public function closeAll()
 {
     $this->closeWriteConnection();
     if ($this->total_read_count >= 1 && is_array(self::$read_conn_pool)) {
         foreach (self::$read_conn_pool as $key => $db) {
             if (!is_null($db)) {
                 $db->close();
                 self::$read_conn_pool[$key] = null;
             }
         }
         self::$read_conn_pool = null;
     }
 }