diff --git a/internal/gen/testing/attribute/attribute.pb.go b/internal/gen/testing/attribute/attribute.pb.go
new file mode 100644
index 0000000000..e63f01a09f
--- /dev/null
+++ b/internal/gen/testing/attribute/attribute.pb.go
@@ -0,0 +1,340 @@
+// Code generated by protoc-gen-go. DO NOT EDIT.
+// versions:
+// 	protoc-gen-go v1.27.1
+// 	protoc        v3.17.3
+// source: testing/attribute/v1/attribute.proto
+
+package attribute
+
+import (
+	_ "github.com/hashicorp/boundary/sdk/pbs/controller/protooptions"
+	protoreflect "google.golang.org/protobuf/reflect/protoreflect"
+	protoimpl "google.golang.org/protobuf/runtime/protoimpl"
+	structpb "google.golang.org/protobuf/types/known/structpb"
+	reflect "reflect"
+	sync "sync"
+)
+
+const (
+	// Verify that this generated code is sufficiently up-to-date.
+	_ = protoimpl.EnforceVersion(20 - protoimpl.MinVersion)
+	// Verify that runtime/protoimpl is sufficiently up-to-date.
+	_ = protoimpl.EnforceVersion(protoimpl.MaxVersion - 20)
+)
+
+type TestResource struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	Id string `protobuf:"bytes,1,opt,name=id,proto3" json:"id,omitempty"`
+	// Types that are assignable to Attrs:
+	//	*TestResource_Attributes
+	//	*TestResource_SubResourceAttributes
+	Attrs isTestResource_Attrs `protobuf_oneof:"attrs"`
+}
+
+func (x *TestResource) Reset() {
+	*x = TestResource{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_testing_attribute_v1_attribute_proto_msgTypes[0]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *TestResource) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*TestResource) ProtoMessage() {}
+
+func (x *TestResource) ProtoReflect() protoreflect.Message {
+	mi := &file_testing_attribute_v1_attribute_proto_msgTypes[0]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use TestResource.ProtoReflect.Descriptor instead.
+func (*TestResource) Descriptor() ([]byte, []int) {
+	return file_testing_attribute_v1_attribute_proto_rawDescGZIP(), []int{0}
+}
+
+func (x *TestResource) GetId() string {
+	if x != nil {
+		return x.Id
+	}
+	return ""
+}
+
+func (m *TestResource) GetAttrs() isTestResource_Attrs {
+	if m != nil {
+		return m.Attrs
+	}
+	return nil
+}
+
+func (x *TestResource) GetAttributes() *structpb.Struct {
+	if x, ok := x.GetAttrs().(*TestResource_Attributes); ok {
+		return x.Attributes
+	}
+	return nil
+}
+
+func (x *TestResource) GetSubResourceAttributes() *TestSubResourceAttributes {
+	if x, ok := x.GetAttrs().(*TestResource_SubResourceAttributes); ok {
+		return x.SubResourceAttributes
+	}
+	return nil
+}
+
+type isTestResource_Attrs interface {
+	isTestResource_Attrs()
+}
+
+type TestResource_Attributes struct {
+	Attributes *structpb.Struct `protobuf:"bytes,10,opt,name=attributes,proto3,oneof"`
+}
+
+type TestResource_SubResourceAttributes struct {
+	SubResourceAttributes *TestSubResourceAttributes `protobuf:"bytes,20,opt,name=sub_resource_attributes,json=subResourceAttributes,proto3,oneof"`
+}
+
+func (*TestResource_Attributes) isTestResource_Attrs() {}
+
+func (*TestResource_SubResourceAttributes) isTestResource_Attrs() {}
+
+type TestSubResourceAttributes struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	Name string `protobuf:"bytes,1,opt,name=name,proto3" json:"name,omitempty"`
+}
+
+func (x *TestSubResourceAttributes) Reset() {
+	*x = TestSubResourceAttributes{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_testing_attribute_v1_attribute_proto_msgTypes[1]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *TestSubResourceAttributes) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*TestSubResourceAttributes) ProtoMessage() {}
+
+func (x *TestSubResourceAttributes) ProtoReflect() protoreflect.Message {
+	mi := &file_testing_attribute_v1_attribute_proto_msgTypes[1]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use TestSubResourceAttributes.ProtoReflect.Descriptor instead.
