## Writing Controllers

The `controllers` package offers a variety of helpers for writing controllers.
These operate in a similar manner as [`controller-runtime`](https://github.com/kubernetes-sigs/controller-runtime) but are *far* smaller and less abstract.

With a few exceptions, Istio controllers typically are split in two phases: construction and running.

### Controllers overview

Below provides a high level overview of controllers in Istiod. For more information about each controller, consulting the controllers Go docs is recommended.

```mermaid
graph BT
    crd("CRD Watcher")
    subgraph Service Discovery
        ksd("Kubernetes Controller")
        sesd("Service Entry Controller")

	if already {
		l.LogAndPrintf("Operator controller is already installed in %s namespace.", oiArgs.common.operatorNamespace)
		l.LogAndPrintf("Upgrading operator controller in namespace: %s using image: %s/operator:%s",
			oiArgs.common.operatorNamespace, oiArgs.common.hub, oiArgs.common.tag)
	} else {
		l.LogAndPrintf("Installing operator controller in namespace: %s using image: %s/operator:%s",

// IngressClassSpec provides information about the class of an Ingress.
message IngressClassSpec {
  // controller refers to the name of the controller that should handle this
  // class. This allows for different "flavors" that are controlled by the
  // same controller. For example, you may have different parameters for the
  // same implementing controller. This should be specified as a
  // domain-prefixed path no more than 250 characters in length, e.g.

### Relationship between the CLI and controller

The CLI and controller share the same API and codebase for generating manifests from the API. You can think of the
controller as the CLI command `istioctl install` running in a loop in a pod in the cluster and using the config
from the in-cluster IstioOperator custom resource (CR).
There are two major differences:

    targetPort: 9443
  selector:
    component: controller
---
apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    app: metallb
    component: controller
  name: controller
  namespace: metallb-system
spec:
  revisionHistoryLimit: 3
  selector:
    matchLabels:
      app: metallb
      component: controller
  template:
    metadata:
      annotations:

  labels:
    app: istiod
    release: {{ .Release.Name }}
rules:
  # sidecar injection controller
  - apiGroups: ["admissionregistration.k8s.io"]
    resources: ["mutatingwebhookconfigurations"]
    verbs: ["get", "list", "watch", "update", "patch"]

  # configuration validation webhook controller
  - apiGroups: ["admissionregistration.k8s.io"]
    resources: ["validatingwebhookconfigurations"]

		FieldSelector: strings.Join(fieldSelectors, ","),
	})
	c.queue = controllers.NewQueue("repair pods",
		controllers.WithReconciler(c.Reconcile),
		controllers.WithMaxAttempts(5))
	c.pods.AddEventHandler(controllers.ObjectHandler(c.queue.AddObject))

	return c, nil
}

func (c *Controller) Run(stop <-chan struct{}) {
	kube.WaitForCacheSync("repair controller", stop, c.pods.HasSynced)
	c.queue.Run(stop)
	c.pods.ShutdownHandlers()

  //
  // More completion modes can be added in the future.
  // If the Job controller observes a mode that it doesn't recognize, which
  // is possible during upgrades due to version skew, the controller
  // skips updates for the Job.
  // +optional
  optional string completionMode = 9;

  // suspend specifies whether the Job controller should create Pods or not. If

// IngressClassSpec provides information about the class of an Ingress.
message IngressClassSpec {
  // controller refers to the name of the controller that should handle this
  // class. This allows for different "flavors" that are controlled by the
  // same controller. For example, you may have different parameters for the
  // same implementing controller. This should be specified as a
  // domain-prefixed path no more than 250 characters in length, e.g.