✨ Add native SSA support

This change adds native server-side apply support to the client by
extending it with an `Apply` method that takes an
`runtime.ApplyConfiguration`.

Author: alvaroaleman

pkg/cache/internal/informers.go

@@ -518,7 +518,7 @@ func (ip *Informers) makeListWatcher(gvk schema.GroupVersionKind, obj runtime.Ob
 	// Structured.
 	//
 	default:
-		client, err := apiutil.RESTClientForGVK(gvk, false, ip.config, ip.codecs, ip.httpClient)
+		client, err := apiutil.RESTClientForGVK(gvk, false, false, ip.config, ip.codecs, ip.httpClient)
 		if err != nil {
 			return nil, err
 		}

pkg/client/apiutil/apimachinery.go

@@ -161,15 +161,27 @@ func GVKForObject(obj runtime.Object, scheme *runtime.Scheme) (schema.GroupVersi
 // RESTClientForGVK constructs a new rest.Interface capable of accessing the resource associated
 // with the given GroupVersionKind. The REST client will be configured to use the negotiated serializer from
 // baseConfig, if set, otherwise a default serializer will be set.
-func RESTClientForGVK(gvk schema.GroupVersionKind, isUnstructured bool, baseConfig *rest.Config, codecs serializer.CodecFactory, httpClient *http.Client) (rest.Interface, error) {
+func RESTClientForGVK(
+	gvk schema.GroupVersionKind,
+	forceDisableProtoBuf bool,
+	isUnstructured bool,
+	baseConfig *rest.Config,
+	codecs serializer.CodecFactory,
+	httpClient *http.Client,
+) (rest.Interface, error) {
 	if httpClient == nil {
 		return nil, fmt.Errorf("httpClient must not be nil, consider using rest.HTTPClientFor(c) to create a client")
 	}
-	return rest.RESTClientForConfigAndClient(createRestConfig(gvk, isUnstructured, baseConfig, codecs), httpClient)
+	return rest.RESTClientForConfigAndClient(createRestConfig(gvk, forceDisableProtoBuf, isUnstructured, baseConfig, codecs), httpClient)
 }
 
 // createRestConfig copies the base config and updates needed fields for a new rest config.
-func createRestConfig(gvk schema.GroupVersionKind, isUnstructured bool, baseConfig *rest.Config, codecs serializer.CodecFactory) *rest.Config {
+func createRestConfig(gvk schema.GroupVersionKind,
+	forceDisableProtoBuf bool,
+	isUnstructured bool,
+	baseConfig *rest.Config,
+	codecs serializer.CodecFactory,
+) *rest.Config {
 	gv := gvk.GroupVersion()
 
 	cfg := rest.CopyConfig(baseConfig)
@@ -183,7 +195,7 @@ func createRestConfig(gvk schema.GroupVersionKind, isUnstructured bool, baseConf
 		cfg.UserAgent = rest.DefaultKubernetesUserAgent()
 	}
 	// TODO(FillZpp): In the long run, we want to check discovery or something to make sure that this is actually true.
-	if cfg.ContentType == "" && !isUnstructured {
+	if cfg.ContentType == "" && !forceDisableProtoBuf {
 		protobufSchemeLock.RLock()
 		if protobufScheme.Recognizes(gvk) {
 			cfg.ContentType = runtime.ContentTypeProtobuf

pkg/client/applyconfigurations.go

@@ -0,0 +1,56 @@
+/*
+Copyright 2025 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package client
+
+import (
+	"k8s.io/apimachinery/pkg/apis/meta/v1/unstructured"
+	"k8s.io/apimachinery/pkg/runtime"
+	"k8s.io/apimachinery/pkg/runtime/schema"
+)
+
+type unstructuredApplyConfiguration struct {
+	*unstructured.Unstructured
+}
+
+func (u *unstructuredApplyConfiguration) IsApplyConfiguration() {}
+
+// ApplyConfigurationFromUnstructured creates a runtime.ApplyConfiguration from an *unstructured.Unstructured object.
+func ApplyConfigurationFromUnstructured(u *unstructured.Unstructured) runtime.ApplyConfiguration {
+	return &unstructuredApplyConfiguration{Unstructured: u}
+}
+
+type applyconfigurationRuntimeObject struct {
+	runtime.ApplyConfiguration
+}
+
+func (a *applyconfigurationRuntimeObject) GetObjectKind() schema.ObjectKind {
+	return a
+}
+
+func (a *applyconfigurationRuntimeObject) GroupVersionKind() schema.GroupVersionKind {
+	return schema.GroupVersionKind{}
+}
+
+func (a *applyconfigurationRuntimeObject) SetGroupVersionKind(gvk schema.GroupVersionKind) {}
+
+func (a *applyconfigurationRuntimeObject) DeepCopyObject() runtime.Object {
+	panic("applyconfigurationRuntimeObject does not support DeepCopyObject")
+}
+
+func runtimeObjectFromApplyConfiguration(ac runtime.ApplyConfiguration) runtime.Object {
+	return &applyconfigurationRuntimeObject{ApplyConfiguration: ac}
+}

