Scheduled Tasks Proposal

Introduction

This proposal is meant both to provide a replacement for the moodle cron job, and provide a means to schedule once off tasks to be run outside of the user's request lifecycle.

Terminology

• *Subtask* an individual piece of cron processing that should be run (equivalent to forum_cron now, or maybe even smaller)
• *Moodle cron instance* a cron.php process

Rationale

The moodle cronjob currently delegates all scheduling to each subtask that is run - for example, the forum cron is responsible for checking when it last run, and making decisions about whether or not it should be run again. This sort of decision process should be centralised, and individual cron subtasks should be called by the central controller.

Additionally, there is not any central locking of subtasks. At the moment, some subtasks that expect that they might take a long time to run implement their own locking (for example statistics), but it's not centralised. Each moodle cron instance runs to completion, no matter how long it takes, and it processes tasks in the order that they're programmed, regardless of if there are any other moodle cron instances running, that might be processing sub tasks in parallel

Finally, we need to be able to run non-related tasks in parallel so that the entire moodle queue isn't held up by single long running jobs.

Goals

• Admin screen to move around scheduling.
• ...

Unresolved issues

• Do we need to allow for scheduling different subtasks on different servers?
• We need to find a way to separate subtasks by some logic so that subtasks that write to the same areas of the database never run at the same time. We could do this by getting each cron job to say what areas of Moodle they write to, but this is problematic.
• We also have to deal with the order of some subtasks - we could maybe do this by introducing dependencies
• When the first cron in a long time is running, we should lock the entire cron and let it run to completeness, because the order is really important then.

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

• /