lib/eaccelerator.class.php

   1 <?php
   2
   3 // This file is part of Moodle - http://moodle.org/
   4 //
   5 // Moodle is free software: you can redistribute it and/or modify
   6 // it under the terms of the GNU General Public License as published by
   7 // the Free Software Foundation, either version 3 of the License, or
   8 // (at your option) any later version.
   9 //
  10 // Moodle is distributed in the hope that it will be useful,
  11 // but WITHOUT ANY WARRANTY; without even the implied warranty of
  12 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13 // GNU General Public License for more details.
  14 //
  15 // You should have received a copy of the GNU General Public License
  16 // along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
  17
  18 /**
  19  * This class abstracts eaccelerator/turckmmcache
  20  * API to provide
  21  *
  22  * - get()
  23  * - set()
  24  * - delete()
  25  * - getforfill()
  26  * - releaseforfill()
  27  *
  28  * Note: do NOT store booleans here. For compatibility with
  29  * memcached, a false value is indistinguisable from a
  30  * "not found in cache" response.
  31  *
  32  * @package    core
  33  * @subpackage lib
  34  * @copyright  Martin Langhoff <martin@catalyst.net.nz>
  35  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  36  */
  37
  38 defined('MOODLE_INTERNAL') || die();
  39
  40 /**
  41  *
  42  * @copyright Martin Langhoff <martin@catalyst.net.nz>
  43  * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  44  * @package moodlecore
  45  */
  46 class eaccelerator {
  47
  48     /**
  49      * @todo Document this function
  50      *
  51      * @global object
  52      */
  53     function eaccelerator() {
  54         global $CFG;
  55         if ( function_exists('eaccelerator_get')) {
  56             $this->mode = 'eaccelerator';
  57         } elseif (function_exists('mmcache_get')) {
  58             $this->mode = 'mmcache';
  59         } else {
  60             debugging("\$CFG->eaccelerator is set to true but the required functions are not available. You need to have either eaccelerator or turckmmcache extensions installed, compiled with the shmem keys option enabled.");
  61         }
  62
  63         $this->prefix = $CFG->dbname .'|' . $CFG->prefix . '|';
  64     }
  65
  66     /**
  67      * The status of the eaccelerator, if it has been established
  68      * this will return true
  69      *
  70      * @return bool
  71      */
  72     function status() {
  73         if (isset($this->mode)) {
  74             return true;
  75         }
  76         return false;
  77     }
  78
  79     /**
  80      * @todo Document this function
  81      *
  82      * @param string $key
  83      * @param string $value
  84      * @param int $ttl
  85      * @return mixed
  86      */
  87     function set($key, $value, $ttl=0) {
  88         $set    = $this->mode . '_put';
  89         $unlock = $this->mode . '_unlock';
  90
  91         // we may have acquired a lock via getforfill
  92         // release if it exists
  93         @$unlock($this->prefix . $key . '_forfill');
  94
  95         return $set($this->prefix . $key, serialize($value), $ttl);
  96     }
  97
  98     /**
  99      * @todo Document this function
 100      *
 101      * @param string $key
 102      * @return string|bool String if success else false
 103      */
 104     function get($key) {
 105         $fn = $this->mode . '_get';
 106         $rec = $fn($this->prefix . $key);
 107         if (is_null($rec)) {
 108             return false;
 109         }
 110         return unserialize($rec);
 111     }
 112
 113     /**
 114      * @todo Document this function
 115      *
 116      * @param string $key
 117      * @return mixed
 118      */
 119     function delete($key) {
 120         $fn = $this->mode . '_rm';
 121         return $fn($this->prefix . $key);
 122     }
 123
 124     /**
 125      * In the simple case, this function will
 126      * get the cached value if available. If the entry
 127      * is not cached, it will try to get an exclusive
 128      * lock that announces that this process will
 129      * populate the cache.
 130      *
 131      * If we fail to get the lock -- this means another
 132      * process is doing it.
 133      * so we wait (block) for a few microseconds while we wait for
 134      * the cache to be filled or the lock to timeout.
 135      *
 136      * If you get a false from this call, you _must_
 137      * populate the cache ASAP or indicate that
 138      * you won't by calling releaseforfill().
 139      *
 140      * This technique forces serialisation and so helps deal
 141      * with thundering herd scenarios where a lot of clients
 142      * ask the for the same idempotent (and costly) operation.
 143      * The implementation is based on suggestions in this message
 144      * http://marc.theaimsgroup.com/?l=git&m=116562052506776&w=2
 145      *
 146      * @param $key string
 147      * @return mixed on cache hit, false otherwise
 148      */
 149     function getforfill ($key) {
 150         $get    = $this->mode . '_get';
 151         $lock   = $this->mode . '_lock';
 152
 153         $rec = $get($this->prefix . $key);
 154         if (!is_null($rec)) {
 155             return unserialize($rec);
 156         }
 157         if ($lock($this->prefix . $key . '_forfill')) {
 158             // we obtained the _forfill lock
 159             // our caller will compute and set the value
 160             return false;
 161         }
 162         // someone else has the lock
 163         // "block" till we can get the value
 164         // actually, loop .05s waiting for it
 165         for ($n=0;$n<5;$n++) {
 166             usleep(10000);
 167             $rec = $get($this->prefix . $key);
 168             if (!is_null($rec)) {
 169                 return unserialize($rec);
 170             }
 171         }
 172         return false;
 173     }
 174
 175     /**
 176      * Release the exclusive lock obtained by
 177      * getforfill(). See getforfill()
 178      * for more details.
 179      *
 180      * @param $key string
 181      * @return bool
 182      */
 183     function releaseforfill ($key) {
 184         $unlock = $this->mode . '_unlock';
 185         return $unlock($this->prefix . $key . '_forfill');
 186     }
 187 }
 188
 189 ?>