pkg/client/client.go

@@ -329,6 +329,16 @@ func (c *client) Patch(ctx context.Context, obj Object, patch Patch, opts ...Pat
 	}
 }
 
+func (c *client) Apply(ctx context.Context, obj runtime.ApplyConfiguration, opts ...ApplyOption) error {
+	switch obj := obj.(type) {
+	case *unstructuredApplyConfiguration:
+		defer c.resetGroupVersionKind(obj, obj.GetObjectKind().GroupVersionKind())
+		return c.unstructuredClient.Apply(ctx, obj, opts...)
+	default:
+		return c.typedClient.Apply(ctx, obj, opts...)
+	}
+}
+
 // Get implements client.Client.
 func (c *client) Get(ctx context.Context, key ObjectKey, obj Object, opts ...GetOption) error {
 	if isUncached, err := c.shouldBypassCache(obj); err != nil {

pkg/client/client_rest_resources.go

@@ -17,16 +17,17 @@ limitations under the License.
 package client
 
 import (
+	"fmt"
 	"net/http"
 	"strings"
 	"sync"
 
 	"k8s.io/apimachinery/pkg/api/meta"
-	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 	"k8s.io/apimachinery/pkg/runtime"
 	"k8s.io/apimachinery/pkg/runtime/schema"
 	"k8s.io/apimachinery/pkg/runtime/serializer"
 	"k8s.io/client-go/rest"
+	"k8s.io/utils/ptr"
 	"sigs.k8s.io/controller-runtime/pkg/client/apiutil"
 )
 
@@ -56,13 +57,17 @@ type clientRestResources struct {
 
 // newResource maps obj to a Kubernetes Resource and constructs a client for that Resource.
 // If the object is a list, the resource represents the item's type instead.
-func (c *clientRestResources) newResource(gvk schema.GroupVersionKind, isList, isUnstructured bool) (*resourceMeta, error) {
+func (c *clientRestResources) newResource(gvk schema.GroupVersionKind,
+	isList bool,
+	forceDisableProtoBuf bool,
+	isUnstructured bool,
+) (*resourceMeta, error) {
 	if strings.HasSuffix(gvk.Kind, "List") && isList {
 		// if this was a list, treat it as a request for the item's resource
 		gvk.Kind = gvk.Kind[:len(gvk.Kind)-4]
 	}
 
-	client, err := apiutil.RESTClientForGVK(gvk, isUnstructured, c.config, c.codecs, c.httpClient)
+	client, err := apiutil.RESTClientForGVK(gvk, forceDisableProtoBuf, isUnstructured, c.config, c.codecs, c.httpClient)
 	if err != nil {
 		return nil, err
 	}
@@ -73,15 +78,44 @@ func (c *clientRestResources) newResource(gvk schema.GroupVersionKind, isList, i
 	return &resourceMeta{Interface: client, mapping: mapping, gvk: gvk}, nil
 }
 
+type applyConfiguration interface {
+	GetName() *string
+	GetNamespace() *string
+	GetKind() *string
+	GetAPIVersion() *string
+}
+
 // getResource returns the resource meta information for the given type of object.
 // If the object is a list, the resource represents the item's type instead.
-func (c *clientRestResources) getResource(obj runtime.Object) (*resourceMeta, error) {
-	gvk, err := apiutil.GVKForObject(obj, c.scheme)
-	if err != nil {
-		return nil, err
+func (c *clientRestResources) getResource(obj any) (*resourceMeta, error) {
+	var gvk schema.GroupVersionKind
+	var err error
+	var isApplyConfiguration bool
+	switch o := obj.(type) {
+	case runtime.Object:
+		gvk, err = apiutil.GVKForObject(o, c.scheme)
+		if err != nil {
+			return nil, err
+		}
+	case runtime.ApplyConfiguration:
+		ac, ok := o.(applyConfiguration)
+		if !ok {
+			return nil, fmt.Errorf("%T is a runtime.ApplyConfiguration but not an applyConfiguration", o)
+		}
+		gv, err := schema.ParseGroupVersion(ptr.Deref(ac.GetAPIVersion(), ""))
+		if err != nil {
+			return nil, fmt.Errorf("failed to parse %q as GroupVersion: %w", ptr.Deref(ac.GetAPIVersion(), ""), err)
+		}
+		gvk.Group = gv.Group
+		gvk.Version = gv.Version
+		gvk.Kind = ptr.Deref(ac.GetKind(), "")
+		isApplyConfiguration = true
+	default:
+		return nil, fmt.Errorf("bug: %T is neither a runtime.Object nor a runtime.ApplyConfiguration", o)
 	}
 
