toughen weedfs clustering, adding synchronizing max volume id among

peers in order to avoid the same volume id being assigned twice
1. moving raft.Server to topology
2. adding max volume id command for raft

go/topology/topology.go

@@ -5,6 +5,7 @@ import (
 	"code.google.com/p/weed-fs/go/sequence"
 	"code.google.com/p/weed-fs/go/storage"
 	"errors"
+	"github.com/goraft/raft"
 	"io/ioutil"
 	"math/rand"
 )
@@ -12,8 +13,6 @@ import (
 type Topology struct {
 	NodeImpl
 
-	IsLeader bool
-
 	collectionMap map[string]*Collection
 
 	pulse int64
@@ -27,6 +26,8 @@ type Topology struct {
 	chanFullVolumes        chan storage.VolumeInfo
 
 	configuration *Configuration
+
+	RaftServer raft.Server
 }
 
 func NewTopology(id string, confFile string, seq sequence.Sequencer, volumeSizeLimit uint64, pulse int) (*Topology, error) {
@@ -50,6 +51,24 @@ func NewTopology(id string, confFile string, seq sequence.Sequencer, volumeSizeL
 	return t, err
 }
 
+func (t *Topology) IsLeader() bool {
+	return t.RaftServer == nil || t.Leader() == t.RaftServer.Name()
+}
+
+func (t *Topology) Leader() string {
+	l := ""
+	if t.RaftServer != nil {
+		l = t.RaftServer.Leader()
+	}
+
+	if l == "" {
+		// We are a single node cluster, we are the leader
+		return t.RaftServer.Name()
+	}
+
+	return l
+}
+
 func (t *Topology) loadConfiguration(configurationFile string) error {
 	b, e := ioutil.ReadFile(configurationFile)
 	if e == nil {
@@ -79,7 +98,9 @@ func (t *Topology) Lookup(collection string, vid storage.VolumeId) []*DataNode {
 
 func (t *Topology) NextVolumeId() storage.VolumeId {
 	vid := t.GetMaxVolumeId()
-	return vid.Next()
+	next := vid.Next()
+	go t.RaftServer.Do(NewMaxVolumeIdCommand(next))
+	return next
 }
 
 func (t *Topology) PickForWrite(collectionName string, rp *storage.ReplicaPlacement, count int, dataCenter string) (string, int, *DataNode, error) {