✨ [Warm Replicas] Implement warm replica support for controllers.

pkg/config/controller.go

@@ -60,6 +60,19 @@ type Controller struct {
 	// Defaults to true, which means the controller will use leader election.
 	NeedLeaderElection *bool
 
+	// EnableWarmup specifies whether the controller should start its sources when the manager is not
+	// the leader. This is useful for cases where sources take a long time to start, as it allows
+	// for the controller to warm up its caches even before it is elected as the leader. This
+	// improves leadership failover time, as the caches will be prepopulated before the controller
+	// transitions to be leader.
+	//
+	// Setting EnableWarmup to true and NeedLeaderElection to true means the controller will start its
+	// sources without waiting to become leader.
+	// Setting EnableWarmup to true and NeedLeaderElection to false is a no-op as controllers without
+	// leader election do not wait on leader election to start their sources.
+	// Defaults to false.
+	EnableWarmup *bool
+
 	// UsePriorityQueue configures the controllers queue to use the controller-runtime provided
 	// priority queue.
 	//

pkg/controller/controller.go

@@ -93,6 +93,19 @@ type TypedOptions[request comparable] struct {
 	//
 	// Note: This flag is disabled by default until a future version. It's currently in beta.
 	UsePriorityQueue *bool
+
+	// EnableWarmup specifies whether the controller should start its sources when the manager is not
+	// the leader. This is useful for cases where sources take a long time to start, as it allows
+	// for the controller to warm up its caches even before it is elected as the leader. This
+	// improves leadership failover time, as the caches will be prepopulated before the controller
+	// transitions to be leader.
+	//
+	// Setting EnableWarmup to true and NeedLeaderElection to true means the controller will start its
+	// sources without waiting to become leader.
+	// Setting EnableWarmup to true and NeedLeaderElection to false is a no-op as controllers without
+	// leader election do not wait on leader election to start their sources.
+	// Defaults to false.
+	EnableWarmup *bool
 }
 
 // DefaultFromConfig defaults the config from a config.Controller
@@ -124,6 +137,10 @@ func (options *TypedOptions[request]) DefaultFromConfig(config config.Controller
 	if options.NeedLeaderElection == nil {
 		options.NeedLeaderElection = config.NeedLeaderElection
 	}
+
+	if options.EnableWarmup == nil {
+		options.EnableWarmup = config.EnableWarmup
+	}
 }
 
 // Controller implements an API. A Controller manages a work queue fed reconcile.Requests
@@ -243,7 +260,7 @@ func NewTypedUnmanaged[request comparable](name string, options TypedOptions[req
 	}
 
 	// Create controller with dependencies set
-	return &controller.Controller[request]{
+	return controller.New[request](controller.ControllerOptions[request]{
 		Do:                      options.Reconciler,
 		RateLimiter:             options.RateLimiter,
 		NewQueue:                options.NewQueue,
@@ -253,7 +270,8 @@ func NewTypedUnmanaged[request comparable](name string, options TypedOptions[req
 		LogConstructor:          options.LogConstructor,
 		RecoverPanic:            options.RecoverPanic,
 		LeaderElected:           options.NeedLeaderElection,
-	}, nil
+		EnableWarmup:            options.EnableWarmup,
+	}), nil
 }
 
 // ReconcileIDFromContext gets the reconcileID from the current context.

