diff --git a/go/topology/collection.go b/go/topology/collection.go
index 376b62405..6368900c3 100644
--- a/go/topology/collection.go
+++ b/go/topology/collection.go
@@ -35,7 +35,7 @@ func (c *Collection) GetOrCreateVolumeLayout(rp *storage.ReplicaPlacement, ttl *
 }
 
 func (c *Collection) Lookup(vid storage.VolumeId) []*DataNode {
-	for _, vl := range c.storageType2VolumeLayout.Items {
+	for _, vl := range c.storageType2VolumeLayout.Items() {
 		if vl != nil {
 			if list := vl.(*VolumeLayout).Lookup(vid); list != nil {
 				return list
@@ -46,7 +46,7 @@ func (c *Collection) Lookup(vid storage.VolumeId) []*DataNode {
 }
 
 func (c *Collection) ListVolumeServers() (nodes []*DataNode) {
-	for _, vl := range c.storageType2VolumeLayout.Items {
+	for _, vl := range c.storageType2VolumeLayout.Items() {
 		if vl != nil {
 			if list := vl.(*VolumeLayout).ListVolumeServers(); list != nil {
 				nodes = append(nodes, list...)
diff --git a/go/topology/data_node.go b/go/topology/data_node.go
index 01419a791..3bad8c188 100644
--- a/go/topology/data_node.go
+++ b/go/topology/data_node.go
@@ -3,7 +3,6 @@ package topology
 import (
 	"fmt"
 	"strconv"
-	"sync"
 
 	"github.com/chrislusf/seaweedfs/go/glog"
 	"github.com/chrislusf/seaweedfs/go/storage"
@@ -11,7 +10,6 @@ import (
 
 type DataNode struct {
 	NodeImpl
-	sync.RWMutex
 	volumes   map[storage.VolumeId]storage.VolumeInfo
 	Ip        string
 	Port      int
diff --git a/go/topology/topology.go b/go/topology/topology.go
index ee1477cd2..088639eef 100644
--- a/go/topology/topology.go
+++ b/go/topology/topology.go
@@ -90,13 +90,13 @@ func (t *Topology) loadConfiguration(configurationFile string) error {
 func (t *Topology) Lookup(collection string, vid storage.VolumeId) []*DataNode {
 	//maybe an issue if lots of collections?
 	if collection == "" {
-		for _, c := range t.collectionMap.Items {
+		for _, c := range t.collectionMap.Items() {
 			if list := c.(*Collection).Lookup(vid); list != nil {
 				return list
 			}
 		}
 	} else {
-		if c, ok := t.collectionMap.Items[collection]; ok {
+		if c, ok := t.collectionMap.Find(collection); ok {
 			return c.(*Collection).Lookup(vid)
 		}
 	}
@@ -130,13 +130,13 @@ func (t *Topology) GetVolumeLayout(collectionName string, rp *storage.ReplicaPla
 	}).(*Collection).GetOrCreateVolumeLayout(rp, ttl)
 }
 
-func (t *Topology) GetCollection(collectionName string) (*Collection, bool) {
-	c, hasCollection := t.collectionMap.Items[collectionName]
+func (t *Topology) FindCollection(collectionName string) (*Collection, bool) {
+	c, hasCollection := t.collectionMap.Find(collectionName)
 	return c.(*Collection), hasCollection
 }
 
 func (t *Topology) DeleteCollection(collectionName string) {
-	delete(t.collectionMap.Items, collectionName)
+	t.collectionMap.Delete(collectionName)
 }
 
