Merge pull request #46 from soorena776/issue_887

Implementating ReferenceResolver in managed reconciler

apis/core/v1alpha1/condition.go

@@ -34,6 +34,9 @@ const (
 	// TypeSynced managed resources are believed to be in sync with the
 	// Kubernetes resources that manage their lifecycle.
 	TypeSynced ConditionType = "Synced"
+
+	// TypeReferencesResolved managed resources' references are resolved
+	TypeReferencesResolved = "ReferencesResolved"
 )
 
 // A ConditionReason represents the reason a resource is in a condition.
@@ -53,6 +56,12 @@ const (
 	ReasonReconcileError   ConditionReason = "Encountered an error during managed resource reconciliation"
 )
 
+// Reason references for a resource are or are not resolved
+const (
+	ReasonReferenceResolveSuccess  ConditionReason = "Successfully resolved managed resource references to other resources"
+	ReasonResolveReferencesBlocked ConditionReason = "One or more of referenced resources do not exist, or are not yet Ready"
+)
+
 // A Condition that may apply to a managed resource.
 type Condition struct {
 	// Type of this condition. At most one of each condition type may apply to
@@ -113,6 +122,18 @@ func NewConditionedStatus(c ...Condition) *ConditionedStatus {
 	return s
 }
 
+// GetCondition returns the condition for the given ConditionType if exists,
+// otherwise returns nil
+func (s *ConditionedStatus) GetCondition(ct ConditionType) Condition {
+	for _, c := range s.Conditions {
+		if c.Type == ct {
+			return c
+		}
+	}
+
+	return Condition{Type: ct, Status: corev1.ConditionUnknown}
+}
+
 // SetConditions sets the supplied conditions, replacing any existing conditions
 // of the same type. This is a no-op if all supplied conditions are identical,
 // ignoring the last transition time, to those already set.
@@ -239,3 +260,28 @@ func ReconcileError(err error) Condition {
 		Message:            err.Error(),
 	}
 }
+
+// ReferenceResolutionSuccess returns a condition indicating that Crossplane
+// successfully resolved the references used in the managed resource
+func ReferenceResolutionSuccess() Condition {
+	return Condition{
+		Type:               TypeReferencesResolved,
+		Status:             corev1.ConditionTrue,
+		LastTransitionTime: metav1.Now(),
+		Reason:             ReasonReferenceResolveSuccess,
+	}
+}
+
+// ReferenceResolutionBlocked returns a condition indicating that Crossplane is
+// unable to resolve the references used in the managed resource. This could
+// mean that one or more of referred resources do not yet exist, or are not yet
+// Ready
+func ReferenceResolutionBlocked(err error) Condition {
+	return Condition{
+		Type:               TypeReferencesResolved,
+		Status:             corev1.ConditionFalse,
+		LastTransitionTime: metav1.Now(),
+		Reason:             ReasonResolveReferencesBlocked,
+		Message:            err.Error(),
+	}
+}

apis/core/v1alpha1/condition_test.go

@@ -153,6 +153,37 @@ func TestSetConditions(t *testing.T) {
 	}
 }
 
+func TestGetCondition(t *testing.T) {
+	cases := map[string]struct {
+		cs   *ConditionedStatus
+		t    ConditionType
+		want Condition
+	}{
+		"ConditionExists": {
+			cs:   NewConditionedStatus(Available()),
+			t:    TypeReady,
+			want: Available(),
+		},
+		"ConditionDoesNotExist": {
+			cs: NewConditionedStatus(Available()),
+			t:  TypeSynced,
+			want: Condition{
+				Type:   TypeSynced,
+				Status: corev1.ConditionUnknown,
+			},
+		},
+	}
+
+	for name, tc := range cases {
+		t.Run(name, func(t *testing.T) {
+			got := tc.cs.GetCondition(tc.t)
+			if diff := cmp.Diff(tc.want, got); diff != "" {
+				t.Errorf("tc.cs.GetConditions(...): -want, +got:\n%s", diff)
+			}
+		})
+	}
+}
+
 func TestConditionWithMessage(t *testing.T) {
 	testMsg := "Something went wrong on cloud side"
 	cases := map[string]struct {

pkg/resource/interfaces.go

@@ -31,11 +31,11 @@ type Bindable interface {
 	GetBindingPhase() v1alpha1.BindingPhase
 }
 
-// A ConditionSetter may have conditions set. Conditions are informational, and
+// A Conditioned may have conditions set or retrieved. Conditions are typically
-// typically indicate the status of both a resource and its reconciliation
+// indicate the status of both a resource and its reconciliation process.
-// process.
+type Conditioned interface {
-type ConditionSetter interface {
 	SetConditions(c ...v1alpha1.Condition)
+	GetCondition(v1alpha1.ConditionType) v1alpha1.Condition
 }
 
 // A ClaimReferencer may reference a resource claim.
@@ -91,7 +91,7 @@ type Claim interface {
 	ManagedResourceReferencer
 	ConnectionSecretWriterTo
 
-	ConditionSetter
+	Conditioned
 	Bindable
 }
 
@@ -115,7 +115,7 @@ type Managed interface {
 	ConnectionSecretWriterTo
 	Reclaimer
 