 	_, isUnstructured := obj.(runtime.Unstructured)
+	disableProtoBuf := isUnstructured || isApplyConfiguration
 
 	// It's better to do creation work twice than to not let multiple
 	// people make requests at once
@@ -97,10 +131,15 @@ func (c *clientRestResources) getResource(obj runtime.Object) (*resourceMeta, er
 		return r, nil
 	}
 
+	var isList bool
+	if runtimeObject, ok := obj.(runtime.Object); ok && meta.IsListType(runtimeObject) {
+		isList = true
+	}
+
 	// Initialize a new Client
 	c.mu.Lock()
 	defer c.mu.Unlock()
-	r, err = c.newResource(gvk, meta.IsListType(obj), isUnstructured)
+	r, err = c.newResource(gvk, isList, disableProtoBuf, isUnstructured)
 	if err != nil {
 		return nil, err
 	}
@@ -109,16 +148,29 @@ func (c *clientRestResources) getResource(obj runtime.Object) (*resourceMeta, er
 }
 
 // getObjMeta returns objMeta containing both type and object metadata and state.
-func (c *clientRestResources) getObjMeta(obj runtime.Object) (*objMeta, error) {
+func (c *clientRestResources) getObjMeta(obj any) (*objMeta, error) {
 	r, err := c.getResource(obj)
 	if err != nil {
 		return nil, err
 	}
-	m, err := meta.Accessor(obj)
-	if err != nil {
-		return nil, err
+	objMeta := &objMeta{resourceMeta: r}
+
+	switch o := obj.(type) {
+	case runtime.Object:
+		m, err := meta.Accessor(obj)
+		if err != nil {
+			return nil, err
+		}
+		objMeta.namespace = m.GetNamespace()
+		objMeta.name = m.GetName()
+	case applyConfiguration:
+		objMeta.namespace = ptr.Deref(o.GetNamespace(), "")
+		objMeta.name = ptr.Deref(o.GetName(), "")
+	default:
+		return nil, fmt.Errorf("object %T is neither a runtime.Object nor a runtime.ApplyConfiguration", obj)
 	}
-	return &objMeta{resourceMeta: r, Object: m}, err
+
+	return objMeta, nil
 }
 
 // resourceMeta stores state for a Kubernetes type.
@@ -146,6 +198,6 @@ type objMeta struct {
 	// resourceMeta contains type information for the object
 	*resourceMeta
 
-	// Object contains meta data for the object instance
-	metav1.Object
+	namespace string
+	name      string
 }

pkg/client/client_test.go

@@ -43,6 +43,7 @@ import (
 	"k8s.io/apimachinery/pkg/runtime"
 	"k8s.io/apimachinery/pkg/runtime/schema"
 	"k8s.io/apimachinery/pkg/types"
+	corev1applyconfigurations "k8s.io/client-go/applyconfigurations/core/v1"
 	kscheme "k8s.io/client-go/kubernetes/scheme"
 	"k8s.io/client-go/rest"
 	"k8s.io/utils/ptr"
@@ -859,6 +860,97 @@ U5wwSivyi7vmegHKmblOzNVKA5qPO8zWzqBC
 		})
 	})
 
