Watching Secondary Resources that are NOT Owned

In some scenarios, a controller may need to watch and respond to changes in resources that it does not Own, meaning those resources are created and managed by another controller.

The following examples demonstrate how a controller can monitor and reconcile resources that it doesn’t directly manage. This applies to any resource not Owned by the controller, including Core Types or Custom Resources managed by other controllers or projects and reconciled in separate processes.

For instance, consider two custom resources—Busybox and BackupBusybox. If changes to Busybox should trigger reconciliation in the BackupBusybox controller, we can configure the BackupBusybox controller to watch for updates in Busybox.

Example: Watching a Non-Owned Busybox Resource to Reconcile BackupBusybox

Consider a controller that manages a custom resource BackupBusybox but also needs to monitor changes to Busybox resources across the cluster. We only want to trigger reconciliation when Busybox instances have the Backup feature enabled.

• Why Watch Secondary Resources?
  • The BackupBusybox controller is not responsible for creating or owning Busybox resources, but changes in these resources (such as updates or deletions) directly affect the primary resource (BackupBusybox).
  • By watching Busybox instances with a specific label, the controller ensures that the necessary actions (e.g., backups) are triggered only for the relevant resources.

Configuration Example

Here’s how to configure the BackupBusyboxReconciler to watch changes in the Busybox resource and trigger reconciliation for BackupBusybox:

// SetupWithManager sets up the controller with the Manager.
// The controller will watch both the BackupBusybox primary resource and the Busybox resource.
func (r *BackupBusyboxReconciler) SetupWithManager(mgr ctrl.Manager) error {
    return ctrl.NewControllerManagedBy(mgr).
        For(&examplecomv1alpha1.BackupBusybox{}).  // Watch the primary resource (BackupBusybox)
        Watches(
            &source.Kind{Type: &examplecomv1alpha1.Busybox{}},  // Watch the Busybox CR
            handler.EnqueueRequestsFromMapFunc(func(obj client.Object) []reconcile.Request {
                // Trigger reconciliation for the BackupBusybox in the same namespace
                return []reconcile.Request{
                    {
                        NamespacedName: types.NamespacedName{
                            Name:      "backupbusybox",  // Reconcile the associated BackupBusybox resource
                            Namespace: obj.GetNamespace(),  // Use the namespace of the changed Busybox
                        },
                    },
                }
            }),
        ).  // Trigger reconciliation when the Busybox resource changes
        Complete(r)
}

Here’s how we can configure the controller to filter and watch for changes to only those Busybox resources that have the specific label:

// SetupWithManager sets up the controller with the Manager.
// The controller will watch both the BackupBusybox primary resource and the Busybox resource, filtering by a label.
func (r *BackupBusyboxReconciler) SetupWithManager(mgr ctrl.Manager) error {
    return ctrl.NewControllerManagedBy(mgr).
        For(&examplecomv1alpha1.BackupBusybox{}).  // Watch the primary resource (BackupBusybox)
        Watches(
            &source.Kind{Type: &examplecomv1alpha1.Busybox{}},  // Watch the Busybox CR
            handler.EnqueueRequestsFromMapFunc(func(obj client.Object) []reconcile.Request {
                // Check if the Busybox resource has the label 'backup-needed: "true"'
                if val, ok := obj.GetLabels()["backup-enable"]; ok && val == "true" {
                    // If the label is present and set to "true", trigger reconciliation for BackupBusybox
                    return []reconcile.Request{
                        {
                            NamespacedName: types.NamespacedName{
                                Name:      "backupbusybox",  // Reconcile the associated BackupBusybox resource
                                Namespace: obj.GetNamespace(),  // Use the namespace of the changed Busybox
                            },
                        },
                    }
                }
                // If the label is not present or doesn't match, don't trigger reconciliation
                return []reconcile.Request{}
            }),
        ).  // Trigger reconciliation when the labeled Busybox resource changes
        Complete(r)
}