+func (*TestSubResourceAttributes) Descriptor() ([]byte, []int) {
+	return file_testing_attribute_v1_attribute_proto_rawDescGZIP(), []int{1}
+}
+
+func (x *TestSubResourceAttributes) GetName() string {
+	if x != nil {
+		return x.Name
+	}
+	return ""
+}
+
+type TestNoAttributes struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	Id string `protobuf:"bytes,1,opt,name=id,proto3" json:"id,omitempty"`
+}
+
+func (x *TestNoAttributes) Reset() {
+	*x = TestNoAttributes{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_testing_attribute_v1_attribute_proto_msgTypes[2]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *TestNoAttributes) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*TestNoAttributes) ProtoMessage() {}
+
+func (x *TestNoAttributes) ProtoReflect() protoreflect.Message {
+	mi := &file_testing_attribute_v1_attribute_proto_msgTypes[2]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use TestNoAttributes.ProtoReflect.Descriptor instead.
+func (*TestNoAttributes) Descriptor() ([]byte, []int) {
+	return file_testing_attribute_v1_attribute_proto_rawDescGZIP(), []int{2}
+}
+
+func (x *TestNoAttributes) GetId() string {
+	if x != nil {
+		return x.Id
+	}
+	return ""
+}
+
+var File_testing_attribute_v1_attribute_proto protoreflect.FileDescriptor
+
+var file_testing_attribute_v1_attribute_proto_rawDesc = []byte{
+	0x0a, 0x24, 0x74, 0x65, 0x73, 0x74, 0x69, 0x6e, 0x67, 0x2f, 0x61, 0x74, 0x74, 0x72, 0x69, 0x62,
+	0x75, 0x74, 0x65, 0x2f, 0x76, 0x31, 0x2f, 0x61, 0x74, 0x74, 0x72, 0x69, 0x62, 0x75, 0x74, 0x65,
+	0x2e, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x12, 0x14, 0x74, 0x65, 0x73, 0x74, 0x69, 0x6e, 0x67, 0x2e,
+	0x61, 0x74, 0x74, 0x72, 0x69, 0x62, 0x75, 0x74, 0x65, 0x2e, 0x76, 0x31, 0x1a, 0x1c, 0x67, 0x6f,
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+	0x72, 0x75, 0x63, 0x74, 0x2e, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x1a, 0x2a, 0x63, 0x6f, 0x6e, 0x74,
+	0x72, 0x6f, 0x6c, 0x6c, 0x65, 0x72, 0x2f, 0x63, 0x75, 0x73, 0x74, 0x6f, 0x6d, 0x5f, 0x6f, 0x70,
+	0x74, 0x69, 0x6f, 0x6e, 0x73, 0x2f, 0x76, 0x31, 0x2f, 0x6f, 0x70, 0x74, 0x69, 0x6f, 0x6e, 0x73,
+	0x2e, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x22, 0xec, 0x01, 0x0a, 0x0c, 0x54, 0x65, 0x73, 0x74, 0x52,
+	0x65, 0x73, 0x6f, 0x75, 0x72, 0x63, 0x65, 0x12, 0x0e, 0x0a, 0x02, 0x69, 0x64, 0x18, 0x01, 0x20,
+	0x01, 0x28, 0x09, 0x52, 0x02, 0x69, 0x64, 0x12, 0x46, 0x0a, 0x0a, 0x61, 0x74, 0x74, 0x72, 0x69,
+	0x62, 0x75, 0x74, 0x65, 0x73, 0x18, 0x0a, 0x20, 0x01, 0x28, 0x0b, 0x32, 0x17, 0x2e, 0x67, 0x6f,
+	0x6f, 0x67, 0x6c, 0x65, 0x2e, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x62, 0x75, 0x66, 0x2e, 0x53, 0x74,
+	0x72, 0x75, 0x63, 0x74, 0x42, 0x0b, 0x9a, 0xe3, 0x29, 0x07, 0x64, 0x65, 0x66, 0x61, 0x75, 0x6c,
+	0x74, 0x48, 0x00, 0x52, 0x0a, 0x61, 0x74, 0x74, 0x72, 0x69, 0x62, 0x75, 0x74, 0x65, 0x73, 0x12,
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+	0x32, 0x2f, 0x2e, 0x74, 0x65, 0x73, 0x74, 0x69, 0x6e, 0x67, 0x2e, 0x61, 0x74, 0x74, 0x72, 0x69,
+	0x62, 0x75, 0x74, 0x65, 0x2e, 0x76, 0x31, 0x2e, 0x54, 0x65, 0x73, 0x74, 0x53, 0x75, 0x62, 0x52,
