Consistent Hashing

一致性哈希算法实现

老外写的，很不错！

<?php

/**
* Flexihash - A simple consistent hashing implementation for PHP.
*
* The MIT License
*
* Copyright (c) 2008 Paul Annesley
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to deal
* in the Software without restriction, including without limitation the rights
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
* copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
* THE SOFTWARE.
*
* @author Paul Annesley
* @link http://paul.annesley.cc/
* @copyright Paul Annesley, 2008
* @comment by MyZ (http://blog.csdn.net/mayongzhan/)
*/

/**
* A simple consistent hashing implementation with pluggable hash algorithms.
*
* @author Paul Annesley
* @package Flexihash
* @licence http://www.opensource.org/licenses/mit-license.php
*/

class Flexihash {

/**
* The number of positions to hash each target to.
*
* @var int
* @comment 虚拟节点数,解决节点分布不均的问题
*/
private $_replicas = 64;

/**
* The hash algorithm, encapsulated in a Flexihash_Hasher implementation.
* @var object Flexihash_Hasher
* @comment 使用的hash方法 : md5,crc32
*/
private $_hasher;

/**
* Internal counter for current number of targets.
* @var int
* @comment 节点记数器
*/
private $_targetCount = 0;

/**
* Internal map of positions (hash outputs) to targets
* @var array { position => target, ... }
* @comment 位置对应节点,用于lookup中根据位置确定要访问的节点
*/
private $_positionToTarget = array();

/**
* Internal map of targets to lists of positions that target is hashed to.
* @var array { target => [ position, position, ... ], ... }
* @comment 节点对应位置,用于删除节点
*/
private $_targetToPositions = array();

/**
* Whether the internal map of positions to targets is already sorted.
* @var boolean
* @comment 是否已排序
*/
private $_positionToTargetSorted = false;

/**
* Constructor
* @param object $hasher Flexihash_Hasher
* @param int $replicas Amount of positions to hash each target to.
* @comment 构造函数,确定要使用的hash方法和需拟节点数,虚拟节点数越多,分布越均匀,但程序的分布式运算越慢
*/
public function __construct(Flexihash_Hasher $hasher = null, $replicas = null) {
    $this->_hasher = $hasher ? $hasher : new Flexihash_Crc32Hasher();
    if (!empty($replicas)) {
        $this->_replicas = $replicas;
    }
}

/**
* Add a target.
* @param string $target
* @chainable
* @comment 添加节点,根据虚拟节点数,将节点分布到多个虚拟位置上
*/
public function addTarget($target) {
    if (isset($this->_targetToPositions[$target])) {
        throw new Flexihash_Exception("Target '$target' already exists.");
    }

    $this->_targetToPositions[$target] = array();

    // hash the target into multiple positions
    for ($i = 0; $i < $this->_replicas; $i++) {
        $position = $this->_hasher->hash($target . $i);
        $this->_positionToTarget[$position] = $target; // lookup
        $this->_targetToPositions[$target] []= $position; // target removal
    }

    $this->_positionToTargetSorted = false;
    $this->_targetCount++;

    return $this;
}

/**
* Add a list of targets.
* @param array $targets
* @chainable
*/
public function addTargets($targets) {
    foreach ($targets as $target) {
        $this->addTarget($target);
    }

    return $this;
}

/**
* Remove a target.
* @param string $target
* @chainable
*/
public function removeTarget($target) {
    if (!isset($this->_targetToPositions[$target])) {
        throw new Flexihash_Exception("Target '$target' does not exist.");
    }

    foreach ($this->_targetToPositions[$target] as $position) {
        unset($this->_positionToTarget[$position]);
    }

    unset($this->_targetToPositions[$target]);

    $this->_targetCount--;

    return $this;
}

/**
* A list of all potential targets
* @return array
*/
public function getAllTargets() {
    return array_keys($this->_targetToPositions);
}

/**
* Looks up the target for the given resource.
* @param string $resource
* @return string
*/
public function lookup($resource) {
    $targets = $this->lookupList($resource, 1);
    if (empty($targets)) {
        throw new Flexihash_Exception('No targets exist');
    }
    return $targets[0];
}

