Refactor cloud provider builder into a dynamic registration pattern

This comprehensive refactoring transitions the Cluster Autoscaler's cloud provider
initialization from a hardcoded, monolithic switch statement to a decoupled,
dynamic registration pattern.

Motivation:
Previously, the core cloud provider builder maintained direct dependencies on every
supported cloud provider implementation. This architectural coupling forced the
Cluster Autoscaler to pull in a massive number of transitive dependencies for
all providers (AWS, Azure, GCE, etc.) regardless of which one was actually used.
This resulted in significant "dependency bloat," unnecessarily large binary sizes,
and long build times. It was particularly burdensome for external forks or
specialized deployments that only required a single provider.

Key Architectural Improvements:

1. Centralized Registration: The cloud provider initialization logic is now
   centralized in the `cloudprovider/builder` package. Each cloud provider
   implementation is responsible for registering its own builder function via
   an `init()` block.

2. True Decoupling: The core builder no longer has any direct knowledge or
   compile-time dependencies on specific provider implementations. It interacts
   solely with a registry of builder functions.

3. Dynamic Provider Discovery: `AvailableCloudProviders()` is now a dynamic
   function that returns only the providers that have been registered in the
   current binary. This ensures that CLI help text (`--help`) and flag validation
   accurately reflect the capabilities of the specific build.

4. Configurable Default Provider: The `DefaultCloudProvider` is no longer a
   hardcoded constant. It can now be set dynamically via the registry, allowing
   custom builds to define their own default provider without modifying core code.

5. Modular Build Support: The set of supported providers in a binary is now
   entirely controlled by blank imports (e.g., in `main.go`). While standard builds
   continue to include all providers, this pattern enables the easy creation of
   optimized, provider-specific binaries.

Impact:
This change significantly improves the maintainability and extensibility of the
Cluster Autoscaler. It paves the way for a more modular architecture where
cloud providers can be treated as optional plugins, reducing the core's
dependency footprint and making it easier for the community to contribute and
maintain provider-specific logic.

Author: Choraden

cluster-autoscaler/cloudprovider/alicloud/alicloud_cloud_provider.go

@@ -24,13 +24,21 @@ import (
 	apiv1 "k8s.io/api/core/v1"
 	"k8s.io/apimachinery/pkg/api/resource"
 	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider"
+	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/builder"
 	"k8s.io/autoscaler/cluster-autoscaler/config/dynamic"
 	coreoptions "k8s.io/autoscaler/cluster-autoscaler/core/options"
 	"k8s.io/autoscaler/cluster-autoscaler/utils/errors"
 	"k8s.io/autoscaler/cluster-autoscaler/utils/gpu"
+	"k8s.io/client-go/informers"
 	klog "k8s.io/klog/v2"
 )
 
+func init() {
+	builder.RegisterCloudProvider(cloudprovider.AlicloudProviderName, func(opts *coreoptions.AutoscalerOptions, do cloudprovider.NodeGroupDiscoveryOptions, rl *cloudprovider.ResourceLimiter, informerFactory informers.SharedInformerFactory) cloudprovider.CloudProvider {
+		return BuildAlicloud(opts, do, rl)
+	})
+}
+
 const (
 	// GPULabel is the label added to nodes with GPU resource.
 	GPULabel = "aliyun.accelerator/nvidia_name"

cluster-autoscaler/cloudprovider/aws/aws_cloud_provider.go

@@ -26,14 +26,22 @@ import (
 	apiv1 "k8s.io/api/core/v1"
 	"k8s.io/apimachinery/pkg/api/resource"
 	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider"
+	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/builder"
 	"k8s.io/autoscaler/cluster-autoscaler/config"
 	coreoptions "k8s.io/autoscaler/cluster-autoscaler/core/options"
 	"k8s.io/autoscaler/cluster-autoscaler/simulator/framework"
 	"k8s.io/autoscaler/cluster-autoscaler/utils/errors"
 	"k8s.io/autoscaler/cluster-autoscaler/utils/gpu"
+	"k8s.io/client-go/informers"
 	klog "k8s.io/klog/v2"
 )
 