-	ConditionSetter
+	Conditioned
 	Bindable
 }
 

pkg/resource/interfaces_test.go

@@ -29,9 +29,12 @@ type MockBindable struct{ Phase v1alpha1.BindingPhase }
 func (m *MockBindable) SetBindingPhase(p v1alpha1.BindingPhase) { m.Phase = p }
 func (m *MockBindable) GetBindingPhase() v1alpha1.BindingPhase  { return m.Phase }
 
-type MockConditionSetter struct{ Conditions []v1alpha1.Condition }
+type MockConditioned struct{ Conditions []v1alpha1.Condition }
 
-func (m *MockConditionSetter) SetConditions(c ...v1alpha1.Condition) { m.Conditions = c }
+func (m *MockConditioned) SetConditions(c ...v1alpha1.Condition) { m.Conditions = c }
+func (m *MockConditioned) GetCondition(ct v1alpha1.ConditionType) v1alpha1.Condition {
+	return v1alpha1.Condition{Type: ct, Status: corev1.ConditionUnknown}
+}
 
 type MockClaimReferencer struct{ Ref *corev1.ObjectReference }
 
@@ -89,7 +92,7 @@ type MockClaim struct {
 	MockPortableClassReferencer
 	MockManagedResourceReferencer
 	MockConnectionSecretWriterTo
-	MockConditionSetter
+	MockConditioned
 	MockBindable
 }
 
@@ -112,7 +115,7 @@ type MockManaged struct {
 	MockClaimReferencer
 	MockConnectionSecretWriterTo
 	MockReclaimer
-	MockConditionSetter
+	MockConditioned
 	MockBindable
 }
 

pkg/resource/managed_reconciler.go

@@ -32,8 +32,10 @@ import (
 )
 
 const (
-	managedControllerName   = "managedresource.crossplane.io"
+	managedControllerName               = "managedresource.crossplane.io"
-	managedFinalizerName    = "finalizer." + managedControllerName
+	managedFinalizerName                = "finalizer." + managedControllerName
+	managedResourceStructTagPackageName = "resource"
+
 	managedReconcileTimeout = 1 * time.Minute
 
 	defaultManagedShortWait = 30 * time.Second
@@ -87,6 +89,14 @@ func (m ManagedInitializerFn) Initialize(ctx context.Context, mg Managed) error
 	return m(ctx, mg)
 }
 
+// ManagedReferenceResolverFn is the pluggable struct to produce objects with ManagedReferenceResolver interface.
+type ManagedReferenceResolverFn func(context.Context, CanReference) error
+
+// ResolveReferences calls ManagedReferenceResolverFn function
+func (m ManagedReferenceResolverFn) ResolveReferences(ctx context.Context, res CanReference) error {
+	return m(ctx, res)
+}
+
 // An ExternalConnecter produces a new ExternalClient given the supplied
 // Managed resource.
 type ExternalConnecter interface {
@@ -224,6 +234,7 @@ type mrManaged struct {
 	ManagedConnectionPublisher
 	ManagedFinalizer
 	ManagedInitializer
+	ManagedReferenceResolver
 }
 
 func defaultMRManaged(m manager.Manager) mrManaged {
@@ -234,6 +245,7 @@ func defaultMRManaged(m manager.Manager) mrManaged {
 			NewManagedNameAsExternalName(m.GetClient()),
 			NewAPIManagedFinalizerAdder(m.GetClient()),
 		},
+		ManagedReferenceResolver: NewAPIManagedReferenceResolver(m.GetClient()),
 	}
 }
 
@@ -360,6 +372,21 @@ func (r *ManagedReconciler) Reconcile(req reconcile.Request) (reconcile.Result,
 		return reconcile.Result{RequeueAfter: r.shortWait}, errors.Wrap(r.client.Status().Update(ctx, managed), errUpdateManagedStatus)
 	}
 
+	if !IsConditionTrue(managed.GetCondition(v1alpha1.TypeReferencesResolved)) {
+		if err := r.managed.ResolveReferences(ctx, managed); err != nil {
+			condition := v1alpha1.ReconcileError(err)
+			if IsReferencesAccessError(err) {
+				condition = v1alpha1.ReferenceResolutionBlocked(err)
+			}
+
+			managed.SetConditions(condition)
+			return reconcile.Result{RequeueAfter: r.longWait}, errors.Wrap(r.client.Status().Update(ctx, managed), errUpdateManagedStatus)
+		}
+
+		// Add ReferenceResolutionSuccess to the conditions
+		managed.SetConditions(v1alpha1.ReferenceResolutionSuccess())
+	}
+
 	observation, err := external.Observe(ctx, managed)
 	if err != nil {
 		// We'll usually hit this case if our Provider credentials are invalid

