diff --git a/go/topology/node.go b/go/topology/node.go
index 356c0abeb..6a84b4b92 100644
--- a/go/topology/node.go
+++ b/go/topology/node.go
@@ -4,6 +4,7 @@ import (
 	"errors"
 	"math/rand"
 	"strings"
+	"sync"
 
 	"github.com/chrislusf/seaweedfs/go/glog"
 	"github.com/chrislusf/seaweedfs/go/storage"
@@ -32,7 +33,7 @@ type Node interface {
 	IsDataNode() bool
 	IsRack() bool
 	IsDataCenter() bool
-	Children() map[NodeId]Node
+	Children() []Node
 	Parent() Node
 
 	GetValue() interface{} //get reference to the topology,dc,rack,datanode
@@ -43,6 +44,7 @@ type NodeImpl struct {
 	activeVolumeCount int
 	maxVolumeCount    int
 	parent            Node
+	sync.RWMutex      // lock children
 	children          map[NodeId]Node
 	maxVolumeId       storage.VolumeId
 
@@ -55,6 +57,7 @@ type NodeImpl struct {
 func (n *NodeImpl) RandomlyPickNodes(numberOfNodes int, filterFirstNodeFn func(dn Node) error) (firstNode Node, restNodes []Node, err error) {
 	candidates := make([]Node, 0, len(n.children))
 	var errs []string
+	n.RLock()
 	for _, node := range n.children {
 		if err := filterFirstNodeFn(node); err == nil {
 			candidates = append(candidates, node)
@@ -62,6 +65,7 @@ func (n *NodeImpl) RandomlyPickNodes(numberOfNodes int, filterFirstNodeFn func(d
 			errs = append(errs, string(node.Id())+":"+err.Error())
 		}
 	}
+	n.RUnlock()
 	if len(candidates) == 0 {
 		return nil, nil, errors.New("No matching data node found! \n" + strings.Join(errs, "\n"))
 	}
@@ -70,6 +74,7 @@ func (n *NodeImpl) RandomlyPickNodes(numberOfNodes int, filterFirstNodeFn func(d
 
 	restNodes = make([]Node, numberOfNodes-1)
 	candidates = candidates[:0]
+	n.RLock()
 	for _, node := range n.children {
 		if node.Id() == firstNode.Id() {
 			continue
@@ -80,6 +85,7 @@ func (n *NodeImpl) RandomlyPickNodes(numberOfNodes int, filterFirstNodeFn func(d
 		glog.V(2).Infoln("select rest node candidate:", node.Id())
 		candidates = append(candidates, node)
 	}
+	n.RUnlock()
 	glog.V(2).Infoln(n.Id(), "picking", numberOfNodes-1, "from rest", len(candidates), "node candidates")
 	ret := len(restNodes) == 0
 	for k, node := range candidates {
@@ -126,8 +132,13 @@ func (n *NodeImpl) FreeSpace() int {
 func (n *NodeImpl) SetParent(node Node) {
 	n.parent = node
 }
-func (n *NodeImpl) Children() map[NodeId]Node {
-	return n.children
+func (n *NodeImpl) Children() (ret []Node) {
+	n.RLock()
+	defer n.RUnlock()
+	for _, c := range n.children {
+		ret = append(ret, c)
+	}
+	return ret
 }
 func (n *NodeImpl) Parent() Node {
 	return n.parent
@@ -136,6 +147,8 @@ func (n *NodeImpl) GetValue() interface{} {
 	return n.value
 }
 func (n *NodeImpl) ReserveOneVolume(r int) (assignedNode *DataNode, err error) {
+	n.RLock()
+	defer n.RUnlock()
 	for _, node := range n.children {
 		freeSpace := node.FreeSpace()
 		// fmt.Println("r =", r, ", node =", node, ", freeSpace =", freeSpace)
@@ -198,6 +211,8 @@ func (n *NodeImpl) GetMaxVolumeCount() int {
 }
 
 func (n *NodeImpl) LinkChildNode(node Node) {
+	n.Lock()
+	defer n.Unlock()
 	if n.children[node.Id()] == nil {
 		n.children[node.Id()] = node
 		n.UpAdjustMaxVolumeCountDelta(node.GetMaxVolumeCount())
@@ -210,6 +225,8 @@ func (n *NodeImpl) LinkChildNode(node Node) {
 }
 
 func (n *NodeImpl) UnlinkChildNode(nodeId NodeId) {
+	n.Lock()
+	defer n.Unlock()
 	node := n.children[nodeId]
 	if node != nil {
 		node.SetParent(nil)