mirror of
https://github.com/moodle/moodle.git
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161 lines
4.9 KiB
PHP
161 lines
4.9 KiB
PHP
<?php
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// This file is part of Moodle - http://moodle.org/
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//
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// Moodle is free software: you can redistribute it and/or modify
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// it under the terms of the GNU General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// Moodle is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU General Public License for more details.
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//
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// You should have received a copy of the GNU General Public License
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// along with Moodle. If not, see <http://www.gnu.org/licenses/>.
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/**
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* @package core
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* @subpackage lib
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* @copyright Martin Langhoff <martin@catalyst.net.nz>
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* @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
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*/
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defined('MOODLE_INTERNAL') || die();
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/**
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* This class abstracts PHP's PECL memcached
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* API to provide
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*
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* - get()
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* - set()
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* - delete()
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* - getforfill()
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* - releaseforfill()
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*
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* Author: Martin Langhoff <martin@catalyst.net.nz>
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*
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* Note: do NOT store booleans here. With memcached, a false value
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* is indistinguisable from a "not found in cache" response.
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*
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* @package moodlecore
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* @copyright Martin Langhoff <martin@catalyst.net.nz>
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* @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
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**/
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class memcached {
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function memcached() {
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global $CFG;
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if (!function_exists('memcache_connect')) {
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debugging("Memcached is set to true but the memcached extension is not installed");
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return false;
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}
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$this->_cache = new Memcache;
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$hosts = explode(',', $CFG->memcachedhosts);
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if (count($hosts) === 1 && !empty($CFG->memcachedpconn)) {
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// the faster pconnect is only available
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// for single-server setups
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// NOTE: PHP-PECL client is buggy and pconnect()
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// will segfault if the server is unavailable
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$this->_cache->pconnect($hosts[0]);
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} else {
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// multi-host setup will share key space
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foreach ($hosts as $host) {
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$host = trim($host);
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$this->_cache->addServer($host);
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}
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}
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$this->prefix = $CFG->dbname .'|' . $CFG->prefix . '|';
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}
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function status() {
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if (is_object($this->_cache)) {
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return true;
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}
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return false;
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}
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function set($key, $value, $ttl=0) {
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// we may have acquired a lock via getforfill
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// release if it exists
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@$this->_cache->delete($this->prefix . $key . '_forfill');
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return $this->_cache->set($this->prefix . $key, $value, false);
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}
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function get($key) {
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$rec = $this->_cache->get($this->prefix . $key);
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return $rec;
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}
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function delete($key) {
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return $this->_cache->delete($this->prefix . $key);
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}
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/**
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* In the simple case, this function will
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* get the cached value if available. If the entry
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* is not cached, it will try to get an exclusive
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* lock that announces that this process will
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* populate the cache.
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*
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* If we fail to get the lock -- this means another
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* process is doing it.
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* so we wait (block) for a few microseconds while we wait for
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* the cache to be filled or the lock to timeout.
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*
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* If you get a false from this call, you _must_
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* populate the cache ASAP or indicate that
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* you won't by calling releaseforfill().
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*
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* This technique forces serialisation and so helps deal
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* with thundering herd scenarios where a lot of clients
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* ask the for the same idempotent (and costly) operation.
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* The implementation is based on suggestions in this message
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* http://marc.theaimsgroup.com/?l=git&m=116562052506776&w=2
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*
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* @param $key string
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* @return mixed on cache hit, NULL otherwise
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*/
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function getforfill ($key) {
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$rec = $this->_cache->get($this->prefix . $key);
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if ($rec) {
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return $rec;
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}
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if ($this->_cache->add($this->prefix . $key . '_forfill', 'true', false, 1)) {
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// we obtained the _forfill lock
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// our caller will compute and set the value
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return false;
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}
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// someone else has the lock
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// "block" till we can get the value
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// actually, loop .05s waiting for it
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for ($n=0;$n<5;$n++) {
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usleep(10000);
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$rec = $this->_cache->get($this->prefix . $key);
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if ($rec) {
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return $rec;
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}
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}
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return false;
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}
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/**
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* Release the exclusive lock obtained by
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* getforfill(). See getforfill()
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* for more details.
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*
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* @param $key string
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* @return bool
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*/
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function releaseforfill ($key) {
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return $this->_cache->delete($this->prefix . $key . '_forfill');
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}
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}
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