diff --git a/pkg/controller/cluster/cluster_controller.go b/pkg/controller/cluster/cluster_controller.go
index 390967d52d1d8c5d60e0607940c7d3293e157515..ea3d021a4a430d36e81b938d4ab09f522b6b5c7b 100644
--- a/pkg/controller/cluster/cluster_controller.go
+++ b/pkg/controller/cluster/cluster_controller.go
@@ -34,6 +34,14 @@ import (
 	"time"
 )
 
+// Cluster controller only runs under multicluster mode. Cluster controller is following below steps,
+//   1. Populates proxy spec if cluster connection type is proxy
+//     1.1 Wait for cluster agent is ready if connection type is proxy
+//   2. Join cluster into federation control plane if kubeconfig is ready.
+//   3. Pull cluster version and configz, set result to cluster status
+// Also put all clusters back into queue every 5 * time.Minute to sync cluster status, this is needed
+// in case there aren't any cluster changes made.
+
 const (
 	// maxRetries is the number of times a service will be retried before it is dropped out of the queue.
 	// With the current rate-limiter in use (5ms*2^(maxRetries-1)) the following numbers represent the
@@ -44,6 +52,7 @@ const (
 
 	kubefedNamespace = "kube-federation-system"
 
+	// Actually host cluster name can be anything, there is only necessary when calling JoinFederation function
 	hostClusterName = "kubesphere"
 
 	// allocate kubernetesAPIServer port in range [portRangeMin, portRangeMax] for agents if port is not specified
@@ -51,7 +60,7 @@ const (
 	portRangeMin = 6000
 	portRangeMax = 7000
 
-	// Service port
+	// Proxy service port
 	kubernetesPort = 6443
 	kubespherePort = 80
 
@@ -307,6 +316,7 @@ func (c *clusterController) syncCluster(key string) error {
 				}
 			}
 
+			// remove our cluster finalizer
 			finalizers := sets.NewString(cluster.ObjectMeta.Finalizers...)
 			finalizers.Delete(clusterv1alpha1.Finalizer)
 			cluster.ObjectMeta.Finalizers = finalizers.List()
@@ -317,10 +327,13 @@ func (c *clusterController) syncCluster(key string) error {
 		return nil
 	}
 
+	// save a old copy of cluster
 	oldCluster := cluster.DeepCopy()
 
 	// prepare for proxy to member cluster
 	if cluster.Spec.Connection.Type == clusterv1alpha1.ConnectionTypeProxy {
+
+		// allocate ports for kubernetes and kubesphere endpoint
 		if cluster.Spec.Connection.KubeSphereAPIServerPort == 0 ||
 			cluster.Spec.Connection.KubernetesAPIServerPort == 0 {
 			port, err := c.allocatePort()
@@ -338,6 +351,7 @@ func (c *clusterController) syncCluster(key string) error {
 			cluster.Spec.Connection.Token = c.generateToken()
 		}
 
+		// create a proxy service spec
 		mcService := v1.Service{
 			ObjectMeta: metav1.ObjectMeta{
 				Name:      serviceName,
@@ -370,7 +384,7 @@ func (c *clusterController) syncCluster(key string) error {
 		}
 
 		service, err := c.client.CoreV1().Services(defaultAgentNamespace).Get(serviceName, metav1.GetOptions{})
-		if err != nil {
+		if err != nil { // proxy service not found
 			if errors.IsNotFound(err) {
 				service, err = c.client.CoreV1().Services(defaultAgentNamespace).Create(&mcService)
 				if err != nil {
@@ -379,7 +393,7 @@ func (c *clusterController) syncCluster(key string) error {
 			}
 
 			return err
-		} else {
+		} else { // update existed proxy service
 			if !reflect.DeepEqual(service.Spec, mcService.Spec) {
 				mcService.ObjectMeta = service.ObjectMeta
 				mcService.Spec.ClusterIP = service.Spec.ClusterIP
@@ -391,7 +405,7 @@ func (c *clusterController) syncCluster(key string) error {
 			}
 		}
 
-		// populated the kubernetes apiEndpoint and kubesphere apiEndpoint
+		// populates the kubernetes apiEndpoint and kubesphere apiEndpoint
 		cluster.Spec.Connection.KubernetesAPIEndpoint = fmt.Sprintf("https://%s:%d", service.Spec.ClusterIP, kubernetesPort)
 		cluster.Spec.Connection.KubeSphereAPIEndpoint = fmt.Sprintf("http://%s:%d", service.Spec.ClusterIP, kubespherePort)
 
@@ -415,10 +429,12 @@ func (c *clusterController) syncCluster(key string) error {
 		}
 	}
 
+	// kubeconfig not ready, nothing to do
 	if len(cluster.Spec.Connection.KubeConfig) == 0 {
 		return nil
 	}
 
+	// build up cached cluster data if there isn't any
 	c.mu.Lock()
 	clusterDt, ok := c.clusterMap[cluster.Name]
 	if !ok || clusterDt == nil || !equality.Semantic.DeepEqual(clusterDt.cachedKubeconfig, cluster.Spec.Connection.KubeConfig) {
@@ -502,6 +518,19 @@ func (c *clusterController) syncCluster(key string) error {
 		c.updateClusterCondition(cluster, clusterReadyCondition)
 	}
 
+	if !isConditionTrue(cluster, clusterv1alpha1.ClusterAgentAvailable) {
+		clusterNotReadyCondition := clusterv1alpha1.ClusterCondition{
+			Type:               clusterv1alpha1.ClusterReady,
+			Status:             v1.ConditionFalse,
+			LastUpdateTime:     metav1.Now(),
+			LastTransitionTime: metav1.Now(),
+			Reason:             "Unable to establish connection with cluster",
+			Message:            "Cluster is not available now",
+		}
+
+		c.updateClusterCondition(cluster, clusterNotReadyCondition)
+	}
+
 	if !reflect.DeepEqual(oldCluster, cluster) {
 		_, err = c.clusterClient.Update(cluster)
 		if err != nil {
@@ -513,6 +542,7 @@ func (c *clusterController) syncCluster(key string) error {
 	return nil
 }
 
+// tryToFetchKubeSphereComponents will send requests to member cluster configz api using kube-apiserver proxy way
 func (c *clusterController) tryToFetchKubeSphereComponents(host string, transport http.RoundTripper) (map[string]bool, error) {
 	client := http.Client{
 		Transport: transport,
@@ -569,6 +599,7 @@ func (c *clusterController) handleErr(err error, key interface{}) {
 	utilruntime.HandleError(err)
 }
 
+// isConditionTrue checks cluster specific condition value is True, return false if condition not exists
 func isConditionTrue(cluster *clusterv1alpha1.Cluster, conditionType clusterv1alpha1.ClusterConditionType) bool {
 	for _, condition := range cluster.Status.Conditions {
 		if condition.Type == conditionType && condition.Status == v1.ConditionTrue {