+	0x65, 0x73, 0x6f, 0x75, 0x72, 0x63, 0x65, 0x41, 0x74, 0x74, 0x72, 0x69, 0x62, 0x75, 0x74, 0x65,
+	0x73, 0x42, 0x10, 0x9a, 0xe3, 0x29, 0x0c, 0x73, 0x75, 0x62, 0x5f, 0x72, 0x65, 0x73, 0x6f, 0x75,
+	0x72, 0x63, 0x65, 0x48, 0x00, 0x52, 0x15, 0x73, 0x75, 0x62, 0x52, 0x65, 0x73, 0x6f, 0x75, 0x72,
+	0x63, 0x65, 0x41, 0x74, 0x74, 0x72, 0x69, 0x62, 0x75, 0x74, 0x65, 0x73, 0x42, 0x07, 0x0a, 0x05,
+	0x61, 0x74, 0x74, 0x72, 0x73, 0x22, 0x2f, 0x0a, 0x19, 0x54, 0x65, 0x73, 0x74, 0x53, 0x75, 0x62,
+	0x52, 0x65, 0x73, 0x6f, 0x75, 0x72, 0x63, 0x65, 0x41, 0x74, 0x74, 0x72, 0x69, 0x62, 0x75, 0x74,
+	0x65, 0x73, 0x12, 0x12, 0x0a, 0x04, 0x6e, 0x61, 0x6d, 0x65, 0x18, 0x01, 0x20, 0x01, 0x28, 0x09,
+	0x52, 0x04, 0x6e, 0x61, 0x6d, 0x65, 0x22, 0x22, 0x0a, 0x10, 0x54, 0x65, 0x73, 0x74, 0x4e, 0x6f,
+	0x41, 0x74, 0x74, 0x72, 0x69, 0x62, 0x75, 0x74, 0x65, 0x73, 0x12, 0x0e, 0x0a, 0x02, 0x69, 0x64,
+	0x18, 0x01, 0x20, 0x01, 0x28, 0x09, 0x52, 0x02, 0x69, 0x64, 0x42, 0x48, 0x5a, 0x46, 0x67, 0x69,
+	0x74, 0x68, 0x75, 0x62, 0x2e, 0x63, 0x6f, 0x6d, 0x2f, 0x68, 0x61, 0x73, 0x68, 0x69, 0x63, 0x6f,
+	0x72, 0x70, 0x2f, 0x62, 0x6f, 0x75, 0x6e, 0x64, 0x61, 0x72, 0x79, 0x2f, 0x69, 0x6e, 0x74, 0x65,
+	0x72, 0x6e, 0x61, 0x6c, 0x2f, 0x67, 0x65, 0x6e, 0x2f, 0x74, 0x65, 0x73, 0x74, 0x69, 0x6e, 0x67,
+	0x2f, 0x61, 0x74, 0x74, 0x72, 0x69, 0x62, 0x75, 0x74, 0x65, 0x3b, 0x61, 0x74, 0x74, 0x72, 0x69,
+	0x62, 0x75, 0x74, 0x65, 0x62, 0x06, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x33,
+}
+
+var (
+	file_testing_attribute_v1_attribute_proto_rawDescOnce sync.Once
+	file_testing_attribute_v1_attribute_proto_rawDescData = file_testing_attribute_v1_attribute_proto_rawDesc
+)
+
+func file_testing_attribute_v1_attribute_proto_rawDescGZIP() []byte {
+	file_testing_attribute_v1_attribute_proto_rawDescOnce.Do(func() {
+		file_testing_attribute_v1_attribute_proto_rawDescData = protoimpl.X.CompressGZIP(file_testing_attribute_v1_attribute_proto_rawDescData)
+	})
+	return file_testing_attribute_v1_attribute_proto_rawDescData
+}
+
+var file_testing_attribute_v1_attribute_proto_msgTypes = make([]protoimpl.MessageInfo, 3)
+var file_testing_attribute_v1_attribute_proto_goTypes = []interface{}{
+	(*TestResource)(nil),              // 0: testing.attribute.v1.TestResource
+	(*TestSubResourceAttributes)(nil), // 1: testing.attribute.v1.TestSubResourceAttributes
+	(*TestNoAttributes)(nil),          // 2: testing.attribute.v1.TestNoAttributes
+	(*structpb.Struct)(nil),           // 3: google.protobuf.Struct
+}
+var file_testing_attribute_v1_attribute_proto_depIdxs = []int32{
+	3, // 0: testing.attribute.v1.TestResource.attributes:type_name -> google.protobuf.Struct
+	1, // 1: testing.attribute.v1.TestResource.sub_resource_attributes:type_name -> testing.attribute.v1.TestSubResourceAttributes
+	2, // [2:2] is the sub-list for method output_type
+	2, // [2:2] is the sub-list for method input_type
+	2, // [2:2] is the sub-list for extension type_name
+	2, // [2:2] is the sub-list for extension extendee
+	0, // [0:2] is the sub-list for field type_name
+}
+