pkg/resource/managed_reconciler_test.go

@@ -51,6 +51,7 @@ func TestManagedReconciler(t *testing.T) {
 	}
 
 	errBoom := errors.New("boom")
+	errNotReady := &referencesAccessErr{[]ReferenceStatus{{Name: "cool-res", Status: ReferenceNotReady}}}
 	now := metav1.Now()
 	testFinalizers := []string{"finalizer.crossplane.io"}
 	testConnectionDetails := ConnectionDetails{
@@ -517,6 +518,82 @@ func TestManagedReconciler(t *testing.T) {
 			},
 			want: want{result: reconcile.Result{RequeueAfter: defaultManagedShortWait}},
 		},
+		"ResolveReferences_NotReadyErr_ReturnsExpected": {
+			args: args{
+				m: &MockManager{
+					c: &test.MockClient{
+						MockGet: test.NewMockGetFn(nil),
+						MockStatusUpdate: test.MockStatusUpdateFn(func(_ context.Context, obj runtime.Object, _ ...client.UpdateOption) error {
+							want := &MockManaged{}
+							want.SetConditions(v1alpha1.ReferenceResolutionBlocked(errNotReady))
+							want.SetFinalizers(testFinalizers)
+							if diff := cmp.Diff(want, obj, test.EquateConditions()); diff != "" {
+								t.Errorf("-want, +got:\n%s", diff)
+							}
+							return nil
+						}),
+					},
+					s: MockSchemeWith(&MockManaged{}),
+				},
+				mg: ManagedKind(MockGVK(&MockManaged{})),
+				e: &ExternalClientFns{
+					ObserveFn: func(_ context.Context, _ Managed) (ExternalObservation, error) {
+						return ExternalObservation{
+							ResourceExists: false,
+						}, nil
+					},
+				},
+				o: []ManagedReconcilerOption{
+					func(r *ManagedReconciler) {
+						r.managed.ManagedInitializer = ManagedInitializerFn(func(_ context.Context, mg Managed) error {
+							mg.SetFinalizers(testFinalizers)
+							return nil
+						})
+					},
+					func(r *ManagedReconciler) {
+						r.managed.ManagedReferenceResolver = ManagedReferenceResolverFn(func(_ context.Context, res CanReference) error {
+							return errNotReady
+						})
+					},
+				},
+			},
+			want: want{result: reconcile.Result{RequeueAfter: defaultManagedLongWait}},
+		},
+		"ResolveReferences_OtherError_ReturnsExpected": {
+			args: args{
+				m: &MockManager{
+					c: &test.MockClient{
+						MockGet: test.NewMockGetFn(nil),
+						MockStatusUpdate: test.MockStatusUpdateFn(func(_ context.Context, obj runtime.Object, _ ...client.UpdateOption) error {
+							want := &MockManaged{}
+							want.SetConditions(v1alpha1.ReconcileError(errBoom))
+							want.SetFinalizers(testFinalizers)
+							if diff := cmp.Diff(want, obj, test.EquateConditions()); diff != "" {
+								t.Errorf("-want, +got:\n%s", diff)
+							}
+							return nil
+						}),
+					},
+					s: MockSchemeWith(&MockManaged{}),
+				},
+				mg: ManagedKind(MockGVK(&MockManaged{})),
+				e:  &ExternalClientFns{},
+				o: []ManagedReconcilerOption{
+					func(r *ManagedReconciler) {
+						r.managed.ManagedInitializer = ManagedInitializerFn(func(_ context.Context, mg Managed) error {
+							mg.SetFinalizers(testFinalizers)
+							return nil
+						})
+					},
+					func(r *ManagedReconciler) {
+						r.managed.ManagedReferenceResolver = ManagedReferenceResolverFn(func(_ context.Context, res CanReference) error {
+							return errBoom
+						})
+					},
+				},
+			},
+			want: want{result: reconcile.Result{RequeueAfter: defaultManagedLongWait}},
+		},
 		"EstablishResourceDoesNotExistError": {
 			args: args{
 				m: &MockManager{
@@ -534,13 +611,7 @@ func TestManagedReconciler(t *testing.T) {
 					s: MockSchemeWith(&MockManaged{}),
 				},
 				mg: ManagedKind(MockGVK(&MockManaged{})),
-				e: &ExternalClientFns{
+				e:  &ExternalClientFns{},
-					ObserveFn: func(_ context.Context, _ Managed) (ExternalObservation, error) {
-						return ExternalObservation{
-							ResourceExists: false,
-						}, nil
-					},
-				},
 				o: []ManagedReconcilerOption{
 					func(r *ManagedReconciler) {
 						r.managed.ManagedConnectionPublisher = ManagedConnectionPublisherFns{