 func (t *Topology) RegisterVolumeLayout(v storage.VolumeInfo, dn *DataNode) {
diff --git a/go/topology/topology_map.go b/go/topology/topology_map.go
index dff11aaad..ce8e9e663 100644
--- a/go/topology/topology_map.go
+++ b/go/topology/topology_map.go
@@ -11,9 +11,9 @@ func (t *Topology) ToMap() interface{} {
 	}
 	m["DataCenters"] = dcs
 	var layouts []interface{}
-	for _, col := range t.collectionMap.Items {
+	for _, col := range t.collectionMap.Items() {
 		c := col.(*Collection)
-		for _, layout := range c.storageType2VolumeLayout.Items {
+		for _, layout := range c.storageType2VolumeLayout.Items() {
 			if layout != nil {
 				tmp := layout.(*VolumeLayout).ToMap()
 				tmp["collection"] = c.Name
diff --git a/go/topology/topology_vacuum.go b/go/topology/topology_vacuum.go
index 48bc8311d..eeb4fef69 100644
--- a/go/topology/topology_vacuum.go
+++ b/go/topology/topology_vacuum.go
@@ -81,10 +81,10 @@ func batchVacuumVolumeCommit(vl *VolumeLayout, vid storage.VolumeId, locationlis
 }
 func (t *Topology) Vacuum(garbageThreshold string) int {
 	glog.V(0).Infoln("Start vacuum on demand")
-	for _, col := range t.collectionMap.Items {
+	for _, col := range t.collectionMap.Items() {
 		c := col.(*Collection)
 		glog.V(0).Infoln("vacuum on collection:", c.Name)
-		for _, vl := range c.storageType2VolumeLayout.Items {
+		for _, vl := range c.storageType2VolumeLayout.Items() {
 			if vl != nil {
 				volumeLayout := vl.(*VolumeLayout)
 				for vid, locationlist := range volumeLayout.vid2location {
diff --git a/go/util/concurrent_read_map.go b/go/util/concurrent_read_map.go
index ca1109f22..bbf303d82 100644
--- a/go/util/concurrent_read_map.go
+++ b/go/util/concurrent_read_map.go
@@ -7,31 +7,53 @@ import (
 // A mostly for read map, which can thread-safely
 // initialize the map entries.
 type ConcurrentReadMap struct {
-	rmutex sync.RWMutex
-	Items  map[string]interface{}
+	sync.RWMutex
+
+	items map[string]interface{}
 }
 
 func NewConcurrentReadMap() *ConcurrentReadMap {
-	return &ConcurrentReadMap{Items: make(map[string]interface{})}
+	return &ConcurrentReadMap{items: make(map[string]interface{})}
 }
 
 func (m *ConcurrentReadMap) initMapEntry(key string, newEntry func() interface{}) (value interface{}) {
-	m.rmutex.Lock()
-	defer m.rmutex.Unlock()
-	if value, ok := m.Items[key]; ok {
+	m.Lock()
+	defer m.Unlock()
+	if value, ok := m.items[key]; ok {
 		return value
 	}
 	value = newEntry()
-	m.Items[key] = value
+	m.items[key] = value
 	return value
 }
 
 func (m *ConcurrentReadMap) Get(key string, newEntry func() interface{}) interface{} {
-	m.rmutex.RLock()
-	if value, ok := m.Items[key]; ok {
-		m.rmutex.RUnlock()
+	m.RLock()
+	if value, ok := m.items[key]; ok {
+		m.RUnlock()
 		return value
 	}
-	m.rmutex.RUnlock()
+	m.RUnlock()
 	return m.initMapEntry(key, newEntry)
 }
+
+func (m *ConcurrentReadMap) Find(key string) (interface{}, bool) {
+	m.RLock()
+	value, ok := m.items[key]
+	m.RUnlock()
+	return value, ok
+}
+
+func (m *ConcurrentReadMap) Items() (itemsCopy []interface{}) {
+	m.RLock()
+	for _, i := range m.items {
+		itemsCopy = append(itemsCopy, i)
+	}
+	return itemsCopy
+}
+
+func (m *ConcurrentReadMap) Delete(key string) {
+	m.Lock()
+	delete(m.items, key)
+	m.Unlock()
+}
diff --git a/go/weed/weed_server/master_server_handlers_admin.go b/go/weed/weed_server/master_server_handlers_admin.go
index 383456356..07399596a 100644
--- a/go/weed/weed_server/master_server_handlers_admin.go
+++ b/go/weed/weed_server/master_server_handlers_admin.go
@@ -19,7 +19,7 @@ import (
 )
 
 func (ms *MasterServer) collectionDeleteHandler(w http.ResponseWriter, r *http.Request) {
-	collection, ok := ms.Topo.GetCollection(r.FormValue("collection"))
+	collection, ok := ms.Topo.FindCollection(r.FormValue("collection"))
 	if !ok {
 		writeJsonError(w, r, http.StatusBadRequest, fmt.Errorf("collection %s does not exist", r.FormValue("collection")))
 		return