diff --git a/weed-fs/src/pkg/topology/data_center.go b/weed-fs/src/pkg/topology/data_center.go
index c274dcf33..8dced9c44 100644
--- a/weed-fs/src/pkg/topology/data_center.go
+++ b/weed-fs/src/pkg/topology/data_center.go
@@ -4,12 +4,13 @@ import (
 )
 
 type DataCenter struct {
-	Node
+	NodeImpl
 	ipRange IpRange
 }
-func NewDataCenter(id NodeId) *DataCenter{
+func NewDataCenter(id string) *DataCenter{
   dc := &DataCenter{}
-  dc.Node = *NewNode()
-  dc.Node.Id = id
+  dc.id = NodeId(id)
+  dc.nodeType = "DataCenter"
+  dc.children = make(map[NodeId]Node)
   return dc
 }
diff --git a/weed-fs/src/pkg/topology/node.go b/weed-fs/src/pkg/topology/node.go
index 4352373cf..cb13a038e 100644
--- a/weed-fs/src/pkg/topology/node.go
+++ b/weed-fs/src/pkg/topology/node.go
@@ -6,93 +6,133 @@ import (
 )
 
 type NodeId string
-type Node struct {
-	Id                  NodeId
-	activeVolumeCount   int
-	maxVolumeCount      int
-	parent              *Node
-	children            map[NodeId]*Node
-	maxVolumeId         storage.VolumeId
+type Node interface {
+	Id() NodeId
+	String() string
+	FreeSpace() int
+	ReserveOneVolume(r int, vid storage.VolumeId) (bool, Node)
+	UpAdjustMaxVolumeCountDelta(maxVolumeCountDelta int)
+	UpAdjustActiveVolumeCountDelta(activeVolumeCountDelta int)
+	UpAdjustMaxVolumeId(vid storage.VolumeId)
+	GetActiveVolumeCount() int
+	GetMaxVolumeCount() int
+	GetMaxVolumeId() storage.VolumeId
+	setParent(Node)
+	LinkChildNode(node Node)
+	UnlinkChildNode(nodeId NodeId)
+}
+type NodeImpl struct {
+	id                NodeId
+	activeVolumeCount int
+	maxVolumeCount    int
+	parent            Node
+	children          map[NodeId]Node
+	maxVolumeId       storage.VolumeId
+
+	//for rack, data center, topology
+	nodeType string
 }
 
-func NewNode() *Node {
-	n := &Node{}
-	n.children = make(map[NodeId]*Node)
-	return n
+func (n *NodeImpl) IsServer() bool {
+	return n.nodeType == "Server"
 }
-func (n *Node) String() string {
-  if n.parent!=nil {
-    return n.parent.String()+":"+string(n.Id)
-  }
-  return string(n.Id)
+func (n *NodeImpl) IsRack() bool {
+	return n.nodeType == "Rack"
 }
-
-func (n *Node) ReserveOneVolume(r int, vid storage.VolumeId) (bool, *Node) {
-  if n.children == nil {
-    return true, n
-  }
-  ret := false
-  var assignedNode *Node
+func (n *NodeImpl) IsDataCenter() bool {
+	return n.nodeType == "DataCenter"
+}
+func (n *NodeImpl) String() string {
+	if n.parent != nil {
+		return n.parent.String() + ":" + string(n.id)
+	}
+	return string(n.id)
+}
+func (n *NodeImpl) Id() NodeId {
+	return n.id
+}
+func (n *NodeImpl) FreeSpace() int {
+	return n.maxVolumeCount - n.activeVolumeCount
+}
+func (n *NodeImpl) setParent(node Node) {
+	n.parent = node
+}
+func (n *NodeImpl) ReserveOneVolume(r int, vid storage.VolumeId) (bool, Node) {
+	if n.IsServer() && n.maxVolumeCount > n.activeVolumeCount {
+		fmt.Println("vid =", vid, " assigned to node =", n, ", freeSpace =", n.maxVolumeCount-n.activeVolumeCount)
+		return true, n
+	}
+	ret := false
+	var assignedNode Node
 	for _, node := range n.children {
-		freeSpace := node.maxVolumeCount - node.activeVolumeCount
-    fmt.Println("r =", r, ", node =", node, ", freeSpace =", freeSpace)
-    if freeSpace <= 0 {
-      continue
-    }
+		freeSpace := node.FreeSpace()
+		fmt.Println("r =", r, ", node =", node, ", freeSpace =", freeSpace)
+		if freeSpace <= 0 {
+			continue
+		}
 		if r >= freeSpace {
 			r -= freeSpace
 		} else {
-		  ret, assignedNode = node.ReserveOneVolume(r, vid)
-		  if ret {
-		    break
-		  }
+			ret, assignedNode = node.ReserveOneVolume(r, vid)
+			if ret {
+				break
+			}
 		}
 	}
 	return ret, assignedNode
 }
 
