/**
  * 返回一个数据库对象,该服务器为写数据准备
  *
  * @return MooMySQL
  */
 public function getWriteConnection()
 {
     if (self::$write_conn != null) {
         return self::$write_conn;
     }
     self::$write_conn = $this->createConnection($this->write_host);
     return self::$write_conn;
 }