includes/PoolCounter.php

   1 <?php
   2
   3 /**
   4  *  When you have many workers (threads/servers) giving service, and a
   5  * cached item expensive to produce expires, you may get several workers
   6  * doing the job at the same time.
   7  *
   8  *  Given enough requests and the item expiring fast (non-cacheable,
   9  * lots of edits...) that single work can end up unfairly using most (all)
  10  * of the cpu of the pool. This is also known as 'Michael Jackson effect'
  11  * since this effect triggered on the english wikipedia on the day Michael
  12  * Jackson died, the biographical article got hit with several edits per
  13  * minutes and hundreds of read hits.
  14  *
  15  *  The PoolCounter provides semaphore semantics for restricting the number
  16  * of workers that may be concurrently performing such single task.
  17  *
  18  *  By default PoolCounter_Stub is used, which provides no locking. You
  19  * can get a useful one in the PoolCounter extension.
  20  */
  21 abstract class PoolCounter {
  22
  23         /* Return codes */
  24         const LOCKED   = 1; /* Lock acquired */
  25         const RELEASED = 2; /* Lock released */
  26         const DONE     = 3; /* Another worker did the work for you */
  27
  28         const ERROR      = -1; /* Indeterminate error */
  29         const NOT_LOCKED = -2; /* Called release() with no lock held */
  30         const QUEUE_FULL = -3; /* There are already maxqueue workers on this lock */
  31         const TIMEOUT    = -4; /* Timeout exceeded */
  32         const LOCK_HELD  = -5; /* Cannot acquire another lock while you have one lock held */
  33
  34         /**
  35          * I want to do this task and I need to do it myself.
  36          *
  37          * @return Locked/Error
  38          */
  39         abstract function acquireForMe();
  40
  41         /**
  42          * I want to do this task, but if anyone else does it
  43          * instead, it's also fine for me. I will read its cached data.
  44          *
  45          * @return Locked/Done/Error
  46          */
  47         abstract function acquireForAnyone();
  48
  49         /**
  50          * I have successfully finished my task.
  51          * Lets another one grab the lock, and returns the workers
  52          * waiting on acquireForAnyone()
  53          *
  54          * @return Released/NotLocked/Error
  55          */
  56         abstract function release();
  57
  58         /**
  59          *  $key: All workers with the same key share the lock.
  60          *  $workers: It wouldn't be a good idea to have more than this number of
  61          * workers doing the task simultaneously.
  62          *  $maxqueue: If this number of workers are already working/waiting,
  63          * fail instead of wait.
  64          *  $timeout: Maximum time in seconds to wait for the lock.
  65          */
  66         protected $key, $workers, $maxqueue, $timeout;
  67
  68         /**
  69          * Create a Pool counter. This should only be called from the PoolWorks.
  70          */
  71         public static function factory( $type, $key ) {
  72                 global $wgPoolCounterConf;
  73                 if ( !isset( $wgPoolCounterConf[$type] ) ) {
  74                         return new PoolCounter_Stub;
  75                 }
  76                 $conf = $wgPoolCounterConf[$type];
  77                 $class = $conf['class'];
  78
  79                 return new $class( $conf, $type, $key );
  80         }
  81
  82         protected function __construct( $conf, $type, $key ) {
  83                 $this->key = $key;
  84                 $this->workers  = $conf['workers'];
  85                 $this->maxqueue = $conf['maxqueue'];
  86                 $this->timeout  = $conf['timeout'];
  87         }
  88 }
  89
  90 class PoolCounter_Stub extends PoolCounter {
  91         function acquireForMe() {
  92                 return Status::newGood( PoolCounter::LOCKED );
  93         }
  94
  95         function acquireForAnyone() {
  96                 return Status::newGood( PoolCounter::LOCKED );
  97         }
  98
  99         function release() {
 100                 return Status::newGood( PoolCounter::RELEASED );
 101         }
 102
 103         public function __construct() {
 104                 /* No parameters needed */
 105         }
 106 }
 107
 108 /**
 109  * Handy class for dealing with PoolCounters using class members instead of callbacks.
 110  */
 111 abstract class PoolCounterWork {
 112         protected $cacheable = false; //Does this override getCachedWork() ?
 113
 114         /**
 115          * Actually perform the work, caching it if needed.
 116          */
 117         abstract function doWork();
 118
 119         /**
 120          * Retrieve the work from cache
 121          * @return mixed work result or false
 122          */
 123         function getCachedWork() {
 124                 return false;
 125         }
 126
 127         /**
 128          * A work not so good (eg. expired one) but better than an error
 129          * message.
 130          * @return mixed work result or false
 131          */
 132         function fallback() {
 133                 return false;
 134         }
 135
 136         /**
 137          * Do something with the error, like showing it to the user.
 138          */
 139         function error( $status ) {
 140                 return false;
 141         }
 142
 143         /**
 144          * Log an error
 145          */
 146         function logError( $status ) {
 147                 wfDebugLog( 'poolcounter', $status->getWikiText() );
 148         }
 149
 150         /**
 151          * Get the result of the work (whatever it is), or false.
 152          */
 153         function execute( $skipcache = false ) {
 154                 if ( $this->cacheable && !$skipcache ) {
 155                         $status = $this->poolCounter->acquireForAnyone();
 156                 } else {
 157                         $status = $this->poolCounter->acquireForMe();
 158                 }
 159
 160                 if ( !$status->isOK() ) {
 161                         // Respond gracefully to complete server breakage: just log it and do the work
 162                         $this->logError( $status );
 163                         return $this->doWork();
 164                 }
 165
 166                 switch ( $status->value ) {
 167                         case PoolCounter::LOCKED:
 168                                 $result = $this->doWork();
 169                                 $this->poolCounter->release();
 170                                 return $result;
 171
 172                         case PoolCounter::DONE:
 173                                 $result = $this->getCachedWork();
 174                                 if ( $result === false ) {
 175                                         /* That someone else work didn't serve us.
 176                                          * Acquire the lock for me
 177                                          */
 178                                         return $this->execute( true );
 179                                 }
 180                                 return $result;
 181
 182                         case PoolCounter::QUEUE_FULL:
 183                         case PoolCounter::TIMEOUT:
 184                                 $result = $this->fallback();
 185
 186                                 if ( $result !== false ) {
 187                                         return $result;
 188                                 }
 189                                 /* no break */
 190
 191                         /* These two cases should never be hit... */
 192                         case PoolCounter::ERROR:
 193                         default:
 194                                 $errors = array( PoolCounter::QUEUE_FULL => 'pool-queuefull', PoolCounter::TIMEOUT => 'pool-timeout' );
 195
 196                                 $status = Status::newFatal( isset($errors[$status->value]) ? $errors[$status->value] : 'pool-errorunknown' );
 197                                 $this->logError( $status );
 198                                 return $this->error( $status );
 199                 }
 200         }
 201
 202         function __construct( $type, $key ) {
 203                 $this->poolCounter = PoolCounter::factory( $type, $key );
 204         }
 205 }