diff --git a/helper/schema/field_writer.go b/helper/schema/field_writer.go
new file mode 100644
index 000000000..9abc41b54
--- /dev/null
+++ b/helper/schema/field_writer.go
@@ -0,0 +1,8 @@
+package schema
+
+// FieldWriters are responsible for writing fields by address into
+// a proper typed representation. ResourceData uses this to write new data
+// into existing sources.
+type FieldWriter interface {
+	WriteField([]string, interface{}) error
+}
diff --git a/helper/schema/field_writer_map.go b/helper/schema/field_writer_map.go
new file mode 100644
index 000000000..fd6796377
--- /dev/null
+++ b/helper/schema/field_writer_map.go
@@ -0,0 +1,303 @@
+package schema
+
+import (
+	"fmt"
+	"reflect"
+	"strconv"
+	"strings"
+	"sync"
+
+	"github.com/mitchellh/mapstructure"
+)
+
+// MapFieldWriter writes data into a single map[string]string structure.
+type MapFieldWriter struct {
+	Schema map[string]*Schema
+
+	lock   sync.Mutex
+	result map[string]string
+}
+
+// Map returns the underlying map that is being written to.
+func (w *MapFieldWriter) Map() map[string]string {
+	w.lock.Lock()
+	defer w.lock.Unlock()
+	if w.result == nil {
+		w.result = make(map[string]string)
+	}
+
+	return w.result
+}
+
+func (w *MapFieldWriter) WriteField(addr []string, value interface{}) error {
+	w.lock.Lock()
+	defer w.lock.Unlock()
+	if w.result == nil {
+		w.result = make(map[string]string)
+	}
+
+	schemaList := addrToSchema(addr, w.Schema)
+	if len(schemaList) == 0 {
+		return fmt.Errorf("Invalid address to set: %#v", addr)
+	}
+
+	// If we're setting anything other than a list root or set root,
+	// then disallow it.
+	for _, schema := range schemaList[:len(schemaList)-1] {
+		if schema.Type == TypeList {
+			return fmt.Errorf(
+				"%s: can only set full list",
+				strings.Join(addr, "."))
+		}
+
+		if schema.Type == TypeMap {
+			return fmt.Errorf(
+				"%s: can only set full map",
+				strings.Join(addr, "."))
+		}
+
+		if schema.Type == TypeSet {
+			return fmt.Errorf(
+				"%s: can only set full set",
+				strings.Join(addr, "."))
+		}
+	}
+
+	return w.set(addr, value)
+}
+
+func (w *MapFieldWriter) set(addr []string, value interface{}) error {
+	schemaList := addrToSchema(addr, w.Schema)
+	if len(schemaList) == 0 {
+		return fmt.Errorf("Invalid address to set: %#v", addr)
+	}
+
+	schema := schemaList[len(schemaList)-1]
+	switch schema.Type {
+	case TypeBool:
+		fallthrough
+	case TypeInt:
+		fallthrough
+	case TypeString:
+		return w.setPrimitive(addr, value, schema)
+	case TypeList:
+		return w.setList(addr, value, schema)
+	case TypeMap:
+		return w.setMap(addr, value, schema)
+	case TypeSet:
+		return w.setSet(addr, value, schema)
+	case typeObject:
+		return w.setObject(addr, value, schema)
+	default:
+		panic(fmt.Sprintf("Unknown type: %#v", schema.Type))
+	}
+}
+
+func (w *MapFieldWriter) setList(
+	addr []string,
+	v interface{},
+	schema *Schema) error {
+	k := strings.Join(addr, ".")
+	setElement := func(idx string, value interface{}) error {
+		addrCopy := make([]string, len(addr), len(addr)+1)
+		copy(addrCopy, addr)
+		return w.set(append(addrCopy, idx), value)
+	}
+
+	var vs []interface{}
+	if err := mapstructure.Decode(v, &vs); err != nil {
+		return fmt.Errorf("%s: %s", k, err)
+	}
+
+	// Set the entire list.