/**
* Get a list of targets for the resource, in order of precedence.
* Up to $requestedCount targets are returned, less if there are fewer in total.
*
* @param string $resource
* @param int $requestedCount The length of the list to return
* @return array List of targets
* @comment 查找当前的资源对应的节点,
*          节点为空则返回空,节点只有一个则返回该节点,
*          对当前资源进行hash,对所有的位置进行排序,在有序的位置列上寻找当前资源的位置
*          当全部没有找到的时候,将资源的位置确定为有序位置的第一个(形成一个环)
*          返回所找到的节点
*/
public function lookupList($resource, $requestedCount) {
    if (!$requestedCount) {
        throw new Flexihash_Exception('Invalid count requested');
    }

    // handle no targets
    if (empty($this->_positionToTarget)) {
        return array();
    }

    // optimize single target
    if ($this->_targetCount == 1) {
        return array_unique(array_values($this->_positionToTarget));
    }

    // hash resource to a position
    $resourcePosition = $this->_hasher->hash($resource);

    $results = array();
    $collect = false;

    $this->_sortPositionTargets();

    // search values above the resourcePosition
    foreach ($this->_positionToTarget as $key => $value) {
        // start collecting targets after passing resource position
        if (!$collect && $key > $resourcePosition) {
            $collect = true;
        }

        // only collect the first instance of any target
        if ($collect && !in_array($value, $results)) {
            $results []= $value;
        }

        // return when enough results, or list exhausted
        if (count($results) == $requestedCount || count($results) == $this->_targetCount) {
            return $results;
        }
    }

    // loop to start - search values below the resourcePosition
    foreach ($this->_positionToTarget as $key => $value) {
        if (!in_array($value, $results)) {
            $results []= $value;
        }

        // return when enough results, or list exhausted
        if (count($results) == $requestedCount || count($results) == $this->_targetCount) {
            return $results;
        }
    }

    // return results after iterating through both "parts"
    return $results;
}

public function __toString() {
    return sprintf(
        '%s{targets:[%s]}',
        get_class($this),
        implode(',', $this->getAllTargets())
    );
}

// ----------------------------------------
// private methods

/**
* Sorts the internal mapping (positions to targets) by position
*/
private function _sortPositionTargets() {
    // sort by key (position) if not already
    if (!$this->_positionToTargetSorted) {
        ksort($this->_positionToTarget, SORT_REGULAR);
        $this->_positionToTargetSorted = true;
    }
}

}

/**
* Hashes given values into a sortable fixed size address space.
*
* @author Paul Annesley
* @package Flexihash
* @licence http://www.opensource.org/licenses/mit-license.php
*/
interface Flexihash_Hasher {

    /**
    * Hashes the given string into a 32bit address space.
    *
    * Note that the output may be more than 32bits of raw data, for example
    * hexidecimal characters representing a 32bit value.
    *
    * The data must have 0xFFFFFFFF possible values, and be sortable by
    * PHP sort functions using SORT_REGULAR.
    *
    * @param string
    * @return mixed A sortable format with 0xFFFFFFFF possible values
    */
    public function hash($string);

}


/**
* Uses CRC32 to hash a value into a signed 32bit int address space.
* Under 32bit PHP this (safely) overflows into negatives ints.
*
* @author Paul Annesley
* @package Flexihash
* @licence http://www.opensource.org/licenses/mit-license.php
*/
class Flexihash_Crc32Hasher
    implements Flexihash_Hasher {

    /* (non-phpdoc)
    * @see Flexihash_Hasher::hash()
    */
    public function hash($string) {
        return crc32($string);
    }

}


/**
* Uses CRC32 to hash a value into a 32bit binary string data address space.
*
* @author Paul Annesley
* @package Flexihash
* @licence http://www.opensource.org/licenses/mit-license.php
*/
class Flexihash_Md5Hasher
    implements Flexihash_Hasher {

    /* (non-phpdoc)
    * @see Flexihash_Hasher::hash()
    */
    public function hash($string) {
        return substr(md5($string), 0, 8); // 8 hexits = 32bit

        // 4 bytes of binary md5 data could also be used, but
        // performance seems to be the same.
    }

}


/**
* An exception thrown by Flexihash.
*
* @author Paul Annesley
* @package Flexihash
* @licence http://www.opensource.org/licenses/mit-license.php
*/
class Flexihash_Exception extends Exception {
}