Add new feature gate to remove subnets minimal count restirction

Author: jerryhe1999

pkg/config/feature_gates.go

@@ -21,6 +21,7 @@ const (
 	SubnetsClusterTagCheck       Feature = "SubnetsClusterTagCheck"
 	NLBHealthCheckAdvancedConfig Feature = "NLBHealthCheckAdvancedConfig"
 	NLBSecurityGroup             Feature = "NLBSecurityGroup"
+	ALBSingleSubnet              Feature = "ALBSingleSubnet"
 )
 
 type FeatureGates interface {
@@ -58,6 +59,7 @@ func NewFeatureGates() FeatureGates {
 			SubnetsClusterTagCheck:       true,
 			NLBHealthCheckAdvancedConfig: true,
 			NLBSecurityGroup:             true,
+			ALBSingleSubnet:              false,
 		},
 	}
 }

pkg/ingress/model_build_load_balancer.go

@@ -215,6 +215,7 @@ func (t *defaultModelBuildTask) buildLoadBalancerSubnetMappings(ctx context.Cont
 			networking.WithSubnetsResolveLBType(elbv2model.LoadBalancerTypeApplication),
 			networking.WithSubnetsResolveLBScheme(scheme),
 			networking.WithSubnetsClusterTagCheck(t.featureGates.Enabled(config.SubnetsClusterTagCheck)),
+			networking.WithALBSingleSubnet(t.featureGates.Enabled(config.ALBSingleSubnet)),
 		)
 		if err != nil {
 			return nil, err
@@ -233,6 +234,7 @@ func (t *defaultModelBuildTask) buildLoadBalancerSubnetMappings(ctx context.Cont
 		chosenSubnets, err := t.subnetsResolver.ResolveViaNameOrIDSlice(ctx, chosenSubnetNameOrIDs,
 			networking.WithSubnetsResolveLBType(elbv2model.LoadBalancerTypeApplication),
 			networking.WithSubnetsResolveLBScheme(scheme),
+			networking.WithALBSingleSubnet(t.featureGates.Enabled(config.ALBSingleSubnet)),
 		)
 		if err != nil {
 			return nil, err

pkg/networking/subnet_resolver.go

@@ -48,6 +48,8 @@ type SubnetsResolveOptions struct {
 	AvailableIPAddressCount int64
 	// whether to check the cluster tag
 	SubnetsClusterTagCheck bool
+	// Disable subnet minimal count restriction
+	ALBSingleSubnet bool
 }
 
 // ApplyOptions applies slice of SubnetsResolveOption.
@@ -95,6 +97,13 @@ func WithSubnetsClusterTagCheck(SubnetsClusterTagCheck bool) SubnetsResolveOptio
 	}
 }
 
+// WithALBSingleSubnet generate an option that foncigure ALBSingleSubnet
+func WithALBSingleSubnet(ALBSingleSubnet bool) SubnetsResolveOption {
+	return func(opts *SubnetsResolveOptions) {
+		opts.ALBSingleSubnet = ALBSingleSubnet
+	}
+}
+
 // SubnetsResolver is responsible for resolve EC2 Subnets for Load Balancers.
 type SubnetsResolver interface {
 	// ResolveViaDiscovery resolve subnets by auto discover matching subnets.
@@ -364,6 +373,9 @@ func (r *defaultSubnetsResolver) validateSubnetsMinimalCount(subnets []*ec2sdk.S
 // computeSubnetsMinimalCount returns the minimal count requirement for subnets.
 func (r *defaultSubnetsResolver) computeSubnetsMinimalCount(subnetLocale subnetLocaleType, resolveOpts SubnetsResolveOptions) int {
 	minimalCount := 1
+	if resolveOpts.ALBSingleSubnet {
+		return minimalCount
+	}
 	if resolveOpts.LBType == elbv2model.LoadBalancerTypeApplication && subnetLocale == subnetLocaleTypeAvailabilityZone {
 		minimalCount = 2
 	}