⚠️ [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 is 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 +93,16 @@ 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.
+	//
+	// When set to true, the controller will start its sources without transitioning to be leader.
+	// Defaults to false.
+	EnableWarmup *bool
 }
 
 // DefaultFromConfig defaults the config from a config.Controller
@@ -124,6 +134,10 @@ 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 +257,7 @@ 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 +267,8 @@ 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()))
+		})
 	})
 })