pkg/controller/controller_test.go

@@ -474,5 +474,71 @@ var _ = Describe("controller.Controller", func() {
 			_, ok = q.(priorityqueue.PriorityQueue[reconcile.Request])
 			Expect(ok).To(BeFalse())
 		})
+
+		It("should set EnableWarmup correctly", func() {
+			m, err := manager.New(cfg, manager.Options{})
+			Expect(err).NotTo(HaveOccurred())
+
+			// Test with EnableWarmup set to true
+			ctrlWithWarmup, err := controller.New("warmup-enabled-ctrl", m, controller.Options{
+				Reconciler:   reconcile.Func(nil),
+				EnableWarmup: ptr.To(true),
+			})
+			Expect(err).NotTo(HaveOccurred())
+
+			internalCtrlWithWarmup, ok := ctrlWithWarmup.(*internalcontroller.Controller[reconcile.Request])
+			Expect(ok).To(BeTrue())
+			Expect(internalCtrlWithWarmup.EnableWarmup).To(HaveValue(BeTrue()))
+
+			// Test with EnableWarmup set to false
+			ctrlWithoutWarmup, err := controller.New("warmup-disabled-ctrl", m, controller.Options{
+				Reconciler:   reconcile.Func(nil),
+				EnableWarmup: ptr.To(false),
+			})
+			Expect(err).NotTo(HaveOccurred())
+
+			internalCtrlWithoutWarmup, ok := ctrlWithoutWarmup.(*internalcontroller.Controller[reconcile.Request])
+			Expect(ok).To(BeTrue())
+			Expect(internalCtrlWithoutWarmup.EnableWarmup).To(HaveValue(BeFalse()))
+
+			// Test with EnableWarmup not set (should default to nil)
+			ctrlWithDefaultWarmup, err := controller.New("warmup-default-ctrl", m, controller.Options{
+				Reconciler: reconcile.Func(nil),
+			})
+			Expect(err).NotTo(HaveOccurred())
+
+			internalCtrlWithDefaultWarmup, ok := ctrlWithDefaultWarmup.(*internalcontroller.Controller[reconcile.Request])
+			Expect(ok).To(BeTrue())
+			Expect(internalCtrlWithDefaultWarmup.EnableWarmup).To(BeNil())
+		})
+
+		It("should inherit EnableWarmup from manager config", func() {
+			// Test with manager default setting EnableWarmup to true
+			managerWithWarmup, err := manager.New(cfg, manager.Options{
+				Controller: config.Controller{
+					EnableWarmup: ptr.To(true),
+				},
+			})
+			Expect(err).NotTo(HaveOccurred())
+			ctrlInheritingWarmup, err := controller.New("inherit-warmup-enabled", managerWithWarmup, controller.Options{
+				Reconciler: reconcile.Func(nil),
+			})
+			Expect(err).NotTo(HaveOccurred())
+
+			internalCtrlInheritingWarmup, ok := ctrlInheritingWarmup.(*internalcontroller.Controller[reconcile.Request])
+			Expect(ok).To(BeTrue())
+			Expect(internalCtrlInheritingWarmup.EnableWarmup).To(HaveValue(BeTrue()))
+
+			// Test that explicit controller setting overrides manager setting
+			ctrlOverridingWarmup, err := controller.New("override-warmup-disabled", managerWithWarmup, controller.Options{
+				Reconciler:   reconcile.Func(nil),
+				EnableWarmup: ptr.To(false),
+			})
+			Expect(err).NotTo(HaveOccurred())
+
+			internalCtrlOverridingWarmup, ok := ctrlOverridingWarmup.(*internalcontroller.Controller[reconcile.Request])
+			Expect(ok).To(BeTrue())
+			Expect(internalCtrlOverridingWarmup.EnableWarmup).To(HaveValue(BeFalse()))
+		})
 	})
 })

pkg/internal/controller/controller.go

@@ -38,6 +38,50 @@ import (
 	"sigs.k8s.io/controller-runtime/pkg/source"
 )
 
+type ControllerOptions[request comparable] struct {
+	// Reconciler is a function that can be called at any time with the Name / Namespace of an object and
+	// ensures that the state of the system matches the state specified in the object.
+	// Defaults to the DefaultReconcileFunc.
+	Do reconcile.TypedReconciler[request]
+
+	// RateLimiter is used to limit how frequently requests may be queued into the work queue.
+	RateLimiter workqueue.TypedRateLimiter[request]
+
+	// NewQueue constructs the queue for this controller once the controller is ready to start.
+	// This is a func because the standard Kubernetes work queues start themselves immediately, which
+	// leads to goroutine leaks if something calls controller.New repeatedly.
+	NewQueue func(controllerName string, rateLimiter workqueue.TypedRateLimiter[request]) workqueue.TypedRateLimitingInterface[request]
+
+	// MaxConcurrentReconciles is the maximum number of concurrent Reconciles which can be run. Defaults to 1.
+	MaxConcurrentReconciles int
+
+	// CacheSyncTimeout refers to the time limit set on waiting for cache to sync
+	// Defaults to 2 minutes if not set.
+	CacheSyncTimeout time.Duration
+
+	// Name is used to uniquely identify a Controller in tracing, logging and monitoring.  Name is required.
+	Name string
+
+	// LogConstructor is used to construct a logger to then log messages to users during reconciliation,
+	// or for example when a watch is started.
+	// Note: LogConstructor has to be able to handle nil requests as we are also using it
+	// outside the context of a reconciliation.
+	LogConstructor func(request *request) logr.Logger
+
+	// RecoverPanic indicates whether the panic caused by reconcile should be recovered.
+	// Defaults to true.
+	RecoverPanic *bool
+
+	// LeaderElected indicates whether the controller is leader elected or always running.
+	LeaderElected *bool
+
+	// EnableWarmup specifies whether the controller should start its sources
+	// when the manager is not the leader.
+	// Defaults to false, which means that the controller will wait for leader election to start
+	// before starting sources.
+	EnableWarmup *bool
+}
+
 // Controller implements controller.Controller.
 type Controller[request comparable] struct {
 	// Name is used to uniquely identify a Controller in tracing, logging and monitoring.  Name is required.
@@ -83,6 +127,9 @@ type Controller[request comparable] struct {
 	// startWatches maintains a list of sources, handlers, and predicates to start when the controller is started.
 	startWatches []source.TypedSource[request]
 
