一致性哈希算法的PHP实现代码

<?php
/**
* 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 ? $hasher : new Flexihash_Crc32Hasher();
if (!emptyempty($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 (emptyempty($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 (emptyempty($this->_positionToTarget)) {
return array();
}
// optimize single target
if ($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 {
}