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
  14 abstract class PoolCounter {
  15
  16         /* Return codes */
  17         const LOCKED = 1;       /* Lock acquired */
  18         const RELEASED = 2; /* Lock released */
  19         const DONE = 3;         /* Another one did the work for you */
  20
  21         const ERROR = -1;               /* Indeterminate error */
  22         const NOT_LOCKED = -2;  /* Called release() with no lock held */
  23         const QUEUE_FULL = -3;  /* There are already maxqueue workers on this lock */
  24         const TIMEOUT = -4;             /* Timeout exceeded */
  25         const LOCK_HELD = -5;   /* Cannot acquire another lock while you have one lock held */
  26
  27         /**
  28          * I want to do this task and I need to do it myself.
  29          *
  30          * @return Locked/Error
  31          */
  32         abstract function acquireForMe();
  33
  34         /**
  35          * I want to do this task, but if anyone else does it
  36          * instead, it's also fine for me. I will read its cached data.
  37          *
  38          * @return Locked/Done/Error
  39          */
  40         abstract function acquireForAnyone();
  41
  42         /**
  43          * I have successfully finished my task.
  44          * Lets another one grab the lock, and returns the workers
  45          * waiting on acquireForAnyone()
  46          *
  47          * @return Released/NotLocked/Error
  48          */
  49         abstract function release();
  50
  51         /**
  52          *  $key: All workers with the same key share the lock.
  53          *  $workers: It wouldn't be a good idea to have more than this number of
  54          * workers doing the task simultaneously.
  55          *  $maxqueue: If this number of workers are already working/waiting,
  56          * fail instead of wait.
  57          *  $timeout: Maximum time in seconds to wait for the lock.
  58          */
  59         protected $key, $workers, $maxqueue, $timeout;
  60
  61         /**
  62          * Create a Pool counter. This should only be called from the PoolWorks.
  63          */
  64         public static function factory( $type, $key ) {
  65                 global $wgPoolCounterConf;
  66                 if ( !isset( $wgPoolCounterConf[$type] ) ) {
  67                         return new PoolCounter_Stub;
  68                 }
  69                 $conf = $wgPoolCounterConf[$type];
  70                 $class = $conf['class'];
  71
  72                 return new $class( $conf, $type, $key );
  73         }
  74
  75         protected function __construct( $conf, $type, $key ) {
  76                 $this->key = $key;
  77                 $this->workers  = $conf['workers'];
  78                 $this->maxqueue = $conf['maxqueue'];
  79                 $this->timeout  = $conf['timeout'];
  80         }
  81 }
  82
  83 class PoolCounter_Stub extends PoolCounter {
  84         function acquireForMe() {
  85                 return PoolCounter::LOCKED;
  86         }
  87
  88         function acquireForAnyone() {
  89                 return PoolCounter::LOCKED;
  90         }
  91
  92         function release() {
  93                 return PoolCounter::RELEASED;
  94         }
  95
  96         public function __construct() {
  97                 /* No parameters needed */
  98         }
  99 }
 100
 101 /**
 102  * Handy class for dealing with PoolCounters using class members instead of callbacks.
 103  */
 104 abstract class PoolCounterWork {
 105         protected $cacheable = false; //Does this override getCachedWork() ?
 106
 107         /**
 108          * Actually perform the work, caching it if needed.
 109          */
 110         abstract function doWork();
 111
 112         /**
 113          * Retrieve the work from cache
 114          * @return mixed work result or false
 115          */
 116         function getCachedWork() {
 117                 return false;
 118         }
 119
 120         /**
 121          * A work not so good (eg. expired one) but better than an error
 122          * message.
 123          * @return mixed work result or false
 124          */
 125         function fallback() {
 126                 return false;
 127         }
 128
 129         /**
 130          * Do something with the error, like showing it to the user.
 131          */
 132         function error( $status ) {
 133                 return false;
 134         }
 135
 136         /**
 137          * Get the result of the work (whatever it is), or false.
 138          */
 139         function execute( $skipcache = false ) {
 140                 if ( $this->cacheable && !$skipcache ) {
 141                         $status = $this->poolCounter->acquireForAnyone();
 142                 } else {
 143                         $status = $this->poolCounter->acquireForMe();
 144                 }
 145
 146                 $result = false;
 147                 switch ( is_int( $status ) ? $status : PoolCounter::ERROR ) {
 148                         case PoolCounter::LOCKED:
 149                                 $result = $this->doWork();
 150                                 $this->poolCounter->release();
 151                                 return $result;
 152
 153                         case PoolCounter::DONE:
 154                                 $result = $this->getCachedWork();
 155                                 if ( $result === false ) {
 156                                         /* That someone else work didn't serve us.
 157                                          * Acquire the lock for me
 158                                          */
 159                                         return $this->execute( true );
 160                                 }
 161                                 return $result;
 162
 163                         case PoolCounter::QUEUE_FULL:
 164                         case PoolCounter::TIMEOUT:
 165                                 $result = $this->fallback();
 166
 167                                 if ( $result !== false ) {
 168                                         return $result;
 169                                 }
 170                                 /* no break */
 171
 172                         case PoolCounter::ERROR:
 173                         default:
 174                                 return $this->error( $status );
 175                 }
 176         }
 177
 178         function __construct( $type, $key ) {
 179                 $this->poolCounter = PoolCounter::factory( $type, $key );
 180         }
 181 }