Example showing how to patch Kubernetes resources in Golang. Companion to the article 'https://dwmkerr.com/patching-kubernetes-resources-in-golang/'.

k8s-patch.go

// Example showing how to patch kubernetes resources.
// This is the companion to my article 'Patching Kubernetes Resources in Golang':
//   https://dwmkerr.com/patching-kubernetes-resources-in-golang/
package main

import (
	"encoding/json"
	"fmt"

	types "k8s.io/apimachinery/pkg/types"
	"k8s.io/client-go/kubernetes"
	_ "k8s.io/client-go/plugin/pkg/client/auth"
	"k8s.io/client-go/tools/clientcmd"
)

var (
	//  Leave blank for the default context in your kube config.
	context = ""

	//  Name of the replication controller to scale, and the desired number of replicas.
	replicationControllerName = "my-rc"
	replicas                  = uint32(3)
)

//  patchStringValue specifies a patch operation for a string.
type patchStringValue struct {
	Op    string `json:"op"`
	Path  string `json:"path"`
	Value string `json:"value"`
}

//  patchUint32Value specifies a patch operation for a uint32.
type patchUInt32Value struct {
	Op    string `json:"op"`
	Path  string `json:"path"`
	Value uint32 `json:"value"`
}

func scaleReplicationController(clientSet *kubernetes.Clientset, replicasetName string, scale uint32) error {
	payload := []patchUInt32Value{{
		Op:    "replace",
		Path:  "/spec/replicas",
		Value: scale,
	}}
	payloadBytes, _ := json.Marshal(payload)
	_, err := clientSet.
		CoreV1().
		ReplicationControllers("default").
		Patch(replicasetName, types.JSONPatchType, payloadBytes)
	return err
}

func main() {
	//  Get the local kube config.
	fmt.Printf("Connecting to Kubernetes Context %v\n", context)
	config, err := clientcmd.NewNonInteractiveDeferredLoadingClientConfig(
		clientcmd.NewDefaultClientConfigLoadingRules(),
		&clientcmd.ConfigOverrides{CurrentContext: context}).ClientConfig()
	if err != nil {
		panic(err.Error())
	}

	// Creates the clientset
	clientset, err := kubernetes.NewForConfig(config)
	if err != nil {
		panic(err.Error())
	}

	//  Scale our replication controller.
	fmt.Printf("Scaling replication controller %v to %v\n", replicationControllerName, replicas)
	err = scaleReplicationController(clientset, replicationControllerName, replicas)
	if err != nil {
		panic(err.Error())
	}
}

If anyone is trying to patch something that is an array, like tolerations, I made a slight modification (thank you dwmkerr for the original code!):

	type tolerationValue struct {
		Key      string `json:"key"`
		Operator string `json:"operator"`
		Value    string `json:"value"`
		Effect   string `json:"effect"`
	}

	type structArrayPayload struct {
		Op    string            `json:"op"`
		Path  string            `json:"path"`
                // tolerations are an array
		Value []tolerationValue `json:"value"`
	}

	patchPayload := []structArrayPayload{
		{
			Op:   "replace",
			Path: "/spec/template/spec/tolerations",
			Value: []tolerationValue{
				{
					Key:      "toleration.key.here",
					Operator: "Equal",
					Value:    "true",
					Effect:   "NoSchedule",
				},
			},
		},
	}

this seems to also work and avoids having to define lots of structs:

type patch struct {
	Op    string `json:"op"`
	Path  string `json:"path"`
	Value interface{} `json:"value"`
}

How can I escape the "statefulset.kubernetes.io/pod-name" if I use patchStringValue() func? API does not accept it. Thanks

Did you find any solutions for this?

How can I escape the "statefulset.kubernetes.io/pod-name" if I use patchStringValue() func? API does not accept it. Thanks

Did you find any solutions for this?

I'm sorry, I can't remember how I solved it.

How can I escape the "statefulset.kubernetes.io/pod-name" if I use patchStringValue() func? API does not accept it. Thanks

For the records, the solution here is, assuming we are changing a pod label,

podLabel := `statefulset.kubernetes.io~1pod-name`

Resulting in path being,

path := fmt.Sprintf("/metadata/labels/%s", podLabel)

My 2 cents.