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          * Get the result of the work (whatever it is), or false.
 140          */
 141         function execute( $skipcache = false ) {
 142                 if ( $this->cacheable && !$skipcache ) {
 143                         $status = $this->poolCounter->acquireForAnyone();
 144                 } else {
 145                         $status = $this->poolCounter->acquireForMe();
 146                 }
 147
 148                 if ( $status->isOK() ) {
 149                         switch ( $status->value ) {
 150                                 case PoolCounter::LOCKED:
 151                                         $result = $this->doWork();
 152                                         $this->poolCounter->release();
 153                                         return $result;
 154
 155                                 case PoolCounter::DONE:
 156                                         $result = $this->getCachedWork();
 157                                         if ( $result === false ) {
 158                                                 /* That someone else work didn't serve us.
 159                                                  * Acquire the lock for me
 160                                                  */
 161                                                 return $this->execute( true );
 162                                         }
 163                                         return $result;
 164
 165                                 case PoolCounter::QUEUE_FULL:
 166                                 case PoolCounter::TIMEOUT:
 167                                         $result = $this->fallback();
 168
 169                                         if ( $result !== false ) {
 170                                                 return $result;
 171                                         }
 172                                         /* no break */
 173
 174                                 /* These two cases should never be hit... */
 175                                 case PoolCounter::ERROR:
 176                                 default:
 177                                         $errors = array( PoolCounter::QUEUE_FULL => 'pool-queuefull', PoolCounter::TIMEOUT => 'pool-timeout' );
 178
 179                                         $status = Status::newFatal( isset($errors[$status->value]) ? $errors[$status->value] : 'pool-errorunknown' );
 180                                         /* continue to the error */
 181                         }
 182                 }
 183                 return $this->error( $status );
 184         }
 185
 186         function __construct( $type, $key ) {
 187                 $this->poolCounter = PoolCounter::factory( $type, $key );
 188         }
 189 }