pkg/resource/reference_resolver.go

@@ -0,0 +1,275 @@
+/*
+Copyright 2019 The Crossplane 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 resource
+
+import (
+	"context"
+	"fmt"
+	"reflect"
+
+	"github.com/pkg/errors"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/runtime"
+
+	"sigs.k8s.io/controller-runtime/pkg/client"
+)
+
+const (
+	attributeReferencerTagName = "attributereferencer"
+)
+
+// Error strings
+const (
+	errTaggedFieldlNotImplemented    = "ManagedReferenceResolver: the field has the %v tag, but has not implemented AttributeReferencer interface"
+	errBuildAttribute                = "ManagedReferenceResolver: could not build the attribute"
+	errAssignAttribute               = "ManagedReferenceResolver: could not assign the attribute"
+	errUpdateResourceAfterAssignment = "ManagedReferenceResolver: could not update the resource after resolving references"
+)
+
+// fieldHasTagPair is used in findAttributeReferencerFields
+type fieldHasTagPair struct {
+	fieldValue reflect.Value
+	hasTheTag  bool
+}
+
+// ReferenceStatusType is an enum type for the possible values for a Reference Status
+type ReferenceStatusType int
+
+const (
+	// ReferenceStatusUnknown is the default value
+	ReferenceStatusUnknown ReferenceStatusType = iota
+	// ReferenceNotFound shows that the reference is not found
+	ReferenceNotFound
+	// ReferenceNotReady shows that the reference is not ready
+	ReferenceNotReady
+	// ReferenceReady shows that the reference is ready
+	ReferenceReady
+)
+
+func (t ReferenceStatusType) String() string {
+	return []string{"Unknown", "NotFound", "NotReady", "Ready"}[t]
+}
+
+// ReferenceStatus has the name and status of a reference
+type ReferenceStatus struct {
+	Name   string
+	Status ReferenceStatusType
+}
+
+func (r ReferenceStatus) String() string {
+	return fmt.Sprintf("{reference:%s status:%s}", r.Name, r.Status)
+}
+
+// referencesAccessErr is used to indicate that one or more references can not
+// be accessed
+type referencesAccessErr struct {
+	statuses []ReferenceStatus
+}
+
+// newReferenceAccessErr returns a referencesAccessErr if any of the given
+// references are not ready
+func newReferenceAccessErr(statuses []ReferenceStatus) error {
+	for _, st := range statuses {
+		if st.Status != ReferenceReady {
+			return &referencesAccessErr{statuses}
+		}
+	}
+
+	return nil
+}
+
+func (r *referencesAccessErr) Error() string {
+	return fmt.Sprintf("%s", r.statuses)
+}
+
+// IsReferencesAccessError returns true if the given error indicates that some
+// of the `AttributeReferencer` fields are referring to objects that are not
+// accessible, either they are not ready or they do not yet exist
+func IsReferencesAccessError(err error) bool {
+	_, result := err.(*referencesAccessErr)
+	return result
+}
+
+// CanReference is a type that is used as ReferenceResolver input
+type CanReference interface {
+	runtime.Object
+	metav1.Object
+}
+
+// AttributeReferencer is an interface for referencing and resolving
+// cross-resource attribute references. See
+// https://github.com/crossplaneio/crossplane/blob/master/design/one-pager-cross-resource-referencing.md
+// for more information
+type AttributeReferencer interface {
+
+	// GetStatus looks up the referenced objects in K8S api and returns a list
+	// of ReferenceStatus
+	GetStatus(context.Context, CanReference, client.Reader) ([]ReferenceStatus, error)
+
+	// Build retrieves referenced resource, as well as other non-managed
+	// resources (like a `Provider`), and builds the referenced attribute
+	Build(context.Context, CanReference, client.Reader) (string, error)
+
+	// Assign accepts a managed resource object, and assigns the given value to the
+	// corresponding property
+	Assign(CanReference, string) error
+}
+
+// A ManagedReferenceResolver resolves the references to other managed
+// resources, by looking them up in the Kubernetes API server. The references
+// are the fields in the managed resource that implement AttributeReferencer
+// interface and have
+// `attributeReferencerTagName:"managedResourceStructTagPackageName"` tag
+type ManagedReferenceResolver interface {
+	ResolveReferences(context.Context, CanReference) error
+}
+
+// APIManagedReferenceResolver resolves implements ManagedReferenceResolver interface
+type APIManagedReferenceResolver struct {
+	client client.Client
+}
+
+// NewAPIManagedReferenceResolver returns a new APIManagedReferenceResolver
+func NewAPIManagedReferenceResolver(c client.Client) *APIManagedReferenceResolver {
+	return &APIManagedReferenceResolver{c}
+}
+
+// ResolveReferences resolves references made to other managed resources
+func (r *APIManagedReferenceResolver) ResolveReferences(ctx context.Context, res CanReference) (err error) {
+	// retrieve all the referencer fields from the managed resource
+	referencers, err := findAttributeReferencerFields(res, false)
+	if err != nil {
+		// if there is an error it should immediately panic, since this means an
+		// attribute is tagged but doesn't implement AttributeReferencer
+		panic(err)
+	}
+
+	// if there are no referencers exit early
+	if len(referencers) == 0 {
+		return nil
+	}
+
+	// make sure that all the references are ready
+	allStatuses := []ReferenceStatus{}
+	for _, referencer := range referencers {
+		statuses, err := referencer.GetStatus(ctx, res, r.client)
+		if err != nil {
+			return err
+		}
+
+		allStatuses = append(allStatuses, statuses...)
+	}
+
+	if err := newReferenceAccessErr(allStatuses); err != nil {
+		return err
+	}
+
+	// build and assign the attributes
+	for _, referencer := range referencers {
+		val, err := referencer.Build(ctx, res, r.client)
+		if err != nil {
+			return errors.Wrap(err, errBuildAttribute)
+		}
+
+		if err := referencer.Assign(res, val); err != nil {
+			return errors.Wrap(err, errAssignAttribute)
+		}
+	}
+
+	// persist the updated managed resource
+	return errors.Wrap(r.client.Update(ctx, res), errUpdateResourceAfterAssignment)
+}
+
+// findAttributeReferencerFields recursively finds all non-nil fields in a struct and its sub types
+// that implement AttributeReferencer and have `attributeReferencerTagName:"managedResourceStructTagPackageName"` tag
+func findAttributeReferencerFields(obj interface{}, objHasTheRightTag bool) ([]AttributeReferencer, error) {
+	pairs := []fieldHasTagPair{}
+	v := reflect.ValueOf(obj)
+
+	switch v.Kind() {
+	case reflect.Ptr:
+		if v.IsNil() {
+			return nil, nil
+		}
+
+		pairs = append(pairs, fieldHasTagPair{v.Elem(), objHasTheRightTag})
+
+	case reflect.Struct:
+		for i := 0; i < v.NumField(); i++ {
+			pairs = append(pairs, fieldHasTagPair{v.Field(i), hasAttrRefTag(reflect.TypeOf(obj).Field(i))})
+		}
+
+	case reflect.Slice:
+		for i := 0; i < v.Len(); i++ {
+			pairs = append(pairs, fieldHasTagPair{v.Index(i), objHasTheRightTag})
+		}
+	}
+
+	return inspectFields(pairs)
+}
+
+// inspectFields along with findAttributeReferencerFields it recursively
+// inspects the extracted struct fields, and returns the ones that are of type
+// `AttributeReferencer`.
+func inspectFields(pairs []fieldHasTagPair) ([]AttributeReferencer, error) {
+	result := []AttributeReferencer{}
+	for _, pair := range pairs {
+		if !pair.fieldValue.CanInterface() {
+			if pair.hasTheTag {
+				// if the field has the tag but its value cannot be converted to
+				// an `Interface{}` (like a struct with private fields) it can't
+				// possibly implement the method sets. returning error here
+				// since, there won't be a recursive call for this value
+				return nil, errors.Errorf(errTaggedFieldlNotImplemented, attributeReferencerTagName)
+			}
+
+			continue
+		}
+
+		if pair.hasTheTag {
+			if ar, implements := pair.fieldValue.Interface().(AttributeReferencer); implements {
+				if !pair.fieldValue.IsNil() {
+					result = append(result, ar)
+				}
+
+				continue
+			}
+
+			// this is for the case where a tag is assigned to a struct, but it
+			// doesn't implement the interface
+			if pair.fieldValue.Kind() == reflect.Struct {
+				return nil, errors.Errorf(errTaggedFieldlNotImplemented, attributeReferencerTagName)
+			}
+		}
+
+		resolvers, err := findAttributeReferencerFields(pair.fieldValue.Interface(), pair.hasTheTag)
+		if err != nil {
+			return nil, err
+		}
+
+		result = append(result, resolvers...)
+	}
+
+	return result, nil
+}
+
+// hasAttrRefTag returns true if the given struct field has the
+// AttributeReference tag
+func hasAttrRefTag(field reflect.StructField) bool {
+	val, ok := field.Tag.Lookup(managedResourceStructTagPackageName)
+	return ok && (val == attributeReferencerTagName)
+}

