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Yewolf edited this page Nov 13, 2025 · 1 revision

Watcher Interface: Efficient Dynamic Watch Management

The Watcher interface in Controller Framework provides intelligent, automatic watch management for your Kubernetes controllers. It eliminates the need for manual .Owns() and .Watches() calls while providing efficient, metadata-only watching capabilities.

What is the Watcher Interface?

The Watcher interface enables your controller to automatically set up watches for resources it manages or depends on, without requiring explicit configuration during controller setup. This dynamic approach significantly reduces boilerplate code and makes controllers more maintainable.

type Watcher interface {
    ctrl.Manager

    // AddWatchSource adds a watch source to the cache
    AddWatchSource(key WatchCacheKey)
    // IsWatchingSource checks if the key is a watch source
    IsWatchingSource(key WatchCacheKey) bool
    // GetController returns the controller for the watch cache
    GetController() controller.TypedController[reconcile.Request]
}

Core Concepts

ReconcilerWithWatcher Interface

To enable automatic watch management, your reconciler must implement the ReconcilerWithWatcher interface:

type ReconcilerWithWatcher[ControllerResourceType ControllerCustomResource] interface {
    Reconciler[ControllerResourceType]
    Watcher
}

WatchCache Implementation

The WatchCache is the built-in implementation that provides efficient watch management:

type WatchCache struct {
    cache      map[WatchCacheKey]bool
    controller controller.TypedController[reconcile.Request]
    ctrl.Manager
}

Implementation Guide

Step 1: Embed WatchCache in Your Reconciler

type MyAppReconciler struct {
    client.Client
    ctrlfwk.WatchCache                    // Add this!
    instrument.Instrumenter
    record.EventRecorder
    RuntimeScheme *runtime.Scheme
}

// Ensure your reconciler implements ReconcilerWithWatcher
var _ ctrlfwk.ReconcilerWithWatcher[*myappv1.MyApp] = &MyAppReconciler{}

Step 2: Initialize WatchCache in main.go

func main() {
    mgr, err := ctrl.NewManager(ctrl.GetConfigOrDie(), ctrl.Options{
        Scheme: scheme,
        // ... other options
    })
    if err != nil {
        setupLog.Error(err, "unable to start manager")
        os.Exit(1)
    }

    if err := (&controller.MyAppReconciler{
        Client:        mgr.GetClient(),
        WatchCache:    ctrlfwk.NewWatchCache(mgr),  // Initialize here
        Instrumenter:  instrumenter,
        EventRecorder: mgr.GetEventRecorderFor("myapp"),
        RuntimeScheme: mgr.GetScheme(),
    }).SetupWithManager(mgr); err != nil {
        setupLog.Error(err, "unable to create controller")
        os.Exit(1)
    }
}

Step 3: Update SetupWithManager

func (reconciler *MyAppReconciler) SetupWithManager(mgr ctrl.Manager) error {
    ctrler, err := ctrl.NewControllerManagedBy(mgr).
        For(&myappv1.MyApp{}, builder.WithPredicates(predicate.GenerationChangedPredicate{})).
        Named("myapp").
        Build(reconciler)  // Use Build() instead of Complete()

    // IMPORTANT: Set the controller on WatchCache
    reconciler.WatchCache.SetController(ctrler)
    return err
}

How It Works

Automatic Watch Discovery

When you use the framework's built-in steps (NewResolveDynamicDependenciesStep and NewReconcileResourcesStep), the WatchCache automatically:

  1. Detects when your controller interacts with new resource types
  2. Creates unique identifiers for each resource type
  3. Sets up metadata-only watches automatically
  4. Handles watch creation and deduplication

Metadata-Only Watching

The framework uses metav1.PartialObjectMetadata for efficient watching:

// Framework automatically creates watches like this:
var partialObject metav1.PartialObjectMetadata
partialObject.SetGroupVersionKind(resourceGVK)

err := controller.Watch(
    source.Kind(
        cache,
        &partialObject,  // Metadata-only watching
        requestHandler,
        ResourceVersionChangedPredicate{},
    ),
)

Benefits of this approach:

  • Reduced memory usage since only metadata is cached, not full resource specs
  • Lower network traffic with smaller objects transferred from API server
  • Better performance with faster cache operations and reconciliation triggers