+func init() {
+	builder.RegisterCloudProvider(cloudprovider.AwsProviderName, func(opts *coreoptions.AutoscalerOptions, do cloudprovider.NodeGroupDiscoveryOptions, rl *cloudprovider.ResourceLimiter, informerFactory informers.SharedInformerFactory) cloudprovider.CloudProvider {
+		return BuildAWS(opts, do, rl)
+	})
+}
+
 const (
 	// GPULabel is the label added to nodes with GPU resource.
 	GPULabel = "k8s.amazonaws.com/accelerator"

cluster-autoscaler/cloudprovider/azure/azure_cloud_provider.go

@@ -25,12 +25,20 @@ import (
 	apiv1 "k8s.io/api/core/v1"
 	"k8s.io/apimachinery/pkg/api/resource"
 	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider"
+	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/builder"
 	coreoptions "k8s.io/autoscaler/cluster-autoscaler/core/options"
 	"k8s.io/autoscaler/cluster-autoscaler/utils/errors"
 	"k8s.io/autoscaler/cluster-autoscaler/utils/gpu"
+	"k8s.io/client-go/informers"
 	klog "k8s.io/klog/v2"
 )
 
+func init() {
+	builder.RegisterCloudProvider(cloudprovider.AzureProviderName, func(opts *coreoptions.AutoscalerOptions, do cloudprovider.NodeGroupDiscoveryOptions, rl *cloudprovider.ResourceLimiter, informerFactory informers.SharedInformerFactory) cloudprovider.CloudProvider {
+		return BuildAzure(opts, do, rl)
+	})
+}
+
 const (
 	// GPULabel is the label added to nodes with GPU resource.
 	GPULabel       = AKSLabelKeyPrefixValue + "accelerator"

cluster-autoscaler/cloudprovider/baiducloud/baiducloud_cloud_provider.go

@@ -25,15 +25,23 @@ import (
 	apiv1 "k8s.io/api/core/v1"
 	"k8s.io/apimachinery/pkg/api/resource"
 	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider"
+	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/builder"
 	"k8s.io/autoscaler/cluster-autoscaler/config"
 	"k8s.io/autoscaler/cluster-autoscaler/config/dynamic"
 	coreoptions "k8s.io/autoscaler/cluster-autoscaler/core/options"
 	"k8s.io/autoscaler/cluster-autoscaler/simulator/framework"
 	"k8s.io/autoscaler/cluster-autoscaler/utils/errors"
 	"k8s.io/autoscaler/cluster-autoscaler/utils/gpu"
+	"k8s.io/client-go/informers"
 	klog "k8s.io/klog/v2"
 )
 
+func init() {
+	builder.RegisterCloudProvider(cloudprovider.BaiducloudProviderName, func(opts *coreoptions.AutoscalerOptions, do cloudprovider.NodeGroupDiscoveryOptions, rl *cloudprovider.ResourceLimiter, informerFactory informers.SharedInformerFactory) cloudprovider.CloudProvider {
+		return BuildBaiducloud(opts, do, rl)
+	})
+}
+
 const (
 	// GPULabel is the label added to nodes with GPU resource.
 	GPULabel = "baidu/nvidia_name"

cluster-autoscaler/cloudprovider/bizflycloud/bizflycloud_cloud_provider.go

@@ -25,12 +25,20 @@ import (
 	apiv1 "k8s.io/api/core/v1"
 	"k8s.io/apimachinery/pkg/api/resource"
 	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider"
+	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/builder"
 	coreoptions "k8s.io/autoscaler/cluster-autoscaler/core/options"
 	"k8s.io/autoscaler/cluster-autoscaler/utils/errors"
 	"k8s.io/autoscaler/cluster-autoscaler/utils/gpu"
+	"k8s.io/client-go/informers"
 	klog "k8s.io/klog/v2"
 )
 