+	var err error
+	for i, elem := range vs {
+		is := strconv.FormatInt(int64(i), 10)
+		err = setElement(is, elem)
+		if err != nil {
+			break
+		}
+	}
+	if err != nil {
+		for i, _ := range vs {
+			is := strconv.FormatInt(int64(i), 10)
+			setElement(is, nil)
+		}
+
+		return err
+	}
+
+	w.result[k+".#"] = strconv.FormatInt(int64(len(vs)), 10)
+	return nil
+}
+
+func (w *MapFieldWriter) setMap(
+	addr []string,
+	value interface{},
+	schema *Schema) error {
+	k := strings.Join(addr, ".")
+	v := reflect.ValueOf(value)
+	vs := make(map[string]interface{})
+
+	if value != nil {
+		if v.Kind() != reflect.Map {
+			return fmt.Errorf("%s: must be a map", k)
+		}
+		if v.Type().Key().Kind() != reflect.String {
+			return fmt.Errorf("%s: keys must strings", k)
+		}
+		for _, mk := range v.MapKeys() {
+			mv := v.MapIndex(mk)
+			vs[mk.String()] = mv.Interface()
+		}
+	}
+
+	if len(vs) == 0 {
+		// The empty string here means the map is removed.
+		w.result[k] = ""
+		return nil
+	}
+
+	// Remove the pure key since we're setting the full map value
+	delete(w.result, k)
+
+	// Set each subkey
+	addrCopy := make([]string, len(addr), len(addr)+1)
+	copy(addrCopy, addr)
+	for subKey, v := range vs {
+		if err := w.set(append(addrCopy, subKey), v); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func (w *MapFieldWriter) setObject(
+	addr []string,
+	value interface{},
+	schema *Schema) error {
+	// Set the entire object. First decode into a proper structure
+	var v map[string]interface{}
+	if err := mapstructure.Decode(value, &v); err != nil {
+		return fmt.Errorf("%s: %s", strings.Join(addr, "."), err)
+	}
+
+	// Make space for additional elements in the address
+	addrCopy := make([]string, len(addr), len(addr)+1)
+	copy(addrCopy, addr)
+
+	// Set each element in turn
+	var err error
+	for k1, v1 := range v {
+		if err = w.set(append(addrCopy, k1), v1); err != nil {
+			break
+		}
+	}
+	if err != nil {
+		for k1, _ := range v {
+			w.set(append(addrCopy, k1), nil)
+		}
+	}
+
+	return err
+}
+
+func (w *MapFieldWriter) setPrimitive(
+	addr []string,
+	v interface{},
+	schema *Schema) error {
+	k := strings.Join(addr, ".")
+
+	if v == nil {
+		delete(w.result, k)
+		return nil
+	}
+
+	var set string
+	switch schema.Type {
+	case TypeBool:
+		var b bool
+		if err := mapstructure.Decode(v, &b); err != nil {
+			return fmt.Errorf("%s: %s", k, err)
+		}
+
+		set = strconv.FormatBool(b)
+	case TypeString:
+		if err := mapstructure.Decode(v, &set); err != nil {
+			return fmt.Errorf("%s: %s", k, err)
+		}
+	case TypeInt:
+		var n int
+		if err := mapstructure.Decode(v, &n); err != nil {
+			return fmt.Errorf("%s: %s", k, err)
+		}
+
+		set = strconv.FormatInt(int64(n), 10)
+	default:
+		return fmt.Errorf("Unknown type: %#v", schema.Type)
+	}
+
+	w.result[k] = set
+	return nil
+}
+
+func (w *MapFieldWriter) setSet(
+	addr []string,
+	value interface{},
+	schema *Schema) error {
+	addrCopy := make([]string, len(addr), len(addr)+1)
+	copy(addrCopy, addr)
+
+	// If it is a slice, then we have to turn it into a *Set so that
+	// we get the proper order back based on the hash code.
