PHP 一致性Hash

一致性HASH

好久没有写文章了，最近忙着公司的事情，也一拖再拖。这篇一致性hash是很久之前就有的一篇算法，记录一下，这周写个基于该算法的Redis中间件。



这个算法有性能问题，因为PHP是解释性语言，每次查找一个key都初始化这个环，最好的办法的把这个方法用在daemon进程里，使之缓存起来，就不必每次都执行一次

<?php
namespace WMRedis;

use RedisException;

/**
* 一致性hash
*
* 网上摘抄的一段代码
*/

class ConsistenHash
{
/**
* 虚拟节点数
*
* @var int
*/
private $_replicas = 64;

/**
* HASH算法对象
* @var object hasher
*/
private $_hasher;

/**
* 当前物理节点数
* @var int
*/
private $_targetCount = 0;

/**
* 虚拟节点到物理节点映射
* @var array { position => target, ... }
*/
private $_positionToTarget = array();

/**
* 物理节点到虚拟节点映射
* @var array { target => [ position, position, ... ], ... }
*/

private $_targetToPositions = array();

/**
* 虚拟节点排序标志位
* @var boolean
*/
private $_positionToTargetSorted = false;

/**
* 构造函数
* @param object $hasher hasher
* @param int $replicas Amount of positions to hash each target to.
*/
public function __construct($hash = 'md5',$replicas = 0)
{
$this->_hasher = strcmp(strtolower($hash),'crc32') === 0 ? new Crc32Hasher() : new Md5Hasher();
$this->_replicas = !$replicas ? $replicas : $this->_replicas;
}

/**
* 添加物理节点
* @param string $target
* @chainable
*/
public function addTarget($target)
{
if (isset($this->_targetToPositions[$target]))
{
throw new RedisException("Target '$target' already exists.");
}
$this->_targetToPositions[$target] = array();
// 打散
for ($i = 0; $i < $this->_replicas; $i++)
{
$position = $this->_hasher->hash($target . $i);

$this->_positionToTarget[$position] = $target;
$this->_targetToPositions[$target][]= $position;
}
$this->_positionToTargetSorted = false;
$this->_targetCount++;
return $this;
}

/**
* 批量添加节点
* @param array $targets
* @chainable
*/
public function addTargets($targets)
{
foreach ($targets as $target)
{
$this->addTarget($target);
}
return $this;
}

/**
* 删除节点
* @param string $target
* @chainable
*/
public function removeTarget($target)
{
if (!isset($this->_targetToPositions[$target]))
{
throw new RedisException("Target '$target' does not exist.");
}
foreach ($this->_targetToPositions[$target] as $position)
{
unset($this->_positionToTarget[$position]);
}
unset($this->_targetToPositions[$target]);
$this->_targetCount--;
return $this;
}

/**
* 返回若有物理节点
* @return array
*/
public function getAllTargets()
{
return array_keys($this->_targetToPositions);
}

/**
* 找到资源所在物理节点
* @param string $resource
* @return string
*/
public function lookup($resource)
{
$targets = $this->lookupList($resource, 1);
if (empty($targets)) throw new RedisException('No targets exist');
return $targets[0];
}
/**
* 需要多个物理节点
*
* @param string $resource
* @param int $requestedCount 节点个数
* @return array
*/
protected function lookupList($resource, $requestedCount)
{
if (!$requestedCount) {
throw new RedisException('Invalid count requested');
}
// 没有节点资源
if (empty($this->_positionToTarget)) {
return array();
}

if ($this->_targetCount == 1) {
return array(array_pop($this->_positionToTarget));
}

$resourcePosition = $this->_hasher->hash($resource);
$results = array();
$collect = false;
$this->_sortPositionTargets();
// 开始搜索
foreach ($this->_positionToTarget as $key => $value)
{
if (!$collect && (string)$key > (string)$resourcePosition)
{
// 找到第一个匹配节点
$collect = true;
}
if ($collect && !in_array($value, $results))
{
$results []= $value;
}
if (count($results) == $requestedCount || count($results) == $this->_targetCount)
{
return $results;
}
}
// 还没有找到足够的节点，则重新开始
foreach ($this->_positionToTarget as $key => $value)
{
if (!in_array($value, $results))
{
$results []= $value;
}
// 已找到足够节点，或所有节点已全部遍历
if (count($results) == $requestedCount || count($results) == $this->_targetCount)
{
return $results;
}
}
// 此时的结果是已遍历完仍然没有足够的节点数
return $results;
}

/**
* 降序排列虚拟节点
*/
private function _sortPositionTargets()
{
// sort by key (position) if not already
if (!$this->_positionToTargetSorted)
{
ksort($this->_positionToTarget, SORT_REGULAR);
$this->_positionToTargetSorted = true;
}
}
}

/**
* Crc32
*/
class Crc32Hasher implements hasher
{
public function hash($string)
{
return (string)hash('crc32', $string);
}
}
/**
* Md5
*/
class Md5Hasher implements hasher
{
public function hash($string)
{
return (string)substr(md5($string), 0, 8);
}
}

/**
* HASH因子接口
*/
interface hasher
{
public function hash($string);
}