+	// didStartEventSourcesOnce is used to ensure that the event sources are only started once.
+	didStartEventSourcesOnce sync.Once
+
 	// LogConstructor is used to construct a logger to then log messages to users during reconciliation,
 	// or for example when a watch is started.
 	// Note: LogConstructor has to be able to handle nil requests as we are also using it
@@ -95,6 +142,34 @@ type Controller[request comparable] struct {
 
 	// LeaderElected indicates whether the controller is leader elected or always running.
 	LeaderElected *bool
+
+	// EnableWarmup specifies whether the controller should start its sources when the manager is not
+	// the leader. This is useful for cases where sources take a long time to start, as it allows
+	// for the controller to warm up its caches even before it is elected as the leader. This
+	// improves leadership failover time, as the caches will be prepopulated before the controller
+	// transitions to be leader.
+	//
+	// Setting EnableWarmup to true and NeedLeaderElection to true means the controller will start its
+	// sources without waiting to become leader.
+	// Setting EnableWarmup to true and NeedLeaderElection to false is a no-op as controllers without
+	// leader election do not wait on leader election to start their sources.
+	// Defaults to false.
+	EnableWarmup *bool
+}
+
+func New[request comparable](options ControllerOptions[request]) *Controller[request] {
+	return &Controller[request]{
+		Do:                      options.Do,
+		RateLimiter:             options.RateLimiter,
+		NewQueue:                options.NewQueue,
+		MaxConcurrentReconciles: options.MaxConcurrentReconciles,
+		CacheSyncTimeout:        options.CacheSyncTimeout,
+		Name:                    options.Name,
+		LogConstructor:          options.LogConstructor,
+		RecoverPanic:            options.RecoverPanic,
+		LeaderElected:           options.LeaderElected,
+		EnableWarmup:            options.EnableWarmup,
+	}
 }
 
 // Reconcile implements reconcile.Reconciler.
@@ -144,6 +219,15 @@ func (c *Controller[request]) NeedLeaderElection() bool {
 	return *c.LeaderElected
 }
 
+// Warmup implements the manager.WarmupRunnable interface.
+func (c *Controller[request]) Warmup(ctx context.Context) error {
+	if c.EnableWarmup == nil || !*c.EnableWarmup {
+		return nil
+	}
+
+	return c.startEventSources(ctx)
+}
+
 // Start implements controller.Controller.
 func (c *Controller[request]) Start(ctx context.Context) error {
 	// use an IIFE to get proper lock handling
@@ -185,12 +269,6 @@ func (c *Controller[request]) Start(ctx context.Context) error {
 
 		c.LogConstructor(nil).Info("Starting Controller")
 