-func (n *Node) AddVolume(v *storage.VolumeInfo) {
-	if n.maxVolumeId < v.Id {
-		n.maxVolumeId = v.Id
-	}
-	n.activeVolumeCount++
-	fmt.Println(n.Id, "adds 1, volumeCount =", n.activeVolumeCount)
+func (n *NodeImpl) UpAdjustMaxVolumeCountDelta(maxVolumeCountDelta int) { //can be negative
+	n.maxVolumeCount += maxVolumeCountDelta
 	if n.parent != nil {
-		n.parent.AddVolume(v)
+		n.parent.UpAdjustMaxVolumeCountDelta(maxVolumeCountDelta)
 	}
 }
-func (n *Node) AddMaxVolumeCount(maxVolumeCount int) {//can be negative
-  n.maxVolumeCount += maxVolumeCount
-  if n.parent != nil {
-    n.parent.AddMaxVolumeCount(maxVolumeCount)
-  }
+func (n *NodeImpl) UpAdjustActiveVolumeCountDelta(activeVolumeCountDelta int) { //can be negative
+	n.activeVolumeCount += activeVolumeCountDelta
+	if n.parent != nil {
+		n.parent.UpAdjustActiveVolumeCountDelta(activeVolumeCountDelta)
+	}
+}
+func (n *NodeImpl) UpAdjustMaxVolumeId(vid storage.VolumeId) { //can be negative
+	if n.maxVolumeId < vid {
+		n.maxVolumeId = vid
+		if n.parent != nil {
+			n.parent.UpAdjustMaxVolumeId(vid)
+		}
+	}
 }
 
-func (n *Node) GetMaxVolumeId() storage.VolumeId {
+func (n *NodeImpl) GetMaxVolumeId() storage.VolumeId {
 	return n.maxVolumeId
 }
-
-func (n *Node) AddNode(node *Node) {
-	if n.children[node.Id] == nil {
-		n.children[node.Id] = node
-		n.activeVolumeCount += node.activeVolumeCount
-		n.maxVolumeCount += node.maxVolumeCount
-		fmt.Println(n.Id, "adds", node.Id, "volumeCount =", n.activeVolumeCount)
-	}
+func (n *NodeImpl) GetActiveVolumeCount() int {
+	return n.activeVolumeCount
+}
+func (n *NodeImpl) GetMaxVolumeCount() int {
+	return n.maxVolumeCount
 }
 