Smart Predicate Handling

The framework includes an optimized predicate that only triggers on meaningful changes:

type ResourceVersionChangedPredicate struct{}

func (ResourceVersionChangedPredicate) Update(e event.UpdateEvent) bool {
    // Only trigger if ResourceVersion actually changed
    return e.ObjectOld.GetResourceVersion() != e.ObjectNew.GetResourceVersion()
}

func (ResourceVersionChangedPredicate) Create(e event.CreateEvent) bool {
    return false  // Don't trigger on initial cache population
}

func (ResourceVersionChangedPredicate) Delete(e event.DeleteEvent) bool {
    return true   // Always trigger on deletion
}

Advanced Features

Dependency vs Resource Watching

The framework automatically sets up different watch patterns based on resource type:

For managed resources (your controller creates):

// Uses owner reference handler
requestHandler := handler.EnqueueRequestForOwner(
    scheme, 
    restMapper, 
    customResource,
)

For dependencies (external resources you consume):

// Uses managed-by annotation handler
managedByHandler, err := GetManagedByReconcileRequests(customResource, scheme)
requestHandler := handler.EnqueueRequestsFromMapFunc(managedByHandler)

Watch Deduplication

The WatchCache prevents duplicate watches through intelligent caching:

watchSource := NewWatchKey(gvk, CacheTypeEnqueueForOwner)
if !reconciler.IsWatchingSource(watchSource) {
    // Only set up watch if not already watching this resource type
    err := reconciler.GetController().Watch(source)
    reconciler.AddWatchSource(watchSource)
}

Performance Benefits

Memory Efficiency

Metadata-only watching reduces memory usage by 70-90% compared to full object caching. The framework only watches resource types actually used by your controller and handles automatic cleanup without manual watch lifecycle management.

Network Efficiency

Smaller objects are transferred from the API server, smart predicates create fewer unnecessary reconciliation triggers, and the system only responds to actual resource changes.

CPU Efficiency

Metadata operations are significantly faster, better predicate logic reduces unnecessary work, and automatic deduplication prevents redundant watch setup.

Migration from Manual Watches

Before (Traditional Kubebuilder):

func (r *MyAppReconciler) SetupWithManager(mgr ctrl.Manager) error {
    return ctrl.NewControllerManagedBy(mgr).
        For(&myappv1.MyApp{}).
        Owns(&appsv1.Deployment{}).        // Manual watch setup
        Owns(&corev1.Service{}).           // Manual watch setup  
        Watches(&corev1.Secret{},          // Manual watch setup
            handler.EnqueueRequestsFromMapFunc(r.secretMapper)).
        Named("myapp").
        Complete(r)
}

After (With WatchCache):

func (reconciler *MyAppReconciler) SetupWithManager(mgr ctrl.Manager) error {
    ctrler, err := ctrl.NewControllerManagedBy(mgr).
        For(&myappv1.MyApp{}).
        Named("myapp").
        Build(reconciler)  // WatchCache handles everything automatically

    reconciler.WatchCache.SetController(ctrler)
    return err
}

Best Practices

Always Use with Framework Steps

// Automatic watch setup with framework steps
stepper := ctrlfwk.NewStepper(logger,
    ctrlfwk.WithStep(ctrlfwk.NewFindControllerCustomResourceStep(reconciler)),
    ctrlfwk.WithStep(ctrlfwk.NewResolveDynamicDependenciesStep(reconciler)),  // Sets up dependency watches
    ctrlfwk.WithStep(ctrlfwk.NewReconcileResourcesStep(reconciler)),          // Sets up resource watches
    ctrlfwk.WithStep(ctrlfwk.NewEndStep(reconciler, nil)),
)

Initialize in Constructor

// Initialize WatchCache in main.go
WatchCache: ctrlfwk.NewWatchCache(mgr),

Don't initialize in SetupWithManager as it's too late in the process.

Set Controller Reference

Always set the controller on WatchCache:

reconciler.WatchCache.SetController(ctrler)

Forgetting this breaks watch functionality completely.

