io/apimachinery/pkg/api/errors"
	"k8s.io/apimachinery/pkg/runtime/schema"
)

func main() {
	// This example demonstrates how to handle common API errors.
	// Create a NotFound error to simulate a failed API request.
	err := errors.NewNotFound(schema.GroupResource{Group: "v1", Resource: "pods"}, "my-pod")

	// The errors.IsNotFound() function checks if an error is a NotFound error.
	if errors.IsNotFound(err) {
		fmt.Println("Pod not found")
	}

	// The errors package provides functions for other common API errors, such as:
	// - errors.IsAlreadyExists(err)
	// - errors.IsConflict(err)
	// - errors.IsServerTimeout(err)

}

Output:

Pod not found

Code:play 

package main

import (
	"context"
	"fmt"

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

func main() {
	// This example demonstrates how to create a clientset for an application
	// running inside a Kubernetes cluster. It uses the pod's service account
	// for authentication.

	// rest.InClusterConfig() returns a configuration object that can be used to
	// create a clientset. It is the recommended way to configure a client for
	// in-cluster applications.
	config, err := rest.InClusterConfig()
	if err != nil {
		fmt.Printf("Error encountered: %v\n", err)
		return
	}

	// Create the clientset.
	clientset, err := kubernetes.NewForConfig(config)
	if err != nil {
		fmt.Printf("Error encountered: %v\n", err)
		return
	}

	// Use the clientset to interact with the API.

	pods, err := clientset.CoreV1().Pods("default").List(context.TODO(), metav1.ListOptions{})
	if err != nil {
		fmt.Printf("Error encountered: %v\n", err)
		return
	}

	fmt.Printf("There are %d pods in the default namespace\n", len(pods.Items))
}

Code:play 

package main

import (
	"context"
	"fmt"
	"log"
	"os"
	"os/signal"
	"path/filepath"
	"syscall"
	"time"

	"k8s.io/client-go/kubernetes"
	"k8s.io/client-go/tools/clientcmd"
	"k8s.io/client-go/tools/leaderelection"
	"k8s.io/client-go/tools/leaderelection/resourcelock"
)

func main() {
	// This example demonstrates the leader election pattern. A controller running
	// multiple replicas uses leader election to ensure that only one replica is
	// active at a time.

	// Configure the client (out-of-cluster for this example).
	var kubeconfig string
	if home := os.Getenv("HOME"); home != "" {
		kubeconfig = filepath.Join(home, ".kube", "config")
	}
	config, err := clientcmd.BuildConfigFromFlags("", kubeconfig)
	if err != nil {
		fmt.Printf("Error building kubeconfig: %s\n", err.Error())
		return
	}
	clientset, err := kubernetes.NewForConfig(config)
	if err != nil {
		fmt.Printf("Error building clientset: %s\n", err.Error())
		return
	}

	// The unique name of the controller writing to the Lease object.
	id := "my-controller"

	// The namespace and name of the Lease object.
	leaseNamespace := "default"
	leaseName := "my-controller-lease"

	// Create a context that can be cancelled to stop the leader election.
	ctx, cancel := context.WithCancel(context.Background())
	defer cancel()

	// Set up a signal handler to cancel the context on termination.
	sigCh := make(chan os.Signal, 1)
	signal.Notify(sigCh, syscall.SIGINT, syscall.SIGTERM)
	go func() {
		<-sigCh
		log.Println("Received termination signal, shutting down...")
		cancel()
	}()

	// Create the lock object.
	lock, err := resourcelock.New(resourcelock.LeasesResourceLock,
		leaseNamespace,
		leaseName,
		clientset.CoreV1(),
		clientset.CoordinationV1(),
		resourcelock.ResourceLockConfig{
			Identity: id,
		})
	if err != nil {
		fmt.Printf("Error creating lock: %v\n", err)
		return
	}

	// Create the leader elector.
	elector, err := leaderelection.NewLeaderElector(leaderelection.LeaderElectionConfig{
		Lock:          lock,
		LeaseDuration: 15 * time.Second,
		RenewDeadline: 10 * time.Second,
		RetryPeriod:   2 * time.Second,
		Callbacks: leaderelection.LeaderCallbacks{
			OnStartedLeading: func(ctx context.Context) {
				// This function is called when the controller becomes the leader.
				// You would start your controller's main logic here.
				log.Println("Became leader, starting controller.")
				// This is a simple placeholder for the controller's work.
				for {
					select {
					case <-time.After(5 * time.Second):
						log.Println("Doing controller work...")
					case <-ctx.Done():
						log.Println("Controller stopped.")
						return
					}
				}
			},
			OnStoppedLeading: func() {
				// This function is called when the controller loses leadership.
				// You should stop any active work and gracefully shut down.
				log.Printf("Lost leadership, shutting down.")
			},
			OnNewLeader: func(identity string) {
				// This function is called when a new leader is elected.
				if identity != id {
					log.Printf("New leader elected: %s", identity)
				}
			},
		},
	})
	if err != nil {
		fmt.Printf("Error creating leader elector: %v\n", err)
		return
	}

	// Start the leader election loop.
	elector.Run(ctx)
}

Code:play 

package main

import (
	"context"
	"fmt"
	"os"
	"path/filepath"

	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
	"k8s.io/client-go/kubernetes"
	"k8s.io/client-go/tools/clientcmd"
)

func main() {
	// This example demonstrates how to create a clientset for an application
	// running outside of a Kubernetes cluster, using a kubeconfig file. This is
	// the standard approach for local development and command-line tools.