+func init() { file_testing_attribute_v1_attribute_proto_init() }
+func file_testing_attribute_v1_attribute_proto_init() {
+	if File_testing_attribute_v1_attribute_proto != nil {
+		return
+	}
+	if !protoimpl.UnsafeEnabled {
+		file_testing_attribute_v1_attribute_proto_msgTypes[0].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*TestResource); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+		file_testing_attribute_v1_attribute_proto_msgTypes[1].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*TestSubResourceAttributes); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+		file_testing_attribute_v1_attribute_proto_msgTypes[2].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*TestNoAttributes); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+	}
+	file_testing_attribute_v1_attribute_proto_msgTypes[0].OneofWrappers = []interface{}{
+		(*TestResource_Attributes)(nil),
+		(*TestResource_SubResourceAttributes)(nil),
+	}
+	type x struct{}
+	out := protoimpl.TypeBuilder{
+		File: protoimpl.DescBuilder{
+			GoPackagePath: reflect.TypeOf(x{}).PkgPath(),
+			RawDescriptor: file_testing_attribute_v1_attribute_proto_rawDesc,
+			NumEnums:      0,
+			NumMessages:   3,
+			NumExtensions: 0,
+			NumServices:   0,
+		},
+		GoTypes:           file_testing_attribute_v1_attribute_proto_goTypes,
+		DependencyIndexes: file_testing_attribute_v1_attribute_proto_depIdxs,
+		MessageInfos:      file_testing_attribute_v1_attribute_proto_msgTypes,
+	}.Build()
+	File_testing_attribute_v1_attribute_proto = out.File
+	file_testing_attribute_v1_attribute_proto_rawDesc = nil
+	file_testing_attribute_v1_attribute_proto_goTypes = nil
+	file_testing_attribute_v1_attribute_proto_depIdxs = nil
+}
diff --git a/internal/proto/local/testing/attribute/v1/attribute.proto b/internal/proto/local/testing/attribute/v1/attribute.proto
new file mode 100644
index 0000000000..6b3d95b04a
--- /dev/null
+++ b/internal/proto/local/testing/attribute/v1/attribute.proto
@@ -0,0 +1,25 @@
+syntax = "proto3";
+
+package testing.attribute.v1;
+
+option go_package = "github.com/hashicorp/boundary/internal/gen/testing/attribute;attribute";
+
+import "google/protobuf/struct.proto";
+import "controller/custom_options/v1/options.proto";
+
+message TestResource {
+  string id = 1;
+
+  oneof attrs {
+    google.protobuf.Struct attributes = 10 [(controller.custom_options.v1.subtype) = "default"];
+    TestSubResourceAttributes sub_resource_attributes = 20 [(controller.custom_options.v1.subtype) = "sub_resource"];
+  }
+}
+
+message TestSubResourceAttributes {
+  string name = 1;
+}
+
+message TestNoAttributes {
+  string id = 1;
+}
diff --git a/internal/types/subtypes/attributes.go b/internal/types/subtypes/attributes.go
new file mode 100644
index 0000000000..95ab53c695
--- /dev/null
+++ b/internal/types/subtypes/attributes.go
@@ -0,0 +1,124 @@
+package subtypes
+
+import (
+	"fmt"
+	"sync"
+
+	"github.com/hashicorp/boundary/sdk/pbs/controller/protooptions"
+	"google.golang.org/protobuf/proto"
+	"google.golang.org/protobuf/reflect/protoreflect"
