feat: validate fallback configuration

Signed-off-by: Guo Peng <370090914@qq.com>

apis/keda/v1alpha1/scaledobject_types.go

@@ -266,3 +266,26 @@ func CheckReplicaCountBoundsAreValid(scaledObject *ScaledObject) error {
 
 	return nil
 }
+
+// CheckFallbackValid checks that the fallback supports scalers with an AverageValue metric target.
+// Consequently, it does not support CPU & memory scalers, or scalers targeting a Value metric type.
+func CheckFallbackValid(scaledObject *ScaledObject) error {
+	if scaledObject.Spec.Fallback == nil {
+		return nil
+	}
+
+	if scaledObject.Spec.Fallback.FailureThreshold < 0 || scaledObject.Spec.Fallback.Replicas < 0 {
+		return fmt.Errorf("FailureThreshold=%d & Replicas=%d must both be greater than or equal to 0",
+			scaledObject.Spec.Fallback.FailureThreshold, scaledObject.Spec.Fallback.Replicas)
+	}
+
+	for _, trigger := range scaledObject.Spec.Triggers {
+		if trigger.Type == cpuString || trigger.Type == memoryString {
+			return fmt.Errorf("type is %s , but fallback it is not supported by the CPU & memory scalers", trigger.Type)
+		}
+		if trigger.MetricType != autoscalingv2.AverageValueMetricType {
+			return fmt.Errorf("MetricType=%s, but Fallback can only be enabled for triggers with metric of type AverageValue", trigger.MetricType)
+		}
+	}
+	return nil
+}

apis/keda/v1alpha1/scaledobject_webhook.go

@@ -135,6 +135,7 @@ func validateWorkload(so *ScaledObject, action string, dryRun bool) (admission.W
 		verifyScaledObjects,
 		verifyHpas,
 		verifyReplicaCount,
+		verifyFallback,
 	}
 
 	for i := range verifyFunctions {
@@ -168,6 +169,15 @@ func verifyReplicaCount(incomingSo *ScaledObject, action string, _ bool) error {
 	return nil
 }
 
+func verifyFallback(incomingSo *ScaledObject, action string, _ bool) error {
+	err := CheckFallbackValid(incomingSo)
+	if err != nil {
+		scaledobjectlog.WithValues("name", incomingSo.Name).Error(err, "validation error")
+		metricscollector.RecordScaledObjectValidatingErrors(incomingSo.Namespace, action, "incorrect-fallback")
+	}
+	return nil
+}
+
 func verifyTriggers(incomingObject interface{}, action string, _ bool) error {
 	var triggers []ScaleTriggers
 	var name string