	// The default location for the kubeconfig file is in the user's home directory.
	var kubeconfig string
	if home := os.Getenv("HOME"); home != "" {
		kubeconfig = filepath.Join(home, ".kube", "config")
	}

	// Create the client configuration from the kubeconfig file.
	config, err := clientcmd.BuildConfigFromFlags("", kubeconfig)
	if err != nil {
		fmt.Printf("Error encountered: %v\n", err)
		return
	}

	// Configure client-side rate limiting.
	config.QPS = 50
	config.Burst = 100

	// A clientset contains clients for all the API groups and versions supported
	// by the cluster.
	clientset, err := kubernetes.NewForConfig(config)
	if err != nil {
		fmt.Printf("Error encountered: %v\n", err)
		return
	}

	// Use the clientset to interact with the API.

	pods, err := clientset.CoreV1().Pods("default").List(context.TODO(), metav1.ListOptions{})
	if err != nil {
		fmt.Printf("Error encountered: %v\n", err)
		return
	}

	fmt.Printf("There are %d pods in the default namespace\n", len(pods.Items))
}

Code:play 

package main

import (
	"context"
	"fmt"
	"os"
	"path/filepath"

	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"

	appsv1ac "k8s.io/client-go/applyconfigurations/apps/v1"

	corev1ac "k8s.io/client-go/applyconfigurations/core/v1"

	metav1ac "k8s.io/client-go/applyconfigurations/meta/v1"
	"k8s.io/client-go/kubernetes"
	"k8s.io/client-go/tools/clientcmd"
)

func main() {
	// This example demonstrates how to use Server-Side Apply to declaratively
	// manage a Deployment object. Server-Side Apply is the recommended approach
	// for controllers and operators to manage objects.

	// Configure the client (out-of-cluster for this example).
	var kubeconfig string
	if home := os.Getenv("HOME"); home != "" {
		kubeconfig = filepath.Join(home, ".kube", "config")
	}
	config, err := clientcmd.BuildConfigFromFlags("", kubeconfig)
	if err != nil {
		fmt.Printf("Error encountered: %v\n", err)
		return
	}
	clientset, err := kubernetes.NewForConfig(config)
	if err != nil {
		fmt.Printf("Error encountered: %v\n", err)
		return
	}

	// Define the desired state of the Deployment using the applyconfigurations package.
	// This provides a typed, structured way to build the patch.
	deploymentName := "my-app"
	replicas := int32(2)
	image := "nginx:1.14.2"

	// The FieldManager is a required field that identifies the controller managing
	// this object's state.
	fieldManager := "my-controller"

	// Build the apply configuration.
	deploymentApplyConfig := appsv1ac.Deployment(deploymentName, "default").
		WithSpec(appsv1ac.DeploymentSpec().
			WithReplicas(replicas).
			WithSelector(metav1ac.LabelSelector().WithMatchLabels(map[string]string{"app": "my-app"})).
			WithTemplate(corev1ac.PodTemplateSpec().
				WithLabels(map[string]string{"app": "my-app"}).
				WithSpec(corev1ac.PodSpec().
					WithContainers(corev1ac.Container().
						WithName("nginx").
						WithImage(image),
					),
				),
			),
		)

	// Perform the Server-Side Apply patch. The PatchType must be types.ApplyPatchType.
	// The context, name, apply configuration, and patch options are required.
	result, err := clientset.AppsV1().Deployments("default").Apply(
		context.TODO(),
		deploymentApplyConfig,
		metav1.ApplyOptions{FieldManager: fieldManager},
	)

	if err != nil {
		fmt.Printf("Error encountered: %v\n", err)
		return
	}

	fmt.Printf("Deployment %q applied successfully.\n", result.Name)
}

Code:play 

package main

import (
	"context"
	"fmt"
	"os"
	"path/filepath"

	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
	"k8s.io/apimachinery/pkg/apis/meta/v1/unstructured"
	"k8s.io/apimachinery/pkg/runtime/schema"
	"k8s.io/client-go/dynamic"
	"k8s.io/client-go/tools/clientcmd"
)

func main() {
	// This example demonstrates how to create and use a dynamic client to work
	// with Custom Resources or other objects without needing their Go type
	// definitions.