+	Describe("Apply", func() {
+		Context("Unstructured Client", func() {
+			It("should create and update a configMap using SSA", func() {
+				cl, err := client.New(cfg, client.Options{})
+				Expect(err).NotTo(HaveOccurred())
+				Expect(cl).NotTo(BeNil())
+
+				data := map[string]any{
+					"some-key": "some-value",
+				}
+				obj := &unstructured.Unstructured{Object: map[string]any{
+					"apiVersion": "v1",
+					"kind":       "ConfigMap",
+					"metadata": map[string]any{
+						"name":      "test-configmap",
+						"namespace": "default",
+					},
+					"data": data,
+				}}
+
+				err = cl.Apply(context.Background(), client.ApplyConfigurationFromUnstructured(obj), &client.ApplyOptions{FieldManager: "test-manager"})
+				Expect(err).NotTo(HaveOccurred())
+
+				cm, err := clientset.CoreV1().ConfigMaps(obj.GetNamespace()).Get(context.Background(), obj.GetName(), metav1.GetOptions{})
+				Expect(err).NotTo(HaveOccurred())
+
+				actualData := map[string]any{}
+				for k, v := range cm.Data {
+					actualData[k] = v
+				}
+
+				Expect(actualData).To(BeComparableTo(data))
+				Expect(actualData).To(BeComparableTo(obj.Object["data"]))
+
+				data["a-new-key"] = "a-new-value"
+				obj.Object["data"] = data
+				unstructured.RemoveNestedField(obj.Object, "metadata", "managedFields")
+
+				err = cl.Apply(context.Background(), client.ApplyConfigurationFromUnstructured(obj), &client.ApplyOptions{FieldManager: "test-manager"})
+				Expect(err).NotTo(HaveOccurred())
+
+				cm, err = clientset.CoreV1().ConfigMaps(obj.GetNamespace()).Get(context.Background(), obj.GetName(), metav1.GetOptions{})
+				Expect(err).NotTo(HaveOccurred())
+
+				actualData = map[string]any{}
+				for k, v := range cm.Data {
+					actualData[k] = v
+				}
+
+				Expect(actualData).To(BeComparableTo(data))
+				Expect(actualData).To(BeComparableTo(obj.Object["data"]))
+			})
+		})
+
+		Context("Structured Client", func() {
+			It("should create and update a configMap using SSA", func() {
+				cl, err := client.New(cfg, client.Options{})
+				Expect(err).NotTo(HaveOccurred())
+				Expect(cl).NotTo(BeNil())
+
+				data := map[string]string{
+					"some-key": "some-value",
+				}
+				obj := corev1applyconfigurations.
+					ConfigMap("test-configmap", "default").
+					WithData(data)
+
+				err = cl.Apply(context.Background(), obj, &client.ApplyOptions{FieldManager: "test-manager"})
+				Expect(err).NotTo(HaveOccurred())
+
+				cm, err := clientset.CoreV1().ConfigMaps(ptr.Deref(obj.GetNamespace(), "")).Get(context.Background(), ptr.Deref(obj.GetName(), ""), metav1.GetOptions{})
+				Expect(err).NotTo(HaveOccurred())
+
+				Expect(cm.Data).To(BeComparableTo(data))
+				Expect(cm.Data).To(BeComparableTo(obj.Data))
+
+				data["a-new-key"] = "a-new-value"
+				obj.Data = data
+
+				err = cl.Apply(context.Background(), obj, &client.ApplyOptions{FieldManager: "test-manager"})
+				Expect(err).NotTo(HaveOccurred())
+
+				cm, err = clientset.CoreV1().ConfigMaps(ptr.Deref(obj.GetNamespace(), "")).Get(context.Background(), ptr.Deref(obj.GetName(), ""), metav1.GetOptions{})
+				Expect(err).NotTo(HaveOccurred())
+
+				Expect(cm.Data).To(BeComparableTo(data))
+				Expect(cm.Data).To(BeComparableTo(obj.Data))
+			})
+		})
+	})
+
 	Describe("SubResourceClient", func() {
 		Context("with structured objects", func() {
 			It("should be able to read the Scale subresource", func() {

pkg/client/dryrun.go

@@ -82,6 +82,10 @@ func (c *dryRunClient) Patch(ctx context.Context, obj Object, patch Patch, opts
 	return c.client.Patch(ctx, obj, patch, append(opts, DryRunAll)...)
 }
 
+func (c *dryRunClient) Apply(ctx context.Context, obj runtime.ApplyConfiguration, opts ...ApplyOption) error {
+	return c.client.Apply(ctx, obj, append(opts, DryRunAll)...)
+}
+
 // Get implements client.Client.
 func (c *dryRunClient) Get(ctx context.Context, key ObjectKey, obj Object, opts ...GetOption) error {
 	return c.client.Get(ctx, key, obj, opts...)