+	"google.golang.org/protobuf/reflect/protoregistry"
+	"google.golang.org/protobuf/types/descriptorpb"
+)
+
+const (
+	defaultSubtype = "default"
+)
+
+func init() {
+	globalAttributeKeys = attributeKeys{
+		m: make(map[protoreflect.FullName]fieldMap),
+	}
+
+	protoregistry.GlobalTypes.RangeMessages(func(m protoreflect.MessageType) bool {
+		d := m.Descriptor()
+		if err := globalAttributeKeys.register(d); err != nil {
+			panic(err)
+		}
+		return true
+	})
+}
+
+type fieldMap map[Subtype]protoreflect.FieldDescriptor
+
+type attributeKeys struct {
+	sync.RWMutex
+	m map[protoreflect.FullName]fieldMap
+}
+
+var globalAttributeKeys attributeKeys
+
+// register examines the given protobuf MessageDescriptor for fields that have
+// the Subtype protobuf extension. It uses these to build a fieldMap of subtype
+// strings to the protobuf FieldDescriptors. If the message does not have any
+// subtypes it will not be registered, but no error is returned, allowing this
+// to be called on any protobuf message. However, if a message has subtypes but
+// does not provide a field with a subtype of "default" an error is returned.
+func (ak attributeKeys) register(d protoreflect.MessageDescriptor) error {
+	ak.Lock()
+	defer ak.Unlock()
+
+	if ak.m == nil {
+		ak.m = make(map[protoreflect.FullName]fieldMap)
+	}
+
+	fn := d.FullName()
+
+	if _, ok := ak.m[fn]; ok {
+		return fmt.Errorf("proto message %s already registered", fn)
+	}
+
+	km := make(fieldMap, 0)
+
+	fields := d.Fields()
+	for i := 0; i < fields.Len(); i++ {
+		f := fields.Get(i)
+
+		opts := f.Options().(*descriptorpb.FieldOptions)
+		st := proto.GetExtension(opts, protooptions.E_Subtype).(string)
+		if st != "" {
+			km[Subtype(st)] = f
+		}
+	}
+
+	// no subtypes were found, so nothing needs to be registered
+	if len(km) <= 0 {
+		return nil
+	}
+
+	// If a message has subtypes, it must provide a "default" to support plugins.
+	if _, ok := km[defaultSubtype]; !ok {
+		return fmt.Errorf("proto message %s with subtype attributes but no 'default'", fn)
+	}
+
+	ak.m[fn] = km
+
+	return nil
+}
+
+// protoAttributeField retrieves the FieldDescriptor for a given subtype's
+// attribute fields. If the corresponding protobuf message has not been
+// registered it will return an error.
+func (ak attributeKeys) protoAttributeField(d protoreflect.MessageDescriptor, t Subtype) (protoreflect.FieldDescriptor, error) {
+	ak.RLock()
+	defer ak.RUnlock()
+
+	fn := d.FullName()
+
+	km, ok := ak.m[fn]
+	if !ok {
+		return nil, fmt.Errorf("proto message %s not registered", fn)
+	}
+
+	tt, ok := km[t]
+	if ok {
+		return tt, nil
+	}
+
+	tt, ok = km[defaultSubtype]
+	if !ok {
+		return nil, fmt.Errorf("missing default for %s", fn)
+	}
+	return tt, nil
+}
+
+// TODO: fix doc
+// protoAttributeField is used by the attrMarshaler to translate between JSON
+// formats for the API and for the protobuf messages. It expects a
+// proto.Message with a OneOf field for the subtype attributes and the subtype
+// string. It returns the string for the JSON key that that should be used for
+// the subtype's attributes fields.