	// Configure the client (out-of-cluster for this example).
	var kubeconfig string
	if home := os.Getenv("HOME"); home != "" {
		kubeconfig = filepath.Join(home, ".kube", "config")
	}
	config, err := clientcmd.BuildConfigFromFlags("", kubeconfig)
	if err != nil {
		fmt.Printf("Error encountered: %v\n", err)
		return
	}

	// Create a dynamic client.
	dynamicClient, err := dynamic.NewForConfig(config)
	if err != nil {
		fmt.Printf("Error encountered: %v\n", err)
		return
	}

	// Define the GroupVersionResource for the object you want to access.
	// For Pods, this is {Group: "", Version: "v1", Resource: "pods"}.
	gvr := schema.GroupVersionResource{Version: "v1", Resource: "pods"}

	// Use the dynamic client to list all pods in the "default" namespace.
	// The result is an UnstructuredList.
	list, err := dynamicClient.Resource(gvr).Namespace("default").List(context.TODO(), metav1.ListOptions{})
	if err != nil {
		fmt.Printf("Error encountered: %v\n", err)
		return
	}

	// Iterate over the list and print the name of each pod.
	for _, item := range list.Items {
		name, found, err := unstructured.NestedString(item.Object, "metadata", "name")
		if err != nil || !found {
			fmt.Printf("Could not find name for pod: %v\n", err)
			continue
		}
		fmt.Printf("Pod Name: %s\n", name)
	}
}

Code:play 

package main

import (
	"fmt"
	"log"
	"os"
	"os/signal"
	"path/filepath"
	"syscall"
	"time"

	"k8s.io/client-go/informers"
	"k8s.io/client-go/kubernetes"
	"k8s.io/client-go/tools/cache"
	"k8s.io/client-go/tools/clientcmd"
)

func main() {
	// This example demonstrates the basic pattern for using an informer to watch
	// for changes to Pods. This is a conceptual example; a real controller would
	// have more robust logic and a workqueue.

	// Configure the client (out-of-cluster for this example).
	var kubeconfig string
	if home := os.Getenv("HOME"); home != "" {
		kubeconfig = filepath.Join(home, ".kube", "config")
	}
	config, err := clientcmd.BuildConfigFromFlags("", kubeconfig)
	if err != nil {
		fmt.Printf("Error encountered: %v\n", err)
		return
	}
	clientset, err := kubernetes.NewForConfig(config)
	if err != nil {
		fmt.Printf("Error encountered: %v\n", err)
		return
	}

	// A SharedInformerFactory provides a shared cache for multiple informers,
	// which reduces memory and network overhead.
	factory := informers.NewSharedInformerFactory(clientset, 10*time.Minute)
	podInformer := factory.Core().V1().Pods().Informer()

	_, err = podInformer.AddEventHandler(cache.ResourceEventHandlerFuncs{
		AddFunc: func(obj interface{}) {
			key, err := cache.MetaNamespaceKeyFunc(obj)
			if err == nil {
				log.Printf("Pod ADDED: %s", key)
			}
		},
		UpdateFunc: func(oldObj, newObj interface{}) {
			key, err := cache.MetaNamespaceKeyFunc(newObj)
			if err == nil {
				log.Printf("Pod UPDATED: %s", key)
			}
		},
		DeleteFunc: func(obj interface{}) {
			key, err := cache.DeletionHandlingMetaNamespaceKeyFunc(obj)
			if err == nil {
				log.Printf("Pod DELETED: %s", key)
			}
		},
	})
	if err != nil {
		fmt.Printf("Error encountered: %v\n", err)
		return
	}

	// Graceful shutdown requires a two-channel pattern.
	//
	// The first channel, `sigCh`, is used by the `signal` package to send us
	// OS signals (e.g., Ctrl+C). This channel must be of type `chan os.Signal`.
	//
	// The second channel, `stopCh`, is used to tell the informer factory to
	// stop. The informer factory's `Start` method expects a channel of type
	// `<-chan struct{}`. It will stop when this channel is closed.
	//
	// The goroutine below is the "translator" that connects these two channels.
	// It waits for a signal on `sigCh`, and when it receives one, it closes
	// `stopCh`, which in turn tells the informer factory to shut down.
	sigCh := make(chan os.Signal, 1)
	signal.Notify(sigCh, syscall.SIGINT, syscall.SIGTERM)
	stopCh := make(chan struct{})
	go func() {
		<-sigCh
		close(stopCh)
	}()

	// Start the informer.
	factory.Start(stopCh)

	// Wait for the initial cache sync.
	if !cache.WaitForCacheSync(stopCh, podInformer.HasSynced) {
		log.Println("Timed out waiting for caches to sync")
		return
	}

	log.Println("Informer has synced. Watching for Pod events...")

	// Wait for the stop signal.
	<-stopCh
	log.Println("Shutting down...")
}