public function read($sessionId){
        //si la sesión está abierta y tengo lock 
        if($this->opened){
            $sql= $this->connection->connection->prepare('SELECT session_data FROM sessions WHERE session_id = :id');
            $sql->bindValue('id', $sessionId);
            $sql->execute();
            $result= $sql->fetch(\PDO::FETCH_ASSOC);
            if($result != NULL){
                return $result['session_data'];                
            }
            return '';
        }
        else{
            return '';
        }
    }