Controller Watch Functions

This chapter describes how to use the controller package functions to configure Controllers to watch Resources.

Link to reference documentation

Watching the Controller Resource

Controllers may watch Resources and trigger Reconcile calls with the key of the object from the watch event.

This example configures a controller to watch for Pod events, and call Reconcile with the Pod key.

If Pod default/foo is created, updated or deleted, then Reconcile will be called with namespace: default, name: foo

// Annotation for generating RBAC role to Watch Pods
// +kubebuilder:rbac:groups="",resources=pods,verbs=get;watch;list
// Watch for Pod events, and enqueue a reconcile.Request to trigger a Reconcile
err := c.Watch(
    &source.Kind{Type: &v1.Pod{}},
    &handler.EnqueueRequestForObject{})
if err != nil {
    return err
}

Watching Created Resources

Controllers may watch Resources of types they create and trigger Reconcile calls with the key of the Owner of the object.

This example configures a Controller to watch for Pod events, and call Reconcile with the Owner ReplicaSet key. This is done by looking up the object referred to by the Owner reference from the watch event object.

  • Define a function to lookup the Owner from the key
  • Call WatchControllerOf with the Owned object and the function to lookup the owner

If Pod default/foo-pod was created by ReplicaSet default/foo-rs, and the Pod is (re)created, updated or deleted, then Reconcile will be called with namespace: default, name: foo-rs

Note: This requires adding the following annotations to your Controller struct to ensure the correct RBAC rules are in place and informers have been started.

// Annotation to generate RBAC roles to watch and update Pods
// +kubebuilder:rbac:groups="",resources=pods,verbs=get;watch;list,create,update,delete
// Watch for Pod events, and enqueue a reconcile.Request for the ReplicaSet in the OwnerReferences
err := c.Watch(
    &source.Kind{Type: &corev1.Pod{}},
    &handler.EnqueueRequestForOwner{
        IsController: true,
        OwnerType:    &appsv1.ReplicaSet{}})
if err != nil {
    return err
}

Watching Arbitrary Resources

Controllers may watch arbitrary Resources and map them to a key of the Resource managed by the controller. Controllers may even map an event to multiple keys, triggering Reconciles for each key.

Example: To respond to cluster scaling events (e.g. the deletion or addition of Nodes), a Controller would watch Nodes and map the watch events to keys of objects managed by the controller.

This simple example configures a Controller to watch for Pod events, and then reconciles objects with names derived from the Pod's name.

If Pod default/foo is created, updated or deleted, then Reconcile will be called for namespace: default, name: foo-parent-1 and for namespace: default, name: foo-parent-2.

Note: This requires adding the following annotations to your Controller struct to ensure the correct RBAC rules are in place and informers have been started.

// +kubebuilder:rbac:groups="",resources=pods,verbs=get;watch;list
// Define a mapping from the object in the event to one or more
// objects to Reconcile
mapFn := handler.ToRequestsFunc(
    func(a handler.MapObject) []reconcile.Request {
        return []reconcile.Request{
            {NamespacedName: types.NamespacedName{
                Name:      a.Meta.GetName() + "-1",
                Namespace: a.Meta.GetNamespace(),
            }},
            {NamespacedName: types.NamespacedName{
                Name:      a.Meta.GetName() + "-2",
                Namespace: a.Meta.GetNamespace(),
            }},
        }
    })


// 'UpdateFunc' and 'CreateFunc' used to judge if a event about the object is
// what we want. If that is true, the event will be processed by the reconciler.
p := predicate.Funcs{
    UpdateFunc: func(e event.UpdateEvent) bool {
        // The object doesn't contain label "foo", so the event will be
        // ignored.
        if _, ok := e.MetaOld.GetLabels()["foo"]; !ok {
            return false
        }
        return e.ObjectOld != e.ObjectNew
    },
    CreateFunc: func(e event.CreateEvent) bool {
        if _, ok := e.Meta.GetLabels()["foo"]; !ok {
            return false
        }
        return true
    },
}

// Watch Deployments and trigger Reconciles for objects
// mapped from the Deployment in the event
err := c.Watch(
    &source.Kind{Type: &appsv1.Deployment{}},
    &handler.EnqueueRequestsFromMapFunc{
        ToRequests: mapFn,
    },
    // Comment it if default predicate fun is used.
    p)
if err != nil {
    return err
}

Watching Channels

Controllers may trigger Reconcile for events written to Channels. This is useful if the Controller needs to trigger a Reconcile in response to something other than a create / update / delete event to a Kubernetes object. Note: in most situations this case is better handled by updating a Kubernetes object with the external state that would trigger the Reconcile.

events := make(chan event.GenericEvent)
err := ctrl.Watch(
    &source.Channel{Source: events},
    &handler.EnqueueRequestForObject{},
)
if err != nil {
    return err
}

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