+	if v := reflect.ValueOf(value); v.Kind() == reflect.Slice {
+		// Build a temp *ResourceData to use for the conversion
+		tempSchema := *schema
+		tempSchema.Type = TypeList
+		tempSchemaMap := map[string]*Schema{addr[0]: &tempSchema}
+		tempW := &MapFieldWriter{Schema: tempSchemaMap}
+
+		// Set the entire list, this lets us get sane values out of it
+		if err := tempW.WriteField(addr, value); err != nil {
+			return err
+		}
+
+		// Build the set by going over the list items in order and
+		// hashing them into the set. The reason we go over the list and
+		// not the `value` directly is because this forces all types
+		// to become []interface{} (generic) instead of []string, which
+		// most hash functions are expecting.
+		s := &Set{F: schema.Set}
+		tempR := &MapFieldReader{
+			Map:    BasicMapReader(tempW.Map()),
+			Schema: tempSchemaMap,
+		}
+		for i := 0; i < v.Len(); i++ {
+			is := strconv.FormatInt(int64(i), 10)
+			result, err := tempR.ReadField(append(addrCopy, is))
+			if err != nil {
+				return err
+			}
+			if !result.Exists {
+				panic("set item just set doesn't exist")
+			}
+
+			s.Add(result.Value)
+		}
+
+		value = s
+	}
+
+	k := strings.Join(addr, ".")
+	for code, elem := range value.(*Set).m {
+		codeStr := strconv.FormatInt(int64(code), 10)
+		if err := w.set(append(addrCopy, codeStr), elem); err != nil {
+			return err
+		}
+	}
+
+	w.result[k+".#"] = strconv.Itoa(value.(*Set).Len())
+	return nil
+
+}
diff --git a/helper/schema/field_writer_map_test.go b/helper/schema/field_writer_map_test.go
new file mode 100644
index 000000000..38eaca94b
--- /dev/null
+++ b/helper/schema/field_writer_map_test.go
@@ -0,0 +1,184 @@
+package schema
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestMapFieldWriter_impl(t *testing.T) {
+	var _ FieldWriter = new(MapFieldWriter)
+}
+
+func TestMapFieldWriter(t *testing.T) {
+	schema := map[string]*Schema{
+		"bool":   &Schema{Type: TypeBool},
+		"int":    &Schema{Type: TypeInt},
+		"string": &Schema{Type: TypeString},
+		"list": &Schema{
+			Type: TypeList,
+			Elem: &Schema{Type: TypeString},
+		},
+		"listInt": &Schema{
+			Type: TypeList,
+			Elem: &Schema{Type: TypeInt},
+		},
+		"map": &Schema{Type: TypeMap},
+		"set": &Schema{
+			Type: TypeSet,
+			Elem: &Schema{Type: TypeInt},
+			Set: func(a interface{}) int {
+				return a.(int)
+			},
+		},
+		"setDeep": &Schema{
+			Type: TypeSet,
+			Elem: &Resource{
+				Schema: map[string]*Schema{
+					"index": &Schema{Type: TypeInt},
+					"value": &Schema{Type: TypeString},
+				},
+			},
+			Set: func(a interface{}) int {
+				return a.(map[string]interface{})["index"].(int)
+			},
+		},
+	}
+
+	cases := map[string]struct {
+		Addr  []string
+		Value interface{}
+		Err   bool
+		Out   map[string]string
+	}{
+		"noexist": {
+			[]string{"noexist"},
+			42,
+			true,
+			map[string]string{},
+		},
+
+		"bool": {
+			[]string{"bool"},
+			false,
+			false,
+			map[string]string{
+				"bool": "false",
+			},
+		},
+
+		"int": {
+			[]string{"int"},
+			42,
+			false,
+			map[string]string{
+				"int": "42",
+			},
+		},
+
+		"string": {
