includes/poolcounter/PoolCounter.php

   1 <?php
   2 /**
   3  * Provides of semaphore semantics for restricting the number
   4  * of workers that may be concurrently performing the same task.
   5  *
   6  * This program is free software; you can redistribute it and/or modify
   7  * it under the terms of the GNU General Public License as published by
   8  * the Free Software Foundation; either version 2 of the License, or
   9  * (at your option) any later version.
  10  *
  11  * This program is distributed in the hope that it will be useful,
  12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  14  * GNU General Public License for more details.
  15  *
  16  * You should have received a copy of the GNU General Public License along
  17  * with this program; if not, write to the Free Software Foundation, Inc.,
  18  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  19  * http://www.gnu.org/copyleft/gpl.html
  20  *
  21  * @file
  22  */
  23
  24 /**
  25  * When you have many workers (threads/servers) giving service, and a
  26  * cached item expensive to produce expires, you may get several workers
  27  * doing the job at the same time.
  28  *
  29  * Given enough requests and the item expiring fast (non-cacheable,
  30  * lots of edits...) that single work can end up unfairly using most (all)
  31  * of the cpu of the pool. This is also known as 'Michael Jackson effect'
  32  * since this effect triggered on the english wikipedia on the day Michael
  33  * Jackson died, the biographical article got hit with several edits per
  34  * minutes and hundreds of read hits.
  35  *
  36  * The PoolCounter provides semaphore semantics for restricting the number
  37  * of workers that may be concurrently performing such single task.
  38  *
  39  * By default PoolCounter_Stub is used, which provides no locking. You
  40  * can get a useful one in the PoolCounter extension.
  41  */
  42 abstract class PoolCounter {
  43         /* Return codes */
  44         const LOCKED = 1; /* Lock acquired */
  45         const RELEASED = 2; /* Lock released */
  46         const DONE = 3; /* Another worker did the work for you */
  47
  48         const ERROR = -1; /* Indeterminate error */
  49         const NOT_LOCKED = -2; /* Called release() with no lock held */
  50         const QUEUE_FULL = -3; /* There are already maxqueue workers on this lock */
  51         const TIMEOUT = -4; /* Timeout exceeded */
  52         const LOCK_HELD = -5; /* Cannot acquire another lock while you have one lock held */
  53
  54         /** @var string All workers with the same key share the lock */
  55         protected $key;
  56         /** @var int Maximum number of workers doing the task simultaneously */
  57         protected $workers;
  58         /** @var int If this number of workers are already working/waiting, fail instead of wait */
  59         protected $maxqueue;
  60         /** @var float Maximum time in seconds to wait for the lock */
  61         protected $timeout;
  62
  63         /**
  64          * @param array $conf
  65          * @param string $type
  66          * @param string $key
  67          */
  68         protected function __construct( $conf, $type, $key ) {
  69                 $this->key = $key;
  70                 $this->workers = $conf['workers'];
  71                 $this->maxqueue = $conf['maxqueue'];
  72                 $this->timeout = $conf['timeout'];
  73         }
  74
  75         /**
  76          * Create a Pool counter. This should only be called from the PoolWorks.
  77          *
  78          * @param string $type
  79          * @param string $key
  80          *
  81          * @return PoolCounter
  82          */
  83         public static function factory( $type, $key ) {
  84                 global $wgPoolCounterConf;
  85                 if ( !isset( $wgPoolCounterConf[$type] ) ) {
  86                         return new PoolCounter_Stub;
  87                 }
  88                 $conf = $wgPoolCounterConf[$type];
  89                 $class = $conf['class'];
  90
  91                 return new $class( $conf, $type, $key );
  92         }
  93
  94         /**
  95          * @return string
  96          */
  97         public function getKey() {
  98                 return $this->key;
  99         }
 100
 101         /**
 102          * I want to do this task and I need to do it myself.
 103          *
 104          * @return Status Value is one of Locked/Error
 105          */
 106         abstract public function acquireForMe();
 107
 108         /**
 109          * I want to do this task, but if anyone else does it
 110          * instead, it's also fine for me. I will read its cached data.
 111          *
 112          * @return Status Value is one of Locked/Done/Error
 113          */
 114         abstract public function acquireForAnyone();
 115
 116         /**
 117          * I have successfully finished my task.
 118          * Lets another one grab the lock, and returns the workers
 119          * waiting on acquireForAnyone()
 120          *
 121          * @return Status value is one of Released/NotLocked/Error
 122          */
 123         abstract public function release();
 124 }
 125
 126 class PoolCounter_Stub extends PoolCounter {
 127         public function __construct() {
 128                 /* No parameters needed */
 129         }
 130
 131         public function acquireForMe() {
 132                 return Status::newGood( PoolCounter::LOCKED );
 133         }
 134
 135         public function acquireForAnyone() {
 136                 return Status::newGood( PoolCounter::LOCKED );
 137         }
 138
 139         public function release() {
 140                 return Status::newGood( PoolCounter::RELEASED );
 141         }
 142 }