+func init() {
+	builder.RegisterCloudProvider(cloudprovider.BizflyCloudProviderName, func(opts *coreoptions.AutoscalerOptions, do cloudprovider.NodeGroupDiscoveryOptions, rl *cloudprovider.ResourceLimiter, informerFactory informers.SharedInformerFactory) cloudprovider.CloudProvider {
+		return BuildBizflyCloud(opts, do, rl)
+	})
+}
+
 var _ cloudprovider.CloudProvider = (*bizflycloudCloudProvider)(nil)
 
 const (

cluster-autoscaler/cloudprovider/brightbox/brightbox_cloud_provider.go

@@ -28,12 +28,20 @@ import (
 	brightbox "k8s.io/autoscaler/cluster-autoscaler/cloudprovider/brightbox/gobrightbox"
 	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/brightbox/gobrightbox/status"
 	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/brightbox/k8ssdk"
+	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/builder"
 	coreoptions "k8s.io/autoscaler/cluster-autoscaler/core/options"
 	"k8s.io/autoscaler/cluster-autoscaler/utils/errors"
 	"k8s.io/autoscaler/cluster-autoscaler/utils/gpu"
+	"k8s.io/client-go/informers"
 	klog "k8s.io/klog/v2"
 )
 
+func init() {
+	builder.RegisterCloudProvider(cloudprovider.BrightboxProviderName, func(opts *coreoptions.AutoscalerOptions, do cloudprovider.NodeGroupDiscoveryOptions, rl *cloudprovider.ResourceLimiter, informerFactory informers.SharedInformerFactory) cloudprovider.CloudProvider {
+		return BuildBrightbox(opts, do, rl)
+	})
+}
+
 const (
 	// GPULabel is added to nodes with GPU resource
 	GPULabel = "cloud.brightbox.com/gpu-node"

cluster-autoscaler/cloudprovider/builder/builder_alicloud.go

@@ -1,6 +1,3 @@
-//go:build !gce && !aws && !azure && !kubemark && !alicloud && !magnum && !digitalocean && !clusterapi && !huaweicloud && !ionoscloud && !linode && !hetzner && !bizflycloud && !brightbox && !equinixmetal && !oci && !vultr && !tencentcloud && !scaleway && !externalgrpc && !civo && !rancher && !volcengine && !baiducloud && !cherry && !cloudstack && !exoscale && !kamatera && !ovhcloud && !kwok && !utho && !coreweave
-// +build !gce,!aws,!azure,!kubemark,!alicloud,!magnum,!digitalocean,!clusterapi,!huaweicloud,!ionoscloud,!linode,!hetzner,!bizflycloud,!brightbox,!equinixmetal,!oci,!vultr,!tencentcloud,!scaleway,!externalgrpc,!civo,!rancher,!volcengine,!baiducloud,!cherry,!cloudstack,!exoscale,!kamatera,!ovhcloud,!kwok,!utho,!coreweave
-
 /*
 Copyright 2018 The Kubernetes Authors.
 
@@ -20,147 +17,74 @@ limitations under the License.
 package builder
 
 import (
+	"sort"
+
 	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/alicloud"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/aws"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/azure"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/baiducloud"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/bizflycloud"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/brightbox"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/cherryservers"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/civo"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/cloudstack"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/clusterapi"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/coreweave"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/digitalocean"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/equinixmetal"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/exoscale"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/externalgrpc"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/gce"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/hetzner"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/huaweicloud"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/ionoscloud"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/kamatera"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/kwok"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/linode"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/magnum"
-	oci "k8s.io/autoscaler/cluster-autoscaler/cloudprovider/oci/instancepools"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/ovhcloud"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/rancher"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/scaleway"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/tencentcloud"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/utho"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/volcengine"
-	"k8s.io/autoscaler/cluster-autoscaler/cloudprovider/vultr"
 	coreoptions "k8s.io/autoscaler/cluster-autoscaler/core/options"
 	"k8s.io/client-go/informers"
+	"k8s.io/klog/v2"
+)
+
+// CloudProviderBuilder builds a cloud provider from provided parameters.
+type CloudProviderBuilder func(opts *coreoptions.AutoscalerOptions, do cloudprovider.NodeGroupDiscoveryOptions, rl *cloudprovider.ResourceLimiter, informerFactory informers.SharedInformerFactory) cloudprovider.CloudProvider
+
+var (
+	cloudProviderBuilders = make(map[string]CloudProviderBuilder)
+	defaultCloudProvider  string
 )
 
