diff --git a/cluster/syncer/statefulset.go b/cluster/syncer/statefulset.go
index 50d726607..5967277a5 100644
--- a/cluster/syncer/statefulset.go
+++ b/cluster/syncer/statefulset.go
@@ -97,6 +97,7 @@ func (s *StatefulSetSyncer) ObjectOwner() runtime.Object { return s.Unwrap() }
 func (s *StatefulSetSyncer) GetOwner() runtime.Object { return s.Unwrap() }
 
 // Sync persists data into the external store.
+// it's called by cluster controller, when return error, retry Reconcile(),when return nil, exit this cycle.
 // See https://github.com/presslabs/controller-util/blob/master/syncer/object.go#L68
 func (s *StatefulSetSyncer) Sync(ctx context.Context) (syncer.SyncResult, error) {
 	var err error
@@ -105,14 +106,19 @@ func (s *StatefulSetSyncer) Sync(ctx context.Context) (syncer.SyncResult, error)
 
 	result.Operation, err = s.createOrUpdate(ctx)
 
+	//get namespace.
 	key := client.ObjectKeyFromObject(s.sfs)
+	//get groupVersionKind.
 	gvk, gvkErr := apiutil.GVKForObject(s.sfs, s.cli.Scheme())
 	if gvkErr != nil {
 		kind = fmt.Sprintf("%T", s.sfs)
 	} else {
 		kind = gvk.String()
 	}
-
+	// print log.
+	// Info: owner is deleted or ignored error.
+	// Waring: other errors.
+	// Normal: no error.
 	switch {
 	case errors.Is(err, syncer.ErrOwnerDeleted):
 		log.Info(string(result.Operation), "key", key, "kind", kind, "error", err)
@@ -136,6 +142,7 @@ func (s *StatefulSetSyncer) Sync(ctx context.Context) (syncer.SyncResult, error)
 // see https://pkg.go.dev/sigs.k8s.io/controller-runtime@v0.9.2/pkg/controller/controllerutil?utm_source=gopls#CreateOrUpdate
 func (s *StatefulSetSyncer) createOrUpdate(ctx context.Context) (controllerutil.OperationResult, error) {
 	var err error
+	// check if sfs exist.
 	if err = s.cli.Get(ctx, client.ObjectKeyFromObject(s.sfs), s.sfs); err != nil {
 		if !k8serrors.IsNotFound(err) {
 			return controllerutil.OperationResultNone, err
@@ -151,32 +158,33 @@ func (s *StatefulSetSyncer) createOrUpdate(ctx context.Context) (controllerutil.
 			return controllerutil.OperationResultCreated, nil
 		}
 	}
-
+	// deep copy the old statefulset from StatefulSetSyncer.
 	existing := s.sfs.DeepCopyObject()
+	// Sync data from cluster.spec to statefulset.
 	if err = s.mutate(); err != nil {
 		return controllerutil.OperationResultNone, err
 	}
-
+	// check if statefulset changed.
 	if equality.Semantic.DeepEqual(existing, s.sfs) {
 		return controllerutil.OperationResultNone, nil
 	}
-
+	// if changed, update statefulset.
 	if err := s.cli.Update(ctx, s.sfs); err != nil {
 		return controllerutil.OperationResultNone, err
 	}
-
+	// update every pods of statefulset.
 	if err := s.updatePod(ctx); err != nil {
 		return controllerutil.OperationResultNone, err
 	}
-
+	// update pvc.
 	if err := s.updatePVC(ctx); err != nil {
 		return controllerutil.OperationResultNone, err
 	}
-
 	return controllerutil.OperationResultUpdated, nil
 }
 
-// updatePod update the pods.
+// updatePod update the pods, update follower nodes first.
+// this can reduce the number of master-slave switching during the update process.
 func (s *StatefulSetSyncer) updatePod(ctx context.Context) error {
 	if s.sfs.Status.UpdatedReplicas >= s.sfs.Status.Replicas {
 		return nil
@@ -188,7 +196,7 @@ func (s *StatefulSetSyncer) updatePod(ctx context.Context) error {
 		log.Info("can't start/continue 'update': waiting for all replicas are ready")
 		return nil
 	}
-
+	// get all pods.
 	pods := corev1.PodList{}
 	if err := s.cli.List(ctx,
 		&pods,
@@ -199,31 +207,33 @@ func (s *StatefulSetSyncer) updatePod(ctx context.Context) error {
 	); err != nil {
 		return err
 	}
-
 	var leaderPod corev1.Pod
 	var followerPods []corev1.Pod
 	for _, pod := range pods.Items {
+		// check if the pod is healthy.
 		if pod.ObjectMeta.Labels["healthy"] != "yes" {
 			return fmt.Errorf("can't start/continue 'update': pod[%s] is unhealthy", pod.Name)
 		}
-
+		// skip if pod is leader.
 		if pod.ObjectMeta.Labels["role"] == "leader" && leaderPod.Name == "" {
 			leaderPod = pod
 			continue
 		}
-
 		followerPods = append(followerPods, pod)
+		// if pod is not leader, direct update.
 		if err := s.applyNWait(ctx, &pod); err != nil {
 			return err
 		}
 	}
-
+	// all followers have been updated now, then update leader.
 	if leaderPod.Name != "" {
+		// when replicas is two (one leader and one follower).
 		if len(followerPods) == 1 {
 			if err := s.preUpdate(ctx, leaderPod.Name, followerPods[0].Name); err != nil {
 				return err
 			}
 		}
+		// replicas greater than two.
 		if err := s.applyNWait(ctx, &leaderPod); err != nil {
 			return err
 		}
@@ -232,7 +242,7 @@ func (s *StatefulSetSyncer) updatePod(ctx context.Context) error {
 	return nil
 }
 
-// preUpdate run before update the leader pod when replica is 2.
+// preUpdate run before update the leader pod when replicas is 2.
 func (s *StatefulSetSyncer) preUpdate(ctx context.Context, leader, follower string) error {
 	if s.sfs.Status.Replicas != 2 {
 		return nil
@@ -364,7 +374,7 @@ func (s *StatefulSetSyncer) mutate() error {
 		}
 	} else if ctime := s.Unwrap().GetCreationTimestamp(); ctime.IsZero() {
 		// the owner is deleted, don't recreate the resource if does not exist, because gc
-		// will not delete it again because has no owner reference set
+		// will not delete it again because has no owner reference set.
 		return fmt.Errorf("owner is deleted")
 	}
 	return nil
@@ -438,6 +448,7 @@ func (s *StatefulSetSyncer) updatePVC(ctx context.Context) error {
 }
 
 func (s *StatefulSetSyncer) applyNWait(ctx context.Context, pod *corev1.Pod) error {
+	//check version, if not latest, delete node.
 	if pod.ObjectMeta.Labels["controller-revision-hash"] == s.sfs.Status.UpdateRevision {
 		log.Info("pod is already updated", "pod name", pod.Name)
 	} else {
@@ -471,9 +482,9 @@ func (s *StatefulSetSyncer) applyNWait(ctx context.Context, pod *corev1.Pod) err
 	})
 }
 
-// retry runs func "f" every "in" time until "limit" is reached
+// retry runs func "f" every "in" time until "limit" is reached.
 // it also doesn't have an extra tail wait after the limit is reached
-// and f func runs first time instantly
+// and f func runs first time instantly.
 func retry(in, limit time.Duration, f func() (bool, error)) error {
 	fdone, err := f()
 	if err != nil {