pkg/resource/reference_resolver_test.go

@@ -0,0 +1,503 @@
+/*
+Copyright 2019 The Crossplane 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 resource
+
+import (
+	"context"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/pkg/errors"
+	corev1 "k8s.io/api/core/v1"
+	"k8s.io/apimachinery/pkg/runtime"
+	"sigs.k8s.io/controller-runtime/pkg/client"
+
+	"github.com/crossplaneio/crossplane-runtime/pkg/test"
+)
+
+type ItemAReferencer struct {
+	*corev1.LocalObjectReference
+
+	getStatusFn     func(context.Context, CanReference, client.Reader) ([]ReferenceStatus, error)
+	getStatusCalled bool
+	buildFn         func(context.Context, CanReference, client.Reader) (string, error)
+	buildCalled     bool
+	assignFn        func(CanReference, string) error
+	assignCalled    bool
+	assignParam     string
+}
+
+func (r *ItemAReferencer) GetStatus(ctx context.Context, res CanReference, reader client.Reader) ([]ReferenceStatus, error) {
+	r.getStatusCalled = true
+	return r.getStatusFn(ctx, res, reader)
+}
+
+func (r *ItemAReferencer) Build(ctx context.Context, res CanReference, reader client.Reader) (string, error) {
+	r.buildCalled = true
+	return r.buildFn(ctx, res, reader)
+}
+
+func (r *ItemAReferencer) Assign(res CanReference, val string) error {
+	r.assignCalled = true
+	r.assignParam = val
+	return r.assignFn(res, val)
+}
+
+type ItemBReferencer struct {
+	*corev1.LocalObjectReference
+}
+
+func (r *ItemBReferencer) GetStatus(context.Context, CanReference, client.Reader) ([]ReferenceStatus, error) {
+	return nil, nil
+}
+
+func (r *ItemBReferencer) Build(context.Context, CanReference, client.Reader) (string, error) {
+	return "", nil
+}
+
+func (r *ItemBReferencer) Assign(CanReference, string) error {
+	return nil
+}
+
+func Test_findAttributeReferencerFields(t *testing.T) {
+	// some structs that are used in this test
+	type mockStruct struct {
+		Name string
+	}
+
+	type MockInnerStruct struct {
+		ItemARef *ItemAReferencer `resource:"attributereferencer"`
+	}
+
+	type args struct {
+		o interface{}
+	}
+
+	type want struct {
+		arrLen int
+		err    error
+	}
+
+	// test cases
+	cases := map[string]struct {
+		args args
+		want want
+	}{
+		"ValidResourceWithSingleReferencer_AsObject_ShouldReturnExpected": {
+			args: args{
+				o: struct {
+					NonReferencerField mockStruct
+					ItemARef           *ItemAReferencer `resource:"attributereferencer"`
+				}{
+					NonReferencerField: mockStruct{},
+					ItemARef:           &ItemAReferencer{LocalObjectReference: &corev1.LocalObjectReference{"item-name"}},
+				},
+			},
+			want: want{
+				arrLen: 1,
+				err:    nil,
+			},
+		},
+		"ValidResourceWithSingleReferencer_AsPointer_ShouldReturnExpected": {
+			args: args{
+				o: &struct {
+					NonReferencerField mockStruct
+					ItemARef           *ItemAReferencer `resource:"attributereferencer"`
+				}{
+					NonReferencerField: mockStruct{},
+					ItemARef:           &ItemAReferencer{LocalObjectReference: &corev1.LocalObjectReference{"item-name"}},
+				},
+			},
+			want: want{
+				arrLen: 1,
+			},
+		},
+		"ValidResourceWithSingleReferencer_NilReferencer_ShouldReturnEmpty": {
+			args: args{
+				o: &struct {
+					NonReferencerField mockStruct
+					ItemARef           *ItemAReferencer `resource:"attributereferencer"`
+				}{
+					NonReferencerField: mockStruct{},
+				},
+			},
+			want: want{},
+		},
+		"NilResource_ShouldReturnEmpty": {
+			args: args{
+				o: nil,
+			},
+			want: want{},
+		},
+		"ValidResourceWithMultipleReferencers_AllReferencersArePopulated_ShouldReturnExpected": {
+			args: args{
+				o: &struct {
+					ItemARef *ItemAReferencer `resource:"attributereferencer"`
+					ItemBRef *ItemBReferencer `resource:"attributereferencer"`
+					AStruct  *MockInnerStruct
+				}{
+					ItemARef: &ItemAReferencer{LocalObjectReference: &corev1.LocalObjectReference{"itemA-name"}},
+					ItemBRef: &ItemBReferencer{LocalObjectReference: &corev1.LocalObjectReference{"itemB-name"}},
+					AStruct: &MockInnerStruct{
+						&ItemAReferencer{LocalObjectReference: &corev1.LocalObjectReference{"itemA-name"}},
+					},
+				},
+			},
+			want: want{
+				arrLen: 3,
+			},
+		},
+		"ValidResourceWithMultipleReferencers_ReferencersArePartiallyPopulated_ShouldReturnExpected": {
+			args: args{
+				o: &struct {
+					ItemARef *ItemAReferencer `resource:"attributereferencer"`
+					ItemBRef *ItemBReferencer `resource:"attributereferencer"`
+					AStruct  *MockInnerStruct
+				}{
+					ItemBRef: &ItemBReferencer{&corev1.LocalObjectReference{"itemB-name"}},
+					AStruct: &MockInnerStruct{
+						&ItemAReferencer{LocalObjectReference: &corev1.LocalObjectReference{"itemA-name"}},
+					},
+				},
+			},