-// AvailableCloudProviders supported by the cloud provider builder.
-var AvailableCloudProviders = []string{
-	cloudprovider.AwsProviderName,
-	cloudprovider.AzureProviderName,
-	cloudprovider.GceProviderName,
-	cloudprovider.AlicloudProviderName,
-	cloudprovider.CherryServersProviderName,
-	cloudprovider.CloudStackProviderName,
-	cloudprovider.BaiducloudProviderName,
-	cloudprovider.MagnumProviderName,
-	cloudprovider.DigitalOceanProviderName,
-	cloudprovider.ExoscaleProviderName,
-	cloudprovider.ExternalGrpcProviderName,
-	cloudprovider.HuaweicloudProviderName,
-	cloudprovider.HetznerProviderName,
-	cloudprovider.OracleCloudProviderName,
-	cloudprovider.OVHcloudProviderName,
-	cloudprovider.ClusterAPIProviderName,
-	cloudprovider.IonoscloudProviderName,
-	cloudprovider.KamateraProviderName,
-	cloudprovider.KwokProviderName,
-	cloudprovider.LinodeProviderName,
-	cloudprovider.BizflyCloudProviderName,
-	cloudprovider.BrightboxProviderName,
-	cloudprovider.EquinixMetalProviderName,
-	cloudprovider.VultrProviderName,
-	cloudprovider.TencentcloudProviderName,
-	cloudprovider.CivoProviderName,
-	cloudprovider.ScalewayProviderName,
-	cloudprovider.RancherProviderName,
-	cloudprovider.VolcengineProviderName,
-	cloudprovider.UthoProviderName,
-	cloudprovider.CoreWeaveProviderName,
+// RegisterCloudProvider registers a cloud provider builder.
+func RegisterCloudProvider(name string, builder CloudProviderBuilder) {
+	if _, ok := cloudProviderBuilders[name]; ok {
+		klog.Fatalf("Cloud provider %s already registered", name)
+	}
+	cloudProviderBuilders[name] = builder
+}
+
+// GetCloudProviderBuilder returns a cloud provider builder by name.
+func GetCloudProviderBuilder(name string) (CloudProviderBuilder, bool) {
+	builder, ok := cloudProviderBuilders[name]
+	return builder, ok
 }
 
-// DefaultCloudProvider is GCE.
-const DefaultCloudProvider = cloudprovider.GceProviderName
+// AvailableCloudProviders returns the list of supported cloud providers.
+func AvailableCloudProviders() []string {
+	providers := make([]string, 0, len(cloudProviderBuilders))
+	for name := range cloudProviderBuilders {
+		providers = append(providers, name)
+	}
+	sort.Strings(providers)
+	return providers
+}
+
+// SetDefaultCloudProvider sets the default cloud provider name.
+func SetDefaultCloudProvider(name string) {
+	defaultCloudProvider = name
+}
+
+// GetDefaultCloudProvider returns the default cloud provider name.
+func GetDefaultCloudProvider() string {
+	return defaultCloudProvider
+}
+
+// DefaultCloudProvider returns the default cloud provider name.
+func DefaultCloudProvider() string {
+	if def := GetDefaultCloudProvider(); def != "" {
+		return def
+	}
+	return cloudprovider.GceProviderName
+}
 
