public static function get_cron($controller,$action){
        $cron = self::find()->where(['controller'=>$controller,'action'=>$action, 'success' => 0])->orderBy('id_cron_job DESC')->one();
        if (is_null($cron)){
            $cron = new CronJob();
            $cron->controller = $controller;
            $cron->action = $action;
        }
        return $cron;
    }