/lib/Varien/Cache/Backend/Memcached.php

<?php
/**
 * Magento
 *
 * NOTICE OF LICENSE
 *
 * This source file is subject to the Open Software License (OSL 3.0)
 * that is bundled with this package in the file LICENSE.txt.
 * It is also available through the world-wide-web at this URL:
 * http://opensource.org/licenses/osl-3.0.php
 * If you did not receive a copy of the license and are unable to
 * obtain it through the world-wide-web, please send an email
 * to license@magentocommerce.com so we can send you a copy immediately.
 *
 * DISCLAIMER
 *
 * Do not edit or add to this file if you wish to upgrade Magento to newer
 * versions in the future. If you wish to customize Magento for your
 * needs please refer to http://www.magentocommerce.com for more information.
 *
 * @category    Varien
 * @package     Varien_Cache
 * @copyright   Copyright (c) 2011 Magento Inc. (http://www.magentocommerce.com)
 * @license     http://opensource.org/licenses/osl-3.0.php  Open Software License (OSL 3.0)
 */

class Varien_Cache_Backend_Memcached 
    extends Zend_Cache_Backend_Memcached
    implements Zend_Cache_Backend_ExtendedInterface
{
    /**
     * Maximum chunk of data that could be saved in one memcache cell (1 MiB)
     */
    const DEFAULT_SLAB_SIZE = 1048576;

    /**
     * Used to tell chunked data from ordinary
     */
    const CODE_WORD = '{splitted}';

    /**
     * Constructor
     *
     * @throws Varien_Exception
     * @param array $options @see Zend_Cache_Backend_Memcached::__construct()
     */
    public function __construct(array $options = array())
    {
        parent::__construct($options);

        if (!isset($options['slab_size']) || !is_numeric($options['slab_size'])) {
            if (isset($options['slab_size'])) {
                throw new Varien_Exception("Invalid value for the node <slab_size>. Expected to be positive integer.");
            }

            $this->_options['slab_size'] = self::DEFAULT_SLAB_SIZE;
        } else {
            $this->_options['slab_size'] = $options['slab_size'];
        }
    }

    /**
     * Returns ID of a specific chunk on the basis of data's ID
     *
     * @param string $id    Main data's ID
     * @param int    $index Particular chunk number to return ID for
     * @return string
     */
    protected function _getChunkId($id, $index)
    {
        return "{$id}[{$index}]";
    }

    /**
     * Remove saved chunks in case something gone wrong (e.g. some chunk from the chain can not be found)
     *
     * @param string $id     ID of data's info cell
     * @param int    $chunks Number of chunks to remove (basically, the number after '{splitted}|')
     * @return null
     */
    protected function _cleanTheMess($id, $chunks)
    {
        for ($i = 0; $i < $chunks; $i++) {
            $this->remove($this->_getChunkId($id, $i));
        }

        $this->remove($id);
    }

    /**
     * Save data to memcached, split it into chunks if data size is bigger than memcached slab size.
     *
     * @param string $data             @see Zend_Cache_Backend_Memcached::save()
     * @param string $id               @see Zend_Cache_Backend_Memcached::save()
     * @param array  $tags             @see Zend_Cache_Backend_Memcached::save()
     * @param bool   $specificLifetime @see Zend_Cache_Backend_Memcached::save()
     * @return bool
     */
    public function save($data, $id, $tags = array(), $specificLifetime = false)
    {
        if (is_string($data) && (strlen($data) > $this->_options['slab_size'])) {
            $dataChunks = str_split($data, $this->_options['slab_size']);

            for ($i = 0, $cnt = count($dataChunks); $i < $cnt; $i++) {
                $chunkId = $this->_getChunkId($id, $i);

                if (!parent::save($dataChunks[$i], $chunkId, $tags, $specificLifetime)) {
                    $this->_cleanTheMess($id, $i + 1);
                    return false;
                }
            }

            $data = self::CODE_WORD . '|' . $i;
        }

        return parent::save($data, $id, $tags, $specificLifetime);
    }

    /**
     * Load data from memcached, glue from several chunks if it was splitted upon save.
     *
     * @param string $id                     @see Zend_Cache_Backend_Memcached::load()
     * @param bool   $doNotTestCacheValidity @see Zend_Cache_Backend_Memcached::load()
     * @return bool|false|string
     */
    public function load($id, $doNotTestCacheValidity = false)
    {
        $data = parent::load($id, $doNotTestCacheValidity);

        if (is_string($data) && (substr($data, 0, strlen(self::CODE_WORD)) == self::CODE_WORD)) {
            // Seems we've got chunked data

            $arr = explode('|', $data);
            $chunks = isset($arr[1]) ? $arr[1] : false;
            $chunkData = array();

            if ($chunks && is_numeric($chunks)) {
                for ($i = 0; $i < $chunks; $i++) {
                    $chunk = parent::load($this->_getChunkId($id, $i), $doNotTestCacheValidity);

                    if (false === $chunk) {
                        // Some chunk in chain was not found, we can not glue-up the data:
                        // clean the mess and return nothing

                        $this->_cleanTheMess($id, $chunks);
                        return false;
                    }

                    $chunkData[] = $chunk;
                }

                return implode('', $chunkData);
            }
        }

        // Data has not been splitted to chunks on save
        return $data;
    }
}