+			[]string{"string"},
+			"42",
+			false,
+			map[string]string{
+				"string": "42",
+			},
+		},
+
+		"list of strings": {
+			[]string{"list"},
+			[]interface{}{"foo", "bar"},
+			false,
+			map[string]string{
+				"list.#": "2",
+				"list.0": "foo",
+				"list.1": "bar",
+			},
+		},
+
+		"list element": {
+			[]string{"list", "0"},
+			"string",
+			true,
+			map[string]string{},
+		},
+
+		"map": {
+			[]string{"map"},
+			map[string]interface{}{"foo": "bar"},
+			false,
+			map[string]string{
+				"map.foo": "bar",
+			},
+		},
+
+		"map delete": {
+			[]string{"map"},
+			nil,
+			false,
+			map[string]string{
+				"map": "",
+			},
+		},
+
+		"map element": {
+			[]string{"map", "foo"},
+			"bar",
+			true,
+			map[string]string{},
+		},
+
+		"set": {
+			[]string{"set"},
+			[]interface{}{1, 2, 5},
+			false,
+			map[string]string{
+				"set.#": "3",
+				"set.1": "1",
+				"set.2": "2",
+				"set.5": "5",
+			},
+		},
+
+		"set resource": {
+			[]string{"setDeep"},
+			[]interface{}{
+				map[string]interface{}{
+					"index": 10,
+					"value": "foo",
+				},
+				map[string]interface{}{
+					"index": 50,
+					"value": "bar",
+				},
+			},
+			false,
+			map[string]string{
+				"setDeep.#":        "2",
+				"setDeep.10.index": "10",
+				"setDeep.10.value": "foo",
+				"setDeep.50.index": "50",
+				"setDeep.50.value": "bar",
+			},
+		},
+
+		"set element": {
+			[]string{"set", "5"},
+			5,
+			true,
+			map[string]string{},
+		},
+	}
+
+	for name, tc := range cases {
+		w := &MapFieldWriter{Schema: schema}
+		err := w.WriteField(tc.Addr, tc.Value)
+		if (err != nil) != tc.Err {
+			t.Fatalf("%s: err: %s", name, err)
+		}
+
+		actual := w.Map()
+		if !reflect.DeepEqual(actual, tc.Out) {
+			t.Fatalf("%s: bad: %#v", name, actual)
+		}
+	}
+}
diff --git a/helper/schema/resource_data.go b/helper/schema/resource_data.go
index 9afd1daa6..0889d294f 100644
--- a/helper/schema/resource_data.go
+++ b/helper/schema/resource_data.go
@@ -8,7 +8,6 @@ import (
 	"sync"
 
 	"github.com/hashicorp/terraform/terraform"
-	"github.com/mitchellh/mapstructure"
 )
 
 // ResourceData is used to query and set the attributes of a resource.
@@ -29,7 +28,7 @@ type ResourceData struct {
 
 	// Don't set
 	multiReader *MultiLevelFieldReader
-	setMap      map[string]string
+	setWriter   *MapFieldWriter
 	newState    *terraform.InstanceState
 	partial     bool
 	partialMap  map[string]struct{}
@@ -156,8 +155,7 @@ func (d *ResourceData) Partial(on bool) {
 // will be returned.
 func (d *ResourceData) Set(key string, value interface{}) error {
 	d.once.Do(d.init)
-	parts := strings.Split(key, ".")
-	return d.setObject("", parts, d.schema, value)
+	return d.setWriter.WriteField(strings.Split(key, "."), value)
 }
 
 // SetPartial adds the key prefix to the final state output while
@@ -243,7 +241,7 @@ func (d *ResourceData) init() {
 	d.newState = &copyState
 
 	// Initialize the map for storing set data
-	d.setMap = make(map[string]string)
+	d.setWriter = &MapFieldWriter{Schema: d.schema}
 
 	// Initialize the reader for getting data from the
 	// underlying sources (config, diff, etc.)