+			want: want{
+				arrLen: 2,
+			},
+		},
+		"ValidResourceWithListOfReferencers_ListIsPopulated_ShouldReturnExpected": {
+			args: args{
+				o: &struct {
+					ItemsRef []*ItemAReferencer `resource:"attributereferencer"`
+				}{
+					ItemsRef: []*ItemAReferencer{
+						{LocalObjectReference: &corev1.LocalObjectReference{"itemA1-name"}},
+						{LocalObjectReference: &corev1.LocalObjectReference{"itemA2-name"}},
+					},
+				},
+			},
+			want: want{
+				arrLen: 2,
+			},
+		},
+		"ValidResourceWithListOfReferencers_ListIsEmpty_ShouldReturnEmpty": {
+			args: args{
+				o: &struct {
+					ItemsRef []*ItemAReferencer `resource:"attributereferencer"`
+				}{
+					ItemsRef: []*ItemAReferencer{},
+				},
+			},
+			want: want{},
+		},
+		"ResourceWithNotImplementingTaggedReferencers_ShouldReturnError": {
+			args: args{
+				o: struct {
+					// InvalidRef is tagged, but mockStruct doesn't implement
+					// the required interface
+					InvalidRef *mockStruct `resource:"attributereferencer"`
+				}{
+					InvalidRef: &mockStruct{"something"},
+				},
+			},
+			want: want{
+				err: errors.Errorf(errTaggedFieldlNotImplemented, attributeReferencerTagName),
+			},
+		},
+		"ResourceWithNotInterfaceableTaggedReferencers_ShouldReturnError": {
+			args: args{
+				o: struct {
+					// since nonReferencerField is not exported, its value is
+					// not interfaceable
+					nonReferencerField mockStruct `resource:"attributereferencer"`
+				}{
+					nonReferencerField: mockStruct{"something else"},
+				},
+			},
+			want: want{
+				err: errors.Errorf(errTaggedFieldlNotImplemented, attributeReferencerTagName),
+			},
+		},
+		"ResourceWithUntaggedReferencers_ShouldReturnEmpty": {
+			args: args{
+				o: struct {
+					// even though UntaggedRef has implemented the interface,
+					// but its not tagged
+					UntaggedRef *ItemAReferencer
+				}{
+					UntaggedRef: &ItemAReferencer{LocalObjectReference: &corev1.LocalObjectReference{"itemA-name"}},
+				},
+			},
+			want: want{},
+		},
+	}
+
+	for name, tc := range cases {
+		t.Run(name, func(t *testing.T) {
+			got, err := findAttributeReferencerFields(tc.args.o, false)
+
+			if diff := cmp.Diff(tc.want.err, err, test.EquateErrors()); diff != "" {
+				t.Errorf("findAttributeReferencerFields(...): -want error, +got error:\n%s", diff)
+			}
+
+			if diff := cmp.Diff(tc.want.arrLen, len(got)); diff != "" {
+				t.Errorf("findAttributeReferencerFields(...): -want len, +got len:\n%s", diff)
+			}
+		})
+	}
+}
+
+func Test_ResolveReferences(t *testing.T) {
+
+	validGetStatusFn := func(context.Context, CanReference, client.Reader) ([]ReferenceStatus, error) { return nil, nil }
+	validBuildFn := func(context.Context, CanReference, client.Reader) (string, error) { return "fakeValue", nil }
+	validAssignFn := func(CanReference, string) error { return nil }
+
+	errBoom := errors.New("boom")
+
+	type managed struct {
+		MockManaged
+		ItemARef *ItemAReferencer `resource:"attributereferencer"`
+	}
+
+	type args struct {
+		field            *ItemAReferencer
+		clientUpdaterErr error
+	}
+
+	type want struct {
+		getStatusCalled bool
+		buildCalled     bool
+		assignCalled    bool
+		assignParam     string
+		err             error
+	}
+
+	for name, tc := range map[string]struct {
+		args args
+		want want
+	}{
+		"ValidAttribute_ReturnsNil": {
+			args: args{
+				field: &ItemAReferencer{
+					getStatusFn: validGetStatusFn,
+					buildFn:     validBuildFn,
+					assignFn:    validAssignFn,
+				},
+			},
+			want: want{
+				getStatusCalled: true,
+				buildCalled:     true,
+				assignCalled:    true,
+				assignParam:     "fakeValue",
+			},
+		},
+		"ValidAttribute_GetStatusError_ReturnsErr": {
+			args: args{
+				field: &ItemAReferencer{
+					getStatusFn: func(context.Context, CanReference, client.Reader) ([]ReferenceStatus, error) {
+						return nil, errBoom
+					},
+					buildFn:  validBuildFn,
+					assignFn: validAssignFn,
+				},
+			},
+			want: want{
+				getStatusCalled: true,
+				err:             errBoom,
+			},
+		},
+		"ValidAttribute_GetStatusReturnsNotReadyStatus_ReturnsErr": {
+			args: args{
+				field: &ItemAReferencer{
+					getStatusFn: func(context.Context, CanReference, client.Reader) ([]ReferenceStatus, error) {
+						return []ReferenceStatus{{"cool-res", ReferenceNotReady}}, nil
+					},
+					buildFn:  validBuildFn,
+					assignFn: validAssignFn,
+				},
+			},
+			want: want{
+				getStatusCalled: true,
+				err:             &referencesAccessErr{[]ReferenceStatus{{"cool-res", ReferenceNotReady}}},
+			},
+		},
+		"ValidAttribute_GetStatusReturnsMixedReadyStatus_ReturnsErr": {
+			args: args{
+				field: &ItemAReferencer{
