memcache的一致性hash算法

<?php
/**
 * 一致性哈希memcache分布式，采用的是虚拟节点的方式解决分布均匀性问题,查找节点采用二分法快速查找
 * the last known user to change this file in the repository  <$LastChangedBy: nash.xiong $>
 * @author nash.xiong <nash.xiong@gmail.com>
 * @copyright Copyright &copy; 2003-2012 phpd.cn
 * @license 
 */

 class memcacheHashMap {

     private $_node = array();
     private $_nodeData = array();
     private $_keyNode = 0;
     private $_memcache = null;

     //每个物理服务器生成虚拟节点个数 [注：节点数越多，cache分布的均匀性越好，同时set get操作时，也更耗资源，10台物理服务器，采用200较为合理]
    private $_virtualNodeNum = 200; 

    private function __construct() {
        /* 放入配置文件 */
        $config = array(
                ‘127.0.0.1:11211‘,
                ‘127.0.0.1:11212‘,
                ‘127.0.0.1:11213‘,
                ‘127.0.0.1:11214‘,
                ‘127.0.0.1:11215‘,
            );

        if(!$config) throw new Exception("Cache config Null");
        foreach ($config as $key => $value) {
            for ($i=0; $i < $this->_virtualNodeNum; $i++) { 
                $this->_node[sprintf("%u",crc32($value . "_" .$i))] = $value . ‘_‘ . $i;
            }
        }
        ksort($this->_node);
    }

    private function __clone() {}

    /**
     * 单例，保证只有一个实例
     */
    static public function getInstance() {
        static $memcacheObj = null;
        if(!is_object($memcacheObj)) {
            $memcacheObj = new self();
        }
        return $memcacheObj;
    }

    /**
     * 根据key做一致性hash后连接到一台物理memcache服务器
     * @param string $key
     */
    private function _connectMemcache($key) {
        $this->_nodeData = array_keys($this->_node);
        $this->_keyNode = sprintf("%u",crc32($key));
        $nodeKey = $this->_findServerNode();
        //如果超出环，从头再用二分法查找一个最近的，然后环的头尾做判断，取最接近的节点
        if($this->_keyNode > end($this->_nodeData)) {
            $this->_keyNode -= end($this->_nodeData);
            $nodekey2 = $this->_findServerNode();
            if(abs($nodekey2 - $this->_keyNode) < abs($nodeKey - $this->_keyNode)) 
                $nodeKey = $nodeKey2;
        }
        var_dump($this->_node[$nodekey]);
        list($config,$num) = explode(‘_‘, $this->_node[$nodeKey]);

        if(!$config) throw new Exception("Cache config Error");

        if(!isset($this->_memcache[$config])) {
            $this->_memcache[$config] = new Memcache;
            list($host,$port) = explode(‘:‘, $config);
            $this->_memcache[$config]->connect($host,$port);
        }

        return $this->_memcache;
    }
    /**
     * 采用二分法从虚拟memcache节点中查找最近的节点
     * @param unknown_type $m
     * @param unknown_type $b
     */
    private function _findServerNode($m = 0,$b = 0) {
        $totle = count($this->_nodeData);
        if($totle != 0 && $b == 0)    $b = $totle - 1;
        if($m < $b) {
            $avg = intval(($m+$b) / 2);
            if($this->_nodeData[$avg] == $this->_keyNode)  return $this->_nodeData[$avg];
            elseif ($this->_keyNode < $this->_nodeData[$avg] && ($avg-1 >= 0)) return $this->_findServerNode($m, $avg-1);
            else return $this->_findServerNode($avg+1, $b);
        }
        if (abs($this->_nodeData[$b] - $this->_keyNode) < abs($this->_nodeData[$m] - $this->_keyNode))  return $this->_nodeData[$b];
        else return $this->_nodeData[$m];
    }

    public function set($key, $value, $expire = 0) {
        return $this->_connectMemcache($key)->set($key, json_encode($value), 0, $expire);
    }
     
    public function add($key, $value, $expire = 0) {
        return $this->_connectMemcache($key)->add($key, json_encode($value), 0, $expire);
    }
     
    public function get($key) {
        return json_decode($this->_connectMemcache($key)->get($key), true);
    }
     
    public function delete($key) {
        return $this->_connectMemcache($key)->delete($key);
    }
 }