@@ -274,7 +272,7 @@ func (d *ResourceData) init() {
 	}
 	readers["set"] = &MapFieldReader{
 		Schema: d.schema,
-		Map:    BasicMapReader(d.setMap),
+		Map:    BasicMapReader(d.setWriter.Map()),
 	}
 	d.multiReader = &MultiLevelFieldReader{
 		Levels: []string{
@@ -383,277 +381,6 @@ func (d *ResourceData) get(
 	}
 }
 
-func (d *ResourceData) set(
-	k string,
-	parts []string,
-	schema *Schema,
-	value interface{}) error {
-	switch schema.Type {
-	case TypeList:
-		return d.setList(k, parts, schema, value)
-	case TypeMap:
-		return d.setMapValue(k, parts, schema, value)
-	case TypeSet:
-		return d.setSet(k, parts, schema, value)
-	case TypeBool:
-		fallthrough
-	case TypeInt:
-		fallthrough
-	case TypeString:
-		return d.setPrimitive(k, schema, value)
-	default:
-		panic(fmt.Sprintf("%s: unknown type %#v", k, schema.Type))
-	}
-}
-
-func (d *ResourceData) setList(
-	k string,
-	parts []string,
-	schema *Schema,
-	value interface{}) error {
-	if len(parts) > 0 {
-		return fmt.Errorf("%s: can only set the full list, not elements", k)
-	}
-
-	setElement := func(k string, idx string, value interface{}) error {
-		if idx == "#" {
-			return fmt.Errorf("%s: can't set count of list", k)
-		}
-
-		key := fmt.Sprintf("%s.%s", k, idx)
-		switch t := schema.Elem.(type) {
-		case *Resource:
-			return d.setObject(key, nil, t.Schema, value)
-		case *Schema:
-			return d.set(key, nil, t, value)
-		}
-
-		return nil
-	}
-
-	var vs []interface{}
-	if err := mapstructure.Decode(value, &vs); err != nil {
-		return fmt.Errorf("%s: %s", k, err)
-	}
-
-	// Set the entire list.
-	var err error
-	for i, elem := range vs {
-		is := strconv.FormatInt(int64(i), 10)
-		err = setElement(k, is, elem)
-		if err != nil {
-			break
-		}
-	}
-	if err != nil {
-		for i, _ := range vs {
-			is := strconv.FormatInt(int64(i), 10)
-			setElement(k, is, nil)
-		}
-
-		return err
-	}
-
-	d.setMap[k+".#"] = strconv.FormatInt(int64(len(vs)), 10)
-	return nil
-}
-
-func (d *ResourceData) setMapValue(
-	k string,
-	parts []string,
-	schema *Schema,
-	value interface{}) error {
-	elemSchema := &Schema{Type: TypeString}
-	if len(parts) > 0 {
-		return fmt.Errorf("%s: full map must be set, no a single element", k)
-	}
-
-	v := reflect.ValueOf(value)
-	if v.Kind() != reflect.Map {
-		return fmt.Errorf("%s: must be a map", k)
-	}
-	if v.Type().Key().Kind() != reflect.String {
-		return fmt.Errorf("%s: keys must strings", k)
-	}
-	vs := make(map[string]interface{})
-	for _, mk := range v.MapKeys() {
-		mv := v.MapIndex(mk)
-		vs[mk.String()] = mv.Interface()
-	}
-
-	if len(vs) == 0 {
-		// The empty string here means the map is removed.
-		d.setMap[k] = ""
-		return nil
-	}
-
-	delete(d.setMap, k)
-	for subKey, v := range vs {
-		err := d.set(fmt.Sprintf("%s.%s", k, subKey), nil, elemSchema, v)
-		if err != nil {
-			return err
-		}
-	}
-
-	return nil
-}
-
-func (d *ResourceData) setObject(
-	k string,
-	parts []string,
-	schema map[string]*Schema,
-	value interface{}) error {
-	if len(parts) > 0 {
-		// We're setting a specific key in an object
-		key := parts[0]
-		parts = parts[1:]
-
-		s, ok := schema[key]
-		if !ok {
-			return fmt.Errorf("%s (internal): unknown key to set: %s", k, key)
-		}
-
-		if k != "" {
-			// If we're not at the root, then we need to append
-			// the key to get the full key path.
