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