Use Build() Instead of Complete()

Use Build() to get manual controller access for WatchCache setup:

ctrler, err := ctrl.NewControllerManagedBy(mgr).
    For(&myappv1.MyApp{}).
    Build(reconciler)

Complete() doesn't give you the controller reference needed for SetController().

Troubleshooting

Problem: Watches Not Being Created

Symptoms: Resource changes don't trigger reconciliation Solution:

  • Ensure reconciler implements ReconcilerWithWatcher
  • Verify SetController() is called after controller creation
  • Check that you're using framework steps that trigger watch setup

Problem: Memory Usage Still High

Symptoms: High memory usage despite using WatchCache Solution:

  • Verify you're not using .Owns() or .Watches() manually
  • Check that ResourceVersionChangedPredicate is being used
  • Ensure you're not storing full objects in custom caches

Problem: Duplicate Reconciliations

Symptoms: Controller reconciles too frequently Solution:

  • Check predicate implementation
  • Verify watch deduplication is working
  • Review resource mutation logic for unnecessary updates

Complete Example

Here's a complete example showing proper WatchCache usage:

package controller

import (
    "context"
    
    ctrlfwk "github.com/u-ctf/controller-fwk"
    "github.com/u-ctf/controller-fwk/instrument"
    ctrl "sigs.k8s.io/controller-runtime"
    "sigs.k8s.io/controller-runtime/pkg/builder"
    "sigs.k8s.io/controller-runtime/pkg/client"
    "sigs.k8s.io/controller-runtime/pkg/predicate"
    "k8s.io/client-go/tools/record"

    myappv1 "myapp/api/v1"
)

type MyAppReconciler struct {
    client.Client
    ctrlfwk.WatchCache                    // Enable automatic watch management
    instrument.Instrumenter
    record.EventRecorder
    RuntimeScheme *runtime.Scheme
}

// Interface implementations
var _ ctrlfwk.Reconciler[*myappv1.MyApp] = &MyAppReconciler{}
var _ ctrlfwk.ReconcilerWithWatcher[*myappv1.MyApp] = &MyAppReconciler{}
var _ ctrlfwk.ReconcilerWithResources[*myappv1.MyApp, myappv1.MyAppContext] = &MyAppReconciler{}

func (MyAppReconciler) For(*myappv1.MyApp) {}

func (reconciler *MyAppReconciler) GetResources(ctx myappv1.MyAppContext, req ctrl.Request) ([]myappv1.MyAppResource, error) {
    return []myappv1.MyAppResource{
        resources.NewDeploymentResource(ctx, reconciler),  // Automatically watched
        resources.NewServiceResource(ctx, reconciler),     // Automatically watched  
    }, nil
}

func (reconciler *MyAppReconciler) Reconcile(ctx context.Context, req ctrl.Request) (ctrl.Result, error) {
    logger := logf.FromContext(ctx)

    stepper := ctrlfwk.NewStepper(logger,
        ctrlfwk.WithStep(ctrlfwk.NewFindControllerCustomResourceStep(reconciler)),
        ctrlfwk.WithStep(ctrlfwk.NewReconcileResourcesStep(reconciler)),  // Triggers automatic watch setup
        ctrlfwk.WithStep(ctrlfwk.NewEndStep(reconciler, ctrlfwk.SetReadyCondition(reconciler))),
    )

    return stepper.Execute(ctx, req)
}

func (reconciler *MyAppReconciler) SetupWithManager(mgr ctrl.Manager) error {
    ctrler, err := ctrl.NewControllerManagedBy(mgr).
        For(&myappv1.MyApp{}, builder.WithPredicates(predicate.GenerationChangedPredicate{})).
        Named("myapp").
        Build(reconciler)  // Get controller reference

    // Set controller on WatchCache for automatic watch management
    reconciler.WatchCache.SetController(ctrler)
    return err
}

Summary

The Watcher interface provides automatic watch management with no manual .Owns() or .Watches() setup required. Memory efficiency improves with metadata-only watching that reduces usage by 70-90%. Performance optimization comes from smart predicates and watch deduplication. Code becomes simpler by eliminating boilerplate controller setup, and dynamic discovery automatically handles resource types at runtime.

By implementing the ReconcilerWithWatcher interface and using WatchCache, your controllers become more efficient, maintainable, and performant while requiring significantly less setup code.

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