lib/memcached.class.php

   1 <?php
   2 /**
   3  ** This class abstracts PHP's PECL memcached
   4  ** API to provide
   5  **
   6  ** - get()
   7  ** - set()
   8  ** - delete()
   9  ** - getforfill()
  10  ** - releaseforfill()
  11  **
  12  ** Author: Martin Langhoff <martin@catalyst.net.nz>
  13  **
  14  ** Note: do NOT store booleans here. With memcached, a false value
  15  ** is indistinguisable from a "not found in cache" response.
  16  **/
  17
  18
  19 class memcached {
  20
  21     function memcached() {
  22         global $CFG;
  23
  24         if (!function_exists('memcache_connect')) {
  25             debugging("Memcached is set to true but the memcached extension is not installed");
  26             return false;
  27         }
  28         $this->_cache = new Memcache;
  29
  30         $hosts = split(',', $CFG->memcachedhosts);
  31         if (count($hosts) === 1 && !empty($CFG->memcachedpconn)) {
  32             // the faster pconnect is only available
  33             // for single-server setups
  34             // NOTE: PHP-PECL client is buggy and pconnect()
  35             // will segfault if the server is unavailable
  36             $this->_cache->pconnect($hosts[0]);
  37         } else {
  38             // multi-host setup will share key space
  39             foreach ($hosts as $host) {
  40                 $host = trim($host);
  41                 $this->_cache->addServer($host);
  42             }
  43         }
  44
  45         $this->prefix = $CFG->dbname .'|' . $CFG->prefix . '|';
  46     }
  47
  48     function status() {
  49         if (is_object($this->_cache)) {
  50             return true;
  51         }
  52         return false;
  53     }
  54
  55     function set($key, $value, $ttl=0) {
  56
  57         // we may have acquired a lock via getforfill
  58         // release if it exists
  59         @$this->_cache->delete($this->prefix . $key . '_forfill');
  60
  61         return $this->_cache->set($this->prefix . $key, $value, false);
  62     }
  63
  64     function get($key) {
  65         $rec = $this->_cache->get($this->prefix . $key);
  66         return $rec;
  67     }
  68
  69     function delete($key) {
  70         return $this->_cache->delete($this->prefix . $key);
  71     }
  72
  73     /**
  74      * In the simple case, this function will
  75      * get the cached value if available. If the entry
  76      * is not cached, it will try to get an exclusive
  77      * lock that announces that this process will
  78      * populate the cache.
  79      *
  80      * If we fail to get the lock -- this means another
  81      * process is doing it.
  82      * so we wait (block) for a few microseconds while we wait for
  83      * the cache to be filled or the lock to timeout.
  84      *
  85      * If you get a false from this call, you _must_
  86      * populate the cache ASAP or indicate that
  87      * you won't by calling releaseforfill().
  88      *
  89      * This technique forces serialisation and so helps deal
  90      * with thundering herd scenarios where a lot of clients
  91      * ask the for the same idempotent (and costly) operation.
  92      * The implementation is based on suggestions in this message
  93      * http://marc.theaimsgroup.com/?l=git&m=116562052506776&w=2
  94      *
  95      * @param $key string
  96      * @return mixed on cache hit, NULL otherwise
  97      */
  98     function getforfill ($key) {
  99
 100         $rec = $this->_cache->get($this->prefix . $key);
 101         if ($rec) {
 102             return $rec;
 103         }
 104         if ($this->_cache->add($this->prefix . $key . '_forfill', 'true', false, 1)) {
 105             // we obtained the _forfill lock
 106             // our caller will compute and set the value
 107             return false;
 108         }
 109         // someone else has the lock
 110         // "block" till we can get the value
 111         // actually, loop .05s waiting for it
 112         for ($n=0;$n<5;$n++) {
 113             usleep(10000);
 114             $rec = $this->_cache->get($this->prefix . $key);
 115             if ($rec) {
 116                 return $rec;
 117             }
 118         }
 119         return false;
 120     }
 121
 122     /**
 123      * Release the exclusive lock obtained by
 124      * getforfill(). See getforfill()
 125      * for more details.
 126      *
 127      * @param $key string
 128      * @return bool
 129      */
 130     function releaseforfill ($key) {
 131         return $this->_cache->delete($this->prefix . $key . '_forfill');
 132     }
 133 }
 134
 135 ?>