Development talk:Scheduled Tasks Proposal

Pseudo code proposal

This is just a rough idea to provoke thought:--Tim Hunt 11:06, 9 December 2009 (UTC)

<?php function handle_cron($maxprocesstime) {

   $timestart = time();

   $runid = cron_record_starting_processing($timenow);

   while (($timenow = time()) < $timestart + $maxprocesstime) {
       try {
           $task = cron_get_next_task($timenow);
           cron_acquire_lock($task);
           $next = $task->execute($timenow);
           cron_record_task_success($task, $next, $timenow);
           cron_release_lock($task);

       } catch (Exception $e) {
           cron_record_task_failure($task, $timenow, $e);
       }
   }

   cron_record_ending_processing($runid, $timenow);

}

/**

* Instances of this class are rows in the scheduled_tasks table.
*/

abstract class scheduled_task {

   protected $id;
   protected $plugin; // = 'mod_quiz', 'qtype_multichoice'.
   protected $function;
   protected $lastruntime;
   protected $nextscheduledtime;
   protected $priority; // LOW, MEDIUM, HIGH, or, a number like nice 19, or something.

   protected abstract function get_start_end_times($timenow);

   protected abstract function schedule_next($timenow);

   public function execute($timenow) {
       $file = get_plugin_dir($this->plugin) . '/cron.php';
       if (!is_readable($file)) {
           throw new cron_exception('required file does not exist.');
       }
       if (include_once($file)) {
           throw new cron_exception('required file could not be included.');
       }
       list($start, $end) = next_times($timenow);
       $this->$function($start, $end);
       return $this->schedule_next();
   }

}

class one_off_task extends scheduled_task {

   protected abstract function get_start_end_times($timenow) {
       return array(null, $timenow);
   }

   protected abstract function schedule_next($timenow) {
       return null;
   }

}

class catchup_task extends scheduled_task {

   protected $desiredinterval; // seconds

   protected abstract function get_start_end_times($timenow) {
       return array($lastruntime, $timenow);
   }

   protected abstract function schedule_next($timenow) {
       return $timenow + $desiredinterval;
   }

}

class every_day_task extends scheduled_task {

   protected $swtichovertime; // Seconds after midnight.
   protected $runtime; // Seconds after midnight.
   // For now this code assumes $runtime > $swtichovertime, but that is just me
   // being lazy. It can be fixed.

   protected abstract function get_start_end_times($timenow) {
       $midnight = get_midnight_before($this->nextscheduledtime);
       $previousmidnight = get_previous_midnight($midnight);
       $starttime = $previousmidnight + $swtichovertime;
       $endtime = $midnight + $swtichovertime;
       return array($starttime, $endtime);
   }

   protected abstract function schedule_next($timenow) {
       $midnight = get_midnight_before($this->nextscheduledtime);
       $nextmidnight = get_following_midnight($midnight);
       return $nextmidnight + $this->runtime;
   }

}

class weekly_task extends scheduled_task {

   // ...

}

/* Approximate database tables.

scheduled_tasks

   id AUTOINCREMENT
   type varchar
   plugin varchar
   function varchar UNIQUE
   lastruntime int 
   nextscheduledtime int
   priority int
   custom1 int
   custom2 int 
   customextra text

scheduled_task_locks

   id int auto
   function fk unique
   locktime int

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