-		// All the watches have been started, we can reset the local slice.
-		//
-		// We should never hold watches more than necessary, each watch source can hold a backing cache,
-		// which won't be garbage collected if we hold a reference to it.
-		c.startWatches = nil
-
 		// Launch workers to process resources
 		c.LogConstructor(nil).Info("Starting workers", "worker count", c.MaxConcurrentReconciles)
 		wg.Add(c.MaxConcurrentReconciles)
@@ -221,60 +299,71 @@ func (c *Controller[request]) Start(ctx context.Context) error {
 // startEventSources launches all the sources registered with this controller and waits
 // for them to sync. It returns an error if any of the sources fail to start or sync.
 func (c *Controller[request]) startEventSources(ctx context.Context) error {
-	errGroup := &errgroup.Group{}
-	for _, watch := range c.startWatches {
-		log := c.LogConstructor(nil)
-		_, ok := watch.(interface {
-			String() string
-		})
-
-		if !ok {
-			log = log.WithValues("source", fmt.Sprintf("%T", watch))
-		} else {
-			log = log.WithValues("source", fmt.Sprintf("%s", watch))
-		}
-		didStartSyncingSource := &atomic.Bool{}
-		errGroup.Go(func() error {
-			// Use a timeout for starting and syncing the source to avoid silently
-			// blocking startup indefinitely if it doesn't come up.
-			sourceStartCtx, cancel := context.WithTimeout(ctx, c.CacheSyncTimeout)
-			defer cancel()
-
-			sourceStartErrChan := make(chan error, 1) // Buffer chan to not leak goroutine if we time out
-			go func() {
-				defer close(sourceStartErrChan)
-				log.Info("Starting EventSource")
-				if err := watch.Start(ctx, c.Queue); err != nil {
-					sourceStartErrChan <- err
-					return
-				}
-				syncingSource, ok := watch.(source.TypedSyncingSource[request])
-				if !ok {
-					return
-				}
-				didStartSyncingSource.Store(true)
-				if err := syncingSource.WaitForSync(sourceStartCtx); err != nil {
-					err := fmt.Errorf("failed to wait for %s caches to sync %v: %w", c.Name, syncingSource, err)
-					log.Error(err, "Could not wait for Cache to sync")
-					sourceStartErrChan <- err
+	var retErr error
+
+	c.didStartEventSourcesOnce.Do(func() {
+		errGroup := &errgroup.Group{}
+		for _, watch := range c.startWatches {
+			log := c.LogConstructor(nil)
+			_, ok := watch.(interface {
+				String() string
+			})
+			if !ok {
+				log = log.WithValues("source", fmt.Sprintf("%T", watch))
+			} else {
+				log = log.WithValues("source", fmt.Sprintf("%s", watch))
+			}
+			didStartSyncingSource := &atomic.Bool{}
+			errGroup.Go(func() error {
+				// Use a timeout for starting and syncing the source to avoid silently
+				// blocking startup indefinitely if it doesn't come up.
+				sourceStartCtx, cancel := context.WithTimeout(ctx, c.CacheSyncTimeout)
+				defer cancel()
+
+				sourceStartErrChan := make(chan error, 1) // Buffer chan to not leak goroutine if we time out
+				go func() {
+					defer close(sourceStartErrChan)
+					log.Info("Starting EventSource")
+					if err := watch.Start(ctx, c.Queue); err != nil {
+						sourceStartErrChan <- err
+						return
+					}
+					syncingSource, ok := watch.(source.TypedSyncingSource[request])
+					if !ok {
+						return
+					}
+					didStartSyncingSource.Store(true)
+					if err := syncingSource.WaitForSync(sourceStartCtx); err != nil {
+						err := fmt.Errorf("failed to wait for %s caches to sync %v: %w", c.Name, syncingSource, err)
+						log.Error(err, "Could not wait for Cache to sync")
+						sourceStartErrChan <- err
+					}
+				}()
+
+				select {
+				case err := <-sourceStartErrChan:
+					return err
+				case <-sourceStartCtx.Done():
+					if didStartSyncingSource.Load() { // We are racing with WaitForSync, wait for it to let it tell us what happened
+						return <-sourceStartErrChan
+					}
+					if ctx.Err() != nil { // Don't return an error if the root context got cancelled
+						return nil
+					}
+					return fmt.Errorf("timed out waiting for source %s to Start. Please ensure that its Start() method is non-blocking", watch)
 				}
-			}()
+			})
+		}
+		retErr = errGroup.Wait()
 
-			select {
-			case err := <-sourceStartErrChan:
-				return err
-			case <-sourceStartCtx.Done():
-				if didStartSyncingSource.Load() { // We are racing with WaitForSync, wait for it to let it tell us what happened
-					return <-sourceStartErrChan
-				}
-				if ctx.Err() != nil { // Don't return an error if the root context got cancelled
-					return nil
-				}
-				return fmt.Errorf("timed out waiting for source %s to Start. Please ensure that its Start() method is non-blocking", watch)
-			}
-		})
-	}
-	return errGroup.Wait()
+		// All the watches have been started, we can reset the local slice.
+		//
+		// We should never hold watches more than necessary, each watch source can hold a backing cache,
+		// which won't be garbage collected if we hold a reference to it.
+		c.startWatches = nil
+	})
+
+	return retErr
 }
 
 // processNextWorkItem will read a single work item off the workqueue and