public function commit()/*# : bool */
    {
        $ends = [ $this->front, $this->back ];
        $res  = false;

        foreach ($ends as $end) {
            // commit failed, set error
            if (!$end->commit()) {
                $this->setError(
                    $end->getError(),
                    $end->getErrorCode()
                );

            // one commit is ok, then all ok
            } else {
                $res = true;
            }
        }

        return $res;
    }