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 working on tasks with the same key simultaneously */
  57         protected $workers;
  58         /**
  59          * Maximum number of workers working on this task type, regardless of key.
  60          * 0 means unlimited. Max allowed value is 65536.
  61          * The way the slot limit is enforced is overzealous - this option should be used with caution.
  62          * @var int
  63          */
  64         protected $slots = 0;
  65         /** @var int If this number of workers are already working/waiting, fail instead of wait */
  66         protected $maxqueue;
  67         /** @var float Maximum time in seconds to wait for the lock */
  68         protected $timeout;
  69
  70         /**
  71          * @param array $conf
  72          * @param string $type
  73          * @param string $key
  74          */
  75         protected function __construct( $conf, $type, $key ) {
  76                 $this->workers = $conf['workers'];
  77                 $this->maxqueue = $conf['maxqueue'];
  78                 $this->timeout = $conf['timeout'];
  79                 if ( isset( $conf['slots'] ) ) {
  80                         $this->slots = $conf['slots'];
  81                 }
  82
  83                 if ( $this->slots ) {
  84                         $key = $this->hashKeyIntoSlots( $key, $this->slots );
  85                 }
  86                 $this->key = $key;
  87         }
  88
  89         /**
  90          * Create a Pool counter. This should only be called from the PoolWorks.
  91          *
  92          * @param string $type
  93          * @param string $key
  94          *
  95          * @return PoolCounter
  96          */
  97         public static function factory( $type, $key ) {
  98                 global $wgPoolCounterConf;
  99                 if ( !isset( $wgPoolCounterConf[$type] ) ) {
 100                         return new PoolCounter_Stub;
 101                 }
 102                 $conf = $wgPoolCounterConf[$type];
 103                 $class = $conf['class'];
 104
 105                 return new $class( $conf, $type, $key );
 106         }
 107
 108         /**
 109          * @return string
 110          */
 111         public function getKey() {
 112                 return $this->key;
 113         }
 114
 115         /**
 116          * I want to do this task and I need to do it myself.
 117          *
 118          * @return Status Value is one of Locked/Error
 119          */
 120         abstract public function acquireForMe();
 121
 122         /**
 123          * I want to do this task, but if anyone else does it
 124          * instead, it's also fine for me. I will read its cached data.
 125          *
 126          * @return Status Value is one of Locked/Done/Error
 127          */
 128         abstract public function acquireForAnyone();
 129
 130         /**
 131          * I have successfully finished my task.
 132          * Lets another one grab the lock, and returns the workers
 133          * waiting on acquireForAnyone()
 134          *
 135          * @return Status Value is one of Released/NotLocked/Error
 136          */
 137         abstract public function release();
 138
 139         /**
 140          * Given a key (any string) and the number of lots, returns a slot number (an integer from the [0..($slots-1)] range).
 141          * This is used for a global limit on the number of instances  of a given type that can acquire a lock.
 142          * The hashing is deterministic so that PoolCounter::$workers is always an upper limit of how many instances with
 143          * the same key can acquire a lock.
 144          *
 145          * @param string $key PoolCounter instance key (any string)
 146          * @param int $slots The number of slots (max allowed value is 65536)
 147          * @return int
 148          */
 149         protected function hashKeyIntoSlots( $key, $slots ) {
 150                 return hexdec( substr( sha1( $key ), 0, 4 ) ) % $slots;
 151         }
 152 }
 153
 154 // @codingStandardsIgnoreStart Squiz.Classes.ValidClassName.NotCamelCaps
 155 class PoolCounter_Stub extends PoolCounter {
 156         // @codingStandardsIgnoreEnd
 157
 158         public function __construct() {
 159                 /* No parameters needed */
 160         }
 161
 162         public function acquireForMe() {
 163                 return Status::newGood( PoolCounter::LOCKED );
 164         }
 165
 166         public function acquireForAnyone() {
 167                 return Status::newGood( PoolCounter::LOCKED );
 168         }
 169
 170         public function release() {
 171                 return Status::newGood( PoolCounter::RELEASED );
 172         }
 173 }