+func protoAttributeField(msg proto.Message, t Subtype) (protoreflect.FieldDescriptor, error) {
+	d := msg.ProtoReflect().Descriptor()
+	return globalAttributeKeys.protoAttributeField(d, t)
+}
diff --git a/internal/types/subtypes/attributes_test.go b/internal/types/subtypes/attributes_test.go
new file mode 100644
index 0000000000..cd39577ad4
--- /dev/null
+++ b/internal/types/subtypes/attributes_test.go
@@ -0,0 +1,119 @@
+package subtypes
+
+import (
+	"testing"
+
+	"github.com/hashicorp/boundary/internal/gen/testing/attribute"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"google.golang.org/protobuf/proto"
+	"google.golang.org/protobuf/reflect/protoreflect"
+)
+
+func TestProtoAttributeKey(t *testing.T) {
+	cases := []struct {
+		name     string
+		msg      proto.Message
+		subtype  Subtype
+		expected protoreflect.FullName
+	}{
+		{
+			"TestResource/sub_resource",
+			&attribute.TestResource{},
+			"sub_resource",
+			"testing.attribute.v1.TestResource.sub_resource_attributes",
+		},
+		{
+			"TestResource/default",
+			&attribute.TestResource{},
+			defaultSubtype,
+			"testing.attribute.v1.TestResource.attributes",
+		},
+		{
+			"TestResource/unknown",
+			&attribute.TestResource{},
+			UnknownSubtype,
+			"testing.attribute.v1.TestResource.attributes",
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			k, err := protoAttributeField(tc.msg, tc.subtype)
+			require.NoError(t, err)
+			assert.Equal(t, tc.expected, k.FullName())
+		})
+	}
+}
+
+func TestProtoAttributeKeyErrors(t *testing.T) {
+	type notproto struct{}
+	cases := []struct {
+		name        string
+		msg         proto.Message
+		subtype     Subtype
+		expectedErr string
+	}{
+		{
+			"TestNoAttributes/sub_resource",
+			&attribute.TestNoAttributes{},
+			"sub_resource",
+			"proto message testing.attribute.v1.TestNoAttributes not registered",
+		},
+		{
+			"TestNoAttributes/default",
+			&attribute.TestNoAttributes{},
+			defaultSubtype,
+			"proto message testing.attribute.v1.TestNoAttributes not registered",
+		},
+		{
+			"TestNoAttributes/unknown",
+			&attribute.TestNoAttributes{},
+			UnknownSubtype,
+			"proto message testing.attribute.v1.TestNoAttributes not registered",
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			_, err := protoAttributeField(tc.msg, tc.subtype)
+			require.EqualError(t, err, tc.expectedErr)
+		})
+	}
+}
+
+func TestRegisterErrors(t *testing.T) {
+	cases := []struct {
+		name        string
+		msg         proto.Message
+		expectedErr string
+	}{
+		{
+			"AlreadyRegistered",
+			&attribute.TestResource{},
+			"proto message testing.attribute.v1.TestResource already registered",
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			err := globalAttributeKeys.register(tc.msg.ProtoReflect().Descriptor())
+			require.EqualError(t, err, tc.expectedErr)
+		})
+	}
+}
+
+func TestRegisterNoSubtypes(t *testing.T) {
+	ak := attributeKeys{
+		m: make(map[protoreflect.FullName]fieldMap),
+	}
+
+	msg := &attribute.TestNoAttributes{}
+	d := msg.ProtoReflect().Descriptor()
+
+	err := ak.register(d)
+	require.NoError(t, err)
+
+	_, ok := ak.m[d.FullName()]
+	require.False(t, ok)
+}