+					getStatusFn: func(context.Context, CanReference, client.Reader) ([]ReferenceStatus, error) {
+						return []ReferenceStatus{
+							{"cool1-res", ReferenceNotFound},
+							{"cool2-res", ReferenceReady},
+						}, nil
+					},
+					buildFn:  validBuildFn,
+					assignFn: validAssignFn,
+				},
+			},
+			want: want{
+				getStatusCalled: true,
+				err: &referencesAccessErr{[]ReferenceStatus{
+					{"cool1-res", ReferenceNotFound},
+					{"cool2-res", ReferenceReady},
+				}},
+			},
+		},
+		"ValidAttribute_GetStatusReturnsReadyStatus_ReturnsErr": {
+			args: args{
+				field: &ItemAReferencer{
+					getStatusFn: func(context.Context, CanReference, client.Reader) ([]ReferenceStatus, error) {
+						return []ReferenceStatus{{"cool-res", ReferenceReady}}, nil
+					},
+					buildFn:  validBuildFn,
+					assignFn: validAssignFn,
+				},
+			},
+			want: want{
+				getStatusCalled: true,
+				buildCalled:     true,
+				assignCalled:    true,
+				assignParam:     "fakeValue",
+			},
+		},
+		"ValidAttribute_BuildError_ReturnsErr": {
+			args: args{
+				field: &ItemAReferencer{
+					getStatusFn: validGetStatusFn,
+					buildFn:     func(context.Context, CanReference, client.Reader) (string, error) { return "", errBoom },
+					assignFn:    validAssignFn,
+				},
+			},
+			want: want{
+				getStatusCalled: true,
+				buildCalled:     true,
+				err:             errors.Wrap(errBoom, errBuildAttribute),
+			},
+		},
+		"ValidAttribute_AssignError_ReturnsErr": {
+			args: args{
+				field: &ItemAReferencer{
+					getStatusFn: validGetStatusFn,
+					buildFn:     validBuildFn,
+					assignFn:    func(CanReference, string) error { return errBoom },
+				},
+			},
+			want: want{
+				getStatusCalled: true,
+				buildCalled:     true,
+				assignCalled:    true,
+				assignParam:     "fakeValue",
+				err:             errors.Wrap(errBoom, errAssignAttribute),
+			},
+		},
+		"ValidAttribute_UpdateResourceError_ReturnsErr": {
+			args: args{
+				field: &ItemAReferencer{
+					getStatusFn: validGetStatusFn,
+					buildFn:     validBuildFn,
+					assignFn:    validAssignFn,
+				},
+				clientUpdaterErr: errBoom,
+			},
+			want: want{
+				getStatusCalled: true,
+				buildCalled:     true,
+				assignCalled:    true,
+				assignParam:     "fakeValue",
+				err:             errors.Wrap(errBoom, errUpdateResourceAfterAssignment),
+			},
+		},
+	} {
+		t.Run(name, func(t *testing.T) {
+
+			c := mockClient{updaterErr: tc.args.clientUpdaterErr}
+			rr := NewAPIManagedReferenceResolver(&c)
+			ctx := context.Background()
+
+			res := managed{
+				ItemARef: tc.args.field,
+			}
+
+			err := rr.ResolveReferences(ctx, &res)
+
+			if diff := cmp.Diff(tc.want.err, err, test.EquateErrors()); diff != "" {
+				t.Errorf("ResolveReferences(...): -want error, +got error:\n%s", diff)
+			}
+
+			gotCalls := []bool{tc.args.field.getStatusCalled, tc.args.field.buildCalled, tc.args.field.assignCalled}
+			wantCalls := []bool{tc.want.getStatusCalled, tc.want.buildCalled, tc.want.assignCalled}
+
+			if diff := cmp.Diff(wantCalls, gotCalls); diff != "" {
+				t.Errorf("ResolveReferences(...) => []{getStatusCalled, buildCalled, assignCalled}, : -want, +got:\n%s", diff)
+			}
+
+			if diff := cmp.Diff(tc.want.assignParam, tc.args.field.assignParam); diff != "" {
+				t.Errorf("ResolveReferences(...) => []{getStatusCalled, buildCalled, assignCalled}, : -want, +got:\n%s", diff)
+			}
+		})
+	}
+}
+
+type mockClient struct {
+	updaterErr error
+	client.Client
+}
+
+func (c *mockClient) Update(ctx context.Context, obj runtime.Object, opts ...client.UpdateOption) error {
+	return c.updaterErr
+}
+
+func Test_ResolveReferences_AttributeNotImplemented_Panics(t *testing.T) {
+	type mockStruct struct {
+		Name string
+	}
+
+	res := struct {
+		MockManaged
+		ItemRef mockStruct `resource:"attributereferencer"`
+	}{}
+
+	paniced := false
+
+	func() {
+		defer func() {
+			if r := recover(); r != nil {
+				paniced = true
+			}
+		}()
+
+		NewAPIManagedReferenceResolver(struct{ client.Client }{}).
+			ResolveReferences(context.Background(), &res)
+	}()
+
+	if diff := cmp.Diff(paniced, true); diff != "" {
+		t.Errorf("ResolveReferences(...) should panic for invalid attributereferencer: -want , +got :\n%s", diff)
+	}
+}
+
+func Test_ResolveReferences_NoReferencersFound_ExitsEarly(t *testing.T) {
+	res := struct {
+		MockManaged
+	}{}
+
+	var wantErr error
+	gotErr := NewAPIManagedReferenceResolver(struct{ client.Client }{}).
+		ResolveReferences(context.Background(), &res)
+
+	if diff := cmp.Diff(wantErr, gotErr, test.EquateErrors()); diff != "" {
+		t.Errorf("ResolveReferences(...) with no referencers: -want error, +got error:\n%s", diff)
+	}
+}