-func (n *Node) RemoveNode(nodeId NodeId) {
-	node := n.children[nodeId]
-	if node != nil {
-		delete(n.children, node.Id)
-		n.activeVolumeCount -= node.activeVolumeCount
-		n.maxVolumeCount -= node.maxVolumeCount
-		p := n.parent
-		for p != nil {
-			p.activeVolumeCount -= node.activeVolumeCount
-			p.maxVolumeCount -= node.maxVolumeCount
-			p = p.parent
+func (n *NodeImpl) LinkChildNode(node Node) {
+	if n.children[node.Id()] == nil {
+		n.children[node.Id()] = node
+		n.activeVolumeCount += node.GetActiveVolumeCount()
+		n.maxVolumeCount += node.GetMaxVolumeCount()
+		node.setParent(n)
+		if n.maxVolumeId < node.GetMaxVolumeId() {
+			n.maxVolumeId = node.GetMaxVolumeId()
 		}
-		fmt.Println(n.Id, "removes", node.Id, "volumeCount =", n.activeVolumeCount)
+		fmt.Println(n, "adds", node, "volumeCount =", n.activeVolumeCount)
+	}
+}
+
+func (n *NodeImpl) UnlinkChildNode(nodeId NodeId) {
+	node := n.children[nodeId]
+	node.setParent(nil)
+	if node != nil {
+		delete(n.children, node.Id())
+		n.UpAdjustActiveVolumeCountDelta(-node.GetActiveVolumeCount())
+		n.UpAdjustMaxVolumeCountDelta(-node.GetMaxVolumeCount())
+		fmt.Println(n, "removes", node, "volumeCount =", n.activeVolumeCount)
 	}
 }
diff --git a/weed-fs/src/pkg/topology/rack.go b/weed-fs/src/pkg/topology/rack.go
index 31dac6218..46e7780dc 100644
--- a/weed-fs/src/pkg/topology/rack.go
+++ b/weed-fs/src/pkg/topology/rack.go
@@ -4,13 +4,14 @@ import (
 )
 
 type Rack struct {
-	Node
+	NodeImpl
 	ipRange IpRange
 }
 
-func NewRack(id NodeId) *Rack {
+func NewRack(id string) *Rack {
 	r := &Rack{}
-	r.Node = *NewNode()
-  r.Node.Id = id
+  r.id = NodeId(id)
+  r.nodeType = "Rack"
+  r.children = make(map[NodeId]Node)
 	return r
 }
diff --git a/weed-fs/src/pkg/topology/server.go b/weed-fs/src/pkg/topology/server.go
index 14aed0e2e..aff1f4f15 100644
--- a/weed-fs/src/pkg/topology/server.go
+++ b/weed-fs/src/pkg/topology/server.go
@@ -6,15 +6,16 @@ import (
 )
 
 type Server struct {
-  Node
+  NodeImpl
 	volumes     map[storage.VolumeId]*storage.VolumeInfo
 	Ip          NodeId
 	Port        int
 	PublicUrl   string
 }
-func NewServer(id NodeId) *Server{
+func NewServer(id string) *Server{
   s := &Server{}
-  s.Node.Id = id
+  s.id = NodeId(id)
+  s.nodeType = "Server"
   s.volumes = make(map[storage.VolumeId]*storage.VolumeInfo)
   return s
 }
@@ -24,5 +25,6 @@ func (s *Server) CreateOneVolume(r int, vid storage.VolumeId) storage.VolumeId {
 }
 func (s *Server) AddVolume(v *storage.VolumeInfo){
   s.volumes[v.Id] = v
-  s.Node.AddVolume(v)
+  s.UpAdjustActiveVolumeCountDelta(1)
+  s.UpAdjustMaxVolumeId(v.Id)
 }
diff --git a/weed-fs/src/pkg/topology/topo_test.go b/weed-fs/src/pkg/topology/topo_test.go
index 5bbc33dd9..cb0551096 100644
--- a/weed-fs/src/pkg/topology/topo_test.go
+++ b/weed-fs/src/pkg/topology/topo_test.go
@@ -80,36 +80,33 @@ func setup() *Topology {
 	printMap(data)
 
 	//need to connect all nodes first before server adding volumes
-	topo := NewTopology(NodeId("mynetwork"))
+	topo := NewTopology("mynetwork")
 	mTopology := data.(map[string]interface{})
 	for dcKey, dcValue := range mTopology {
-		dc := NewDataCenter(NodeId(dcKey))
-		dc.Node.parent = &topo.Node
+		dc := NewDataCenter(dcKey)
 		dcMap := dcValue.(map[string]interface{})
-		topo.Node.AddNode(&dc.Node)
+		topo.LinkChildNode(dc)
 		for rackKey, rackValue := range dcMap {
-			rack := NewRack(NodeId(rackKey))
-			rack.Node.parent = &dc.Node
+			rack := NewRack(rackKey)
 			rackMap := rackValue.(map[string]interface{})
-			dc.Node.AddNode(&rack.Node)
+			dc.LinkChildNode(rack)
 			for serverKey, serverValue := range rackMap {
-				server := NewServer(NodeId(serverKey))
-				server.Node.parent = &rack.Node
+				server := NewServer(serverKey)
 				serverMap := serverValue.(map[string]interface{})
-				rack.Node.AddNode(&server.Node)
+				rack.LinkChildNode(server)
 				for _, v := range serverMap["volumes"].([]interface{}) {
 					m := v.(map[string]interface{})
 					vi := &storage.VolumeInfo{Id: storage.VolumeId(int64(m["id"].(float64))), Size: int64(m["size"].(float64))}
 					server.AddVolume(vi)
 				}
-				server.Node.AddMaxVolumeCount(int(serverMap["limit"].(float64)))
+				server.UpAdjustMaxVolumeCountDelta(int(serverMap["limit"].(float64)))
 			}
 		}
 	}
 
