diff --git a/.github/workflows/coverage.yml b/.github/workflows/coverage.yml
index 4a135f9dd39..4babfd8f55e 100644
--- a/.github/workflows/coverage.yml
+++ b/.github/workflows/coverage.yml
@@ -30,7 +30,7 @@ on:
 
 jobs:
   coverage:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     env:
       CTEST_PARALLEL_LEVEL: 1 # parallel test level for ctest (make test)
       CMAKE_BUILD_TYPE: Debug
diff --git a/src/sdk/db_sdk.cc b/src/sdk/db_sdk.cc
index 39f791e9602..27d56901027 100644
--- a/src/sdk/db_sdk.cc
+++ b/src/sdk/db_sdk.cc
@@ -194,12 +194,19 @@ ClusterSDK::~ClusterSDK() {
 }
 
 void ClusterSDK::CheckZk() {
-    if (session_id_ == 0) {
-        WatchNotify();
-    } else if (session_id_ != zk_client_->GetSessionTerm()) {
-        LOG(WARNING) << "session changed, re-watch notify";
-        WatchNotify();
+    // ensure that zk client is alive
+    if (zk_client_->EnsureConnected()) {
+        if (session_id_ == 0) {
+            WatchNotify();
+        } else if (session_id_ != zk_client_->GetSessionTerm()) {
+            LOG(WARNING) << "session changed, re-watch notify";
+            WatchNotify();
+        }
+    } else {
+        // 5min print once
+        LOG_EVERY_N(WARNING, 150) << "zk client is not connected, reconnect later";
     }
+
     pool_.DelayTask(2000, [this] { CheckZk(); });
 }
 
@@ -380,7 +387,7 @@ bool ClusterSDK::InitTabletClient() {
     std::vector<std::string> tablets;
     bool ok = zk_client_->GetNodes(tablets);
     if (!ok) {
-        LOG(WARNING) << "fail to get tablet";
+        LOG(WARNING) << "fail to get tablets from zk";
         return false;
     }
     std::map<std::string, std::string> real_ep_map;
diff --git a/src/sdk/db_sdk.h b/src/sdk/db_sdk.h
index 48bb1ea80ab..88a9fd8a62d 100644
--- a/src/sdk/db_sdk.h
+++ b/src/sdk/db_sdk.h
@@ -170,7 +170,8 @@ class ClusterSDK : public DBSDK {
     std::string globalvar_changed_notify_path_;
     std::string leader_path_;
     std::string taskmanager_leader_path_;
-
+    // CheckZk will be called periodically, so we don't need to check zk_client_ before using it
+    // if failed, just retry
     ::openmldb::zk::ZkClient* zk_client_;
     ::baidu::common::ThreadPool pool_;
 };
diff --git a/src/zk/zk_client.cc b/src/zk/zk_client.cc
index e1044db73ec..8bbd265071d 100644
--- a/src/zk/zk_client.cc
+++ b/src/zk/zk_client.cc
@@ -55,7 +55,7 @@ void NodeWatcher(zhandle_t* zh, int type, int state, const char* path, void* wat
 }
 
 void ItemWatcher(zhandle_t* zh, int type, int state, const char* path, void* watcher_ctx) {
-    PDLOG(INFO, "node watcher with event type %d, state %d", type, state);
+    PDLOG(INFO, "item watcher with event type %d, state %d", type, state);
     if (zoo_get_context(zh)) {
         ZkClient* client = const_cast<ZkClient*>(reinterpret_cast<const ZkClient*>(zoo_get_context(zh)));
         std::string path_str(path);
@@ -565,8 +565,13 @@ void ZkClient::LogEvent(int type, int state, const char* path) {
     if (type == ZOO_SESSION_EVENT) {
         if (state == ZOO_CONNECTED_STATE) {
             Connected();
+        } else if (state == ZOO_CONNECTING_STATE || state == ZOO_ASSOCIATING_STATE) {
+            // just wait
         } else if (state == ZOO_EXPIRED_SESSION_STATE) {
             connected_ = false;
+        } else {
+            // unknow state, should retry
+            connected_ = false;
         }
     }
 }
@@ -612,5 +617,18 @@ bool ZkClient::Mkdir(const std::string& path) {
     return MkdirNoLock(path);
 }
 
+bool ZkClient::EnsureConnected() {
+    if (!IsConnected()) {
+        LOG(WARNING) << "reconnect zk";
+        if (Reconnect()) {
+            LOG(INFO) << "reconnect zk ok";
+        } else {
+            LOG(WARNING) << "reconnect zk failed";
+            return false;
+        }
+    }
+    return true;
+}
+
 }  // namespace zk
 }  // namespace openmldb
diff --git a/src/zk/zk_client.h b/src/zk/zk_client.h
index e06c0de7e6a..19f86d3f631 100644
--- a/src/zk/zk_client.h
+++ b/src/zk/zk_client.h
@@ -136,6 +136,11 @@ class ZkClient {
     // when reconnect, need Register and Watchnodes again
     bool Reconnect();
 
+    // ensure that zk client is connected:
+    // if not, try to reconnect, return false if reconnect failed
+    // DON'T use zk client if this function return false
+    bool EnsureConnected();
+
  private:
     void Connected();