pkg/resource/resource.go

@@ -138,3 +138,8 @@ func IsBindable(b Bindable) bool {
 func IsBound(b Bindable) bool {
 	return b.GetBindingPhase() == v1alpha1.BindingPhaseBound
 }
+
+// IsConditionTrue returns if condition status is true
+func IsConditionTrue(c v1alpha1.Condition) bool {
+	return c.Status == corev1.ConditionTrue
+}

pkg/resource/resource_test.go

@@ -326,3 +326,36 @@ func TestSetBindable(t *testing.T) {
 		})
 	}
 }
+
+func TestIsConditionTrue(t *testing.T) {
+	cases := map[string]struct {
+		c    v1alpha1.Condition
+		want bool
+	}{
+		"IsTrue": {
+			c:    v1alpha1.Condition{Status: corev1.ConditionTrue},
+			want: true,
+		},
+		"IsFalse": {
+			c:    v1alpha1.Condition{Status: corev1.ConditionFalse},
+			want: false,
+		},
+		"IsUnknown": {
+			c:    v1alpha1.Condition{Status: corev1.ConditionUnknown},
+			want: false,
+		},
+		"IsUnset": {
+			c:    v1alpha1.Condition{},
+			want: false,
+		},
+	}
+
+	for name, tc := range cases {
+		t.Run(name, func(t *testing.T) {
+			got := IsConditionTrue(tc.c)
+			if diff := cmp.Diff(tc.want, got); diff != "" {
+				t.Errorf("IsConditionTrue(...): -want, +got:\n%s", diff)
+			}
+		})
+	}
+}