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Hello World

A new project may be scaffolded for a user by running kubebuilder init and then scaffolding a new API with kubebuilder create api. More on this topic in Project Creation and Structure

This chapter shows a simple Controller implementation using the controller-runtime builder libraries to do most of the Controller configuration.

While Kubernetes APIs have typically have 3 components, (Resource, Controller, Manager), this example uses an existing Resource (ReplicaSet) and the builder package to hide many of the setup details.

For a more detailed look at creating Resources and Controllers that may be more complex, see the Resource, Controller and Manager examples.

ReplicaSet Controller Setup

The example main program configures a new ReplicaSetController to watch for create/update/delete events for ReplicaSets and Pods.

  • On ReplicaSet create/update/delete events - Reconcile the ReplicaSet
  • On Pod create/update/delete events - Reconcile the ReplicaSet that created the Pod
  • Reconcile by calling ReplicaSetController.Reconcile with the Namespace and Name of ReplicaSet
func main() {
  a, err := builder.SimpleController()
    // ReplicaSet is the Application type that
    // is Reconciled Respond to ReplicaSet events.
    // ReplicaSet creates Pods. Trigger
    // ReplicaSet Reconciles for Pod events.
    // Call ReplicaSetController with the
    // Namespace / Name of the ReplicaSet
  if err != nil {

// ReplicaSetController is a simple Controller example implementation.
type ReplicaSetController struct {

ReplicaSet Implementation

ReplicaSetController implements reconcile.Reconciler. It takes the Namespace and Name for a ReplicaSet object and makes the state of the cluster match what is specified in the ReplicaSet at the time Reconcile is called. This typically means using a client.Client to read the same of multiple objects, and perform create / update / delete as needed.

  • Implement InjectClient to get a client.Client from the application.Builder
  • Read the ReplicaSet object using the provided Namespace and Name
  • List the Pods matching the ReplicaSet selector
  • Set a Label on the ReplicaSet with the matching Pod count

Because the Controller watches for Pod events, the count will be updated any time a Pod is created or deleted.

// InjectClient is called by the application.Builder
// to provide a client.Client
func (a *ReplicaSetController) InjectClient(
  c client.Client) error {
  a.Client = c
  return nil

// Reconcile reads the Pods for a ReplicaSet and writes
// the count back as an annotation
func (a *ReplicaSetController) Reconcile(
  req reconcile.Request) (reconcile.Result, error) {
  // Read the ReplicaSet
  rs := &appsv1.ReplicaSet{}
  err := a.Get(context.TODO(), req.NamespacedName, rs)
  if err != nil {
    return reconcile.Result{}, err

  // List the Pods matching the PodTemplate Labels
  pods := &corev1.PodList{}
  err = a.List(context.TODO(), 
  if err != nil {
    return reconcile.Result{}, err

  // Update the ReplicaSet
  rs.Labels["selector-pod-count"] = 
    fmt.Sprintf("%v", len(pods.Items))
  err = a.Update(context.TODO(), rs)
  if err != nil {
    return reconcile.Result{}, err

  return reconcile.Result{}, nil