-			key = fmt.Sprintf("%s.%s", k, key)
-		}
-
-		return d.set(key, parts, s, value)
-	}
-
-	// Set the entire object. First decode into a proper structure
-	var v map[string]interface{}
-	if err := mapstructure.Decode(value, &v); err != nil {
-		return fmt.Errorf("%s: %s", k, err)
-	}
-
-	// Set each element in turn
-	var err error
-	for k1, v1 := range v {
-		err = d.setObject(k, []string{k1}, schema, v1)
-		if err != nil {
-			break
-		}
-	}
-	if err != nil {
-		for k1, _ := range v {
-			d.setObject(k, []string{k1}, schema, nil)
-		}
-	}
-
-	return err
-}
-
-func (d *ResourceData) setPrimitive(
-	k string,
-	schema *Schema,
-	v interface{}) error {
-	if v == nil {
-		delete(d.setMap, k)
-		return nil
-	}
-
-	var set string
-	switch schema.Type {
-	case TypeBool:
-		var b bool
-		if err := mapstructure.Decode(v, &b); err != nil {
-			return fmt.Errorf("%s: %s", k, err)
-		}
-
-		set = strconv.FormatBool(b)
-	case TypeString:
-		if err := mapstructure.Decode(v, &set); err != nil {
-			return fmt.Errorf("%s: %s", k, err)
-		}
-	case TypeInt:
-		var n int
-		if err := mapstructure.Decode(v, &n); err != nil {
-			return fmt.Errorf("%s: %s", k, err)
-		}
-
-		set = strconv.FormatInt(int64(n), 10)
-	default:
-		return fmt.Errorf("Unknown type: %#v", schema.Type)
-	}
-
-	d.setMap[k] = set
-	return nil
-}
-
-func (d *ResourceData) setSet(
-	k string,
-	parts []string,
-	schema *Schema,
-	value interface{}) error {
-	if len(parts) > 0 {
-		return fmt.Errorf("%s: can only set the full set, not elements", k)
-	}
-
-	// If it is a slice, then we have to turn it into a *Set so that
-	// we get the proper order back based on the hash code.
-	if v := reflect.ValueOf(value); v.Kind() == reflect.Slice {
-		// Build a temp *ResourceData to use for the conversion
-		tempD := &ResourceData{
-			setMap: make(map[string]string),
-			schema: map[string]*Schema{k: schema},
-		}
-		tempD.once.Do(tempD.init)
-
-		// Set the entire list, this lets us get sane values out of it
-		if err := tempD.setList(k, nil, schema, value); err != nil {
-			return err
-		}
-
-		// Build the set by going over the list items in order and
-		// hashing them into the set. The reason we go over the list and
-		// not the `value` directly is because this forces all types
-		// to become []interface{} (generic) instead of []string, which
-		// most hash functions are expecting.
-		s := &Set{F: schema.Set}
-		source := getSourceSet | getSourceExact
-		for i := 0; i < v.Len(); i++ {
-			is := strconv.FormatInt(int64(i), 10)
-			result := tempD.get(k, []string{is}, schema, source)
-			if !result.Exists {
-				panic("just set item doesn't exist")
-			}
-
-			s.Add(result.Value)
-		}
-
-		value = s
-	}
-
-	switch t := schema.Elem.(type) {
-	case *Resource:
-		for code, elem := range value.(*Set).m {
-			for field, _ := range t.Schema {
-				subK := fmt.Sprintf("%s.%d", k, code)
-				value := elem.(map[string]interface{})[field]
-				err := d.setObject(subK, []string{field}, t.Schema, value)
-				if err != nil {
-					return err
-				}
-			}
-		}
-	case *Schema:
-		for code, elem := range value.(*Set).m {
-			subK := fmt.Sprintf("%s.%d", k, code)
-			err := d.set(subK, nil, t, elem)
-			if err != nil {
-				return err
-			}
-		}
-	default:
-		return fmt.Errorf("%s: unknown element type (internal)", k)
-	}
-
-	d.setMap[k+".#"] = strconv.Itoa(value.(*Set).Len())
-	return nil
-}
-
 func (d *ResourceData) stateList(
 	prefix string,
 	schema *Schema) map[string]string {