+// buildCloudProvider creates a cloud provider instance using the registration registry.
+// This function is only used when no specific cloud provider build tag is set.
+// It requires that the desired cloud provider has been registered via its init() function,
+// which typically happens when the cloud provider's package is imported.
 func buildCloudProvider(opts *coreoptions.AutoscalerOptions,
 	do cloudprovider.NodeGroupDiscoveryOptions,
 	rl *cloudprovider.ResourceLimiter,
 	informerFactory informers.SharedInformerFactory) cloudprovider.CloudProvider {
-	switch opts.CloudProviderName {
-	case cloudprovider.BizflyCloudProviderName:
-		return bizflycloud.BuildBizflyCloud(opts, do, rl)
-	case cloudprovider.GceProviderName:
-		return gce.BuildGCE(opts, do, rl)
-	case cloudprovider.AwsProviderName:
-		return aws.BuildAWS(opts, do, rl)
-	case cloudprovider.AzureProviderName:
-		return azure.BuildAzure(opts, do, rl)
-	case cloudprovider.AlicloudProviderName:
-		return alicloud.BuildAlicloud(opts, do, rl)
-	case cloudprovider.CherryServersProviderName:
-		return cherryservers.BuildCherry(opts, do, rl)
-	case cloudprovider.CloudStackProviderName:
-		return cloudstack.BuildCloudStack(opts, do, rl)
-	case cloudprovider.BaiducloudProviderName:
-		return baiducloud.BuildBaiducloud(opts, do, rl)
-	case cloudprovider.BrightboxProviderName:
-		return brightbox.BuildBrightbox(opts, do, rl)
-	case cloudprovider.DigitalOceanProviderName:
-		return digitalocean.BuildDigitalOcean(opts, do, rl)
-	case cloudprovider.ExoscaleProviderName:
-		return exoscale.BuildExoscale(opts, do, rl)
-	case cloudprovider.ExternalGrpcProviderName:
-		return externalgrpc.BuildExternalGrpc(opts, do, rl)
-	case cloudprovider.MagnumProviderName:
-		return magnum.BuildMagnum(opts, do, rl)
-	case cloudprovider.HuaweicloudProviderName:
-		return huaweicloud.BuildHuaweiCloud(opts, do, rl)
-	case cloudprovider.OVHcloudProviderName:
-		return ovhcloud.BuildOVHcloud(opts, do, rl)
-	case cloudprovider.HetznerProviderName:
-		return hetzner.BuildHetzner(opts, do, rl)
-	case cloudprovider.PacketProviderName, cloudprovider.EquinixMetalProviderName:
-		return equinixmetal.BuildCloudProvider(opts, do, rl)
-	case cloudprovider.ClusterAPIProviderName:
-		return clusterapi.BuildClusterAPI(opts, do, rl)
-	case cloudprovider.IonoscloudProviderName:
-		return ionoscloud.BuildIonosCloud(opts, do, rl)
-	case cloudprovider.KamateraProviderName:
-		return kamatera.BuildKamatera(opts, do, rl)
-	case cloudprovider.KwokProviderName:
-		return kwok.BuildKwok(opts, do, rl, informerFactory)
-	case cloudprovider.LinodeProviderName:
-		return linode.BuildLinode(opts, do, rl)
-	case cloudprovider.OracleCloudProviderName:
-		return oci.BuildOCI(opts, do, rl)
-	case cloudprovider.VultrProviderName:
-		return vultr.BuildVultr(opts, do, rl)
-	case cloudprovider.TencentcloudProviderName:
-		return tencentcloud.BuildTencentcloud(opts, do, rl)
-	case cloudprovider.CivoProviderName:
-		return civo.BuildCivo(opts, do, rl)
-	case cloudprovider.ScalewayProviderName:
-		return scaleway.BuildScaleway(opts, do, rl)
-	case cloudprovider.RancherProviderName:
-		return rancher.BuildRancher(opts, do, rl)
-	case cloudprovider.VolcengineProviderName:
-		return volcengine.BuildVolcengine(opts, do, rl)
-	case cloudprovider.UthoProviderName:
-		return utho.BuildUtho(opts, do, rl)
-	case cloudprovider.CoreWeaveProviderName:
-		return coreweave.BuildCoreWeave(opts, do, rl)
+	if builder, ok := GetCloudProviderBuilder(opts.CloudProviderName); ok {
+		return builder(opts, do, rl, informerFactory)
 	}
 	return nil
 }