diff --git a/server/id.go b/server/id.go
index 4e4b6145589..38785f3a782 100644
--- a/server/id.go
+++ b/server/id.go
@@ -39,13 +39,9 @@ func (alloc *idAllocator) Alloc() (uint64, error) {
 	defer alloc.mu.Unlock()
 
 	if alloc.base == alloc.end {
-		end, err := alloc.generate()
-		if err != nil {
+		if err := alloc.generateLocked(); err != nil {
 			return 0, err
 		}
-
-		alloc.end = end
-		alloc.base = alloc.end - allocStep
 	}
 
 	alloc.base++
@@ -53,11 +49,19 @@ func (alloc *idAllocator) Alloc() (uint64, error) {
 	return alloc.base, nil
 }
 
-func (alloc *idAllocator) generate() (uint64, error) {
+// Generate synchronizes and generates id range.
+func (alloc *idAllocator) Generate() error {
+	alloc.mu.Lock()
+	defer alloc.mu.Unlock()
+
+	return alloc.generateLocked()
+}
+
+func (alloc *idAllocator) generateLocked() error {
 	key := alloc.s.getAllocIDPath()
 	value, err := getValue(alloc.s.client, key)
 	if err != nil {
-		return 0, err
+		return err
 	}
 
 	var (
@@ -72,7 +76,7 @@ func (alloc *idAllocator) generate() (uint64, error) {
 		// update the key
 		end, err = bytesToUint64(value)
 		if err != nil {
-			return 0, err
+			return err
 		}
 
 		cmp = clientv3.Compare(clientv3.Value(key), "=", string(value))
@@ -82,13 +86,15 @@ func (alloc *idAllocator) generate() (uint64, error) {
 	value = uint64ToBytes(end)
 	resp, err := alloc.s.leaderTxn(cmp).Then(clientv3.OpPut(key, string(value))).Commit()
 	if err != nil {
-		return 0, err
+		return err
 	}
 	if !resp.Succeeded {
-		return 0, errors.New("generate id failed, we may not leader")
+		return errors.New("generate id failed, we may not leader")
 	}
 
 	log.Info("idAllocator allocates a new id", zap.Uint64("alloc-id", end))
 	metadataGauge.WithLabelValues("idalloc").Set(float64(end))
-	return end, nil
+	alloc.end = end
+	alloc.base = end - allocStep
+	return nil
 }
diff --git a/server/leader.go b/server/leader.go
index 419fe8c0612..a34cdfaf048 100644
--- a/server/leader.go
+++ b/server/leader.go
@@ -275,6 +275,11 @@ func (s *Server) campaignLeader() error {
 	}
 	defer s.stopRaftCluster()
 
+	log.Info("sync id from etcd")
+	if err = s.idAlloc.Generate(); err != nil {
+		return err
+	}
+
 	s.enableLeader()
 	defer s.disableLeader()
 
diff --git a/server/server.go b/server/server.go
index 63e978b9aba..6c200fdb825 100644
--- a/server/server.go
+++ b/server/server.go
@@ -495,6 +495,11 @@ func (s *Server) GetClassifier() namespace.Classifier {
 	return s.classifier
 }
 
+// GetAllocator returns the id allocator of this server.
+func (s *Server) GetAllocator() core.IDAllocator {
+	return s.idAlloc
+}
+
 // txn returns an etcd client transaction wrapper.
 // The wrapper will set a request timeout to the context and log slow transactions.
 func (s *Server) txn() clientv3.Txn {
diff --git a/tests/server/server_test.go b/tests/server/server_test.go
index 4553a51a774..5004b7487c3 100644
--- a/tests/server/server_test.go
+++ b/tests/server/server_test.go
@@ -14,6 +14,7 @@
 package server_test
 
 import (
+	"context"
 	"testing"
 
 	. "github.com/pingcap/check"
@@ -125,3 +126,79 @@ func (s *serverTestSuite) TestLeader(c *C) {
 		return leader != leader1
 	})
 }
+
+func (s *serverTestSuite) TestMonotonicID(c *C) {
+	var err error
+	cluster, err := tests.NewTestCluster(2)
+	defer cluster.Destroy()
+	c.Assert(err, IsNil)
+
+	err = cluster.RunInitialServers()
+	c.Assert(err, IsNil)
+	cluster.WaitLeader()
+
+	leaderServer := cluster.GetServer(cluster.GetLeader()).GetServer()
+	var last1 uint64
+	for i := uint64(0); i < 10; i++ {
+		id, err := leaderServer.GetAllocator().Alloc()
+		c.Assert(err, IsNil)
+		c.Assert(id, Greater, last1)
+		last1 = id
+	}
+	err = cluster.ResignLeader()
+	c.Assert(err, IsNil)
+	cluster.WaitLeader()
+	leaderServer = cluster.GetServer(cluster.GetLeader()).GetServer()
+	var last2 uint64
+	for i := uint64(0); i < 10; i++ {
+		id, err := leaderServer.GetAllocator().Alloc()
+		c.Assert(err, IsNil)
+		c.Assert(id, Greater, last2)
+		last2 = id
+	}
+	err = cluster.ResignLeader()
+	c.Assert(err, IsNil)
+	cluster.WaitLeader()
+	leaderServer = cluster.GetServer(cluster.GetLeader()).GetServer()
+	id, err := leaderServer.GetAllocator().Alloc()
+	c.Assert(err, IsNil)
+	c.Assert(id, Greater, last2)
+	var last3 uint64
+	for i := uint64(0); i < 1000; i++ {
+		id, err := leaderServer.GetAllocator().Alloc()
+		c.Assert(err, IsNil)
+		c.Assert(id, Greater, last3)
+		last3 = id
+	}
+}
+
+func (s *serverTestSuite) TestPDRestart(c *C) {
+	cluster, err := tests.NewTestCluster(1)
+	c.Assert(err, IsNil)
+	defer cluster.Destroy()
+
+	err = cluster.RunInitialServers()
+	c.Assert(err, IsNil)
+	cluster.WaitLeader()
+	leaderServer := cluster.GetServer(cluster.GetLeader())
+	leader := leaderServer.GetServer()
+
+	var last uint64
+	for i := uint64(0); i < 10; i++ {
+		id, err := leader.GetAllocator().Alloc()
+		c.Assert(err, IsNil)
+		c.Assert(id, Greater, last)
+		last = id
+	}
+
+	c.Assert(leaderServer.Stop(), IsNil)
+	c.Assert(leaderServer.Run(context.TODO()), IsNil)
+	cluster.WaitLeader()
+
+	for i := uint64(0); i < 10; i++ {
+		id, err := leader.GetAllocator().Alloc()
+		c.Assert(err, IsNil)
+		c.Assert(id, Greater, last)
+		last = id
+	}
+}