 	fmt.Println("topology:", *topo)
 
-	bytes, err := json.Marshal(topo.Node.children)
+	bytes, err := json.Marshal(topo.children)
 	if err != nil {
 		fmt.Println("json error:", err)
 	}
@@ -140,20 +137,14 @@ func printMap(mm interface{}) {
 	}
 }
 
-func TestAddVolume(t *testing.T) {
-	topo := setup()
-	v := &storage.VolumeInfo{}
-	topo.AddVolume(v)
-}
-
 func TestRemoveDataCenter(t *testing.T) {
 	topo := setup()
-	topo.RemoveNode(NodeId("dc2"))
-	if topo.activeVolumeCount != 15 {
+	topo.UnlinkChildNode(NodeId("dc2"))
+	if topo.GetActiveVolumeCount() != 15 {
 		t.Fail()
 	}
-	topo.RemoveNode(NodeId("dc3"))
-	if topo.activeVolumeCount != 12 {
+	topo.UnlinkChildNode(NodeId("dc3"))
+	if topo.GetActiveVolumeCount() != 12 {
 		t.Fail()
 	}
 }
@@ -161,8 +152,9 @@ func TestRemoveDataCenter(t *testing.T) {
 func TestReserveOneVolume(t *testing.T) {
 	topo := setup()
   rand.Seed(time.Now().UnixNano())
+  rand.Seed(1)
 	ret, node, vid := topo.RandomlyReserveOneVolume()
-	fmt.Println("topology:", topo.Node)
+	fmt.Println("topology:", topo)
   fmt.Println("assigned :", ret)
   fmt.Println("assigned node :", node)
 	fmt.Println("assigned volume id:", vid)
diff --git a/weed-fs/src/pkg/topology/topology.go b/weed-fs/src/pkg/topology/topology.go
index 79f0fac67..085031c5d 100644
--- a/weed-fs/src/pkg/topology/topology.go
+++ b/weed-fs/src/pkg/topology/topology.go
@@ -7,24 +7,25 @@ import (
 )
 
 type Topology struct {
-  Node
-}
-func NewTopology(id NodeId) *Topology{
-  t := &Topology{}
-  t.Node = *NewNode()
-  t.Node.Id = id
-  return t
+  NodeImpl
 }
 
-func (t *Topology) RandomlyReserveOneVolume() (bool, *Node, storage.VolumeId) {
-  slots := t.Node.maxVolumeCount-t.Node.activeVolumeCount
+func NewTopology(id string) *Topology {
+  t := &Topology{}
+  t.id = NodeId(id)
+  t.nodeType = "Topology"
+  t.children = make(map[NodeId]Node)
+  return t
+}
+func (t *Topology) RandomlyReserveOneVolume() (bool, Node, storage.VolumeId) {
+  slots := t.maxVolumeCount-t.activeVolumeCount
   r := rand.Intn(slots)
   vid := t.nextVolumeId()
-  ret, node := t.Node.ReserveOneVolume(r,vid)
+  ret, node := t.ReserveOneVolume(r,vid)
   return ret, node, vid
 }
 
 func (t *Topology) nextVolumeId() storage.VolumeId {
-  vid := t.Node.GetMaxVolumeId()
+  vid := t.GetMaxVolumeId()
 	return vid.Next()
 }