memcache雖然是分佈式的應用服務，但分佈的原則是由client端的api來決定的，api根據存儲用的key以及已知的服務器列表，根據key的hash計算將指定的key存儲到對應的服務器列表上。
通常使用的散列方法是根據 key的hash值%服務器數取餘數的方法來決定當前這個key的內容發往哪一個memcache服務器。但這樣的算法在服務實例本身發生變動的時候，服務列表的變動會造成幾乎大部分數據都會需要遷移到另外的服務實例上。這樣在大型服務在線時，瞬時對後端數據庫/硬盤照成的壓力很可能導致整個服務的crash。
在此可以採用一致性哈希（Consistent Hashing）來解決這個問題，處理服務器的選擇不再僅僅依賴key的hash本身而是將服務實例（節點）的配置也進行hash運算。
一致性哈希（Consistent Hashing）算法簡述如下：
1. 求出每個服務節點的hash，並將其配置到一個0~2^32的圓環（continuum）區間上。
2. 使用同樣的方法求出你所需要存儲的key的hash，也將其配置到這個圓環（continuum）上。
3. 從數據映射到的位置開始順時針查找，將數據保存到找到的第一個服務節點上。如果超過2^32仍然找不到服務節點，就會保存到第一個服務節點上。
整個數據的圖例：
當增加服務器節點時：
這樣做可以最大程度的避免了key在服務節點列表上的重新分佈，同時還增加了虛擬服務節點的方法，也就是一個服務節點在環上有多個映射點，這樣就能抑制分佈不均勻，最大限度地減小服務節點增減時的緩存重新分佈。具體來說，只有在圓環上增加服務節點的位置爲逆時針方向的第一個服務節點上的鍵會受到影響。
以下部分爲一致性哈希算法的一種PHP實現。
from: http://paul.annesley.cc/
comment: http://blog.csdn.net/mayongzhan/
<?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 虛擬節點數,解決節點分佈不均的問題
*/
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 節點記數器
*/
private $_targetCount = 0;
/**
* Internal map of positions (hash outputs) to targets
* @var array { position => target, ... }
* @comment 位置對應節點,用於lookup中根據位置確定要訪問的節點
*/
private $_positionToTarget = array();
/**
* Internal map of targets to lists of positions that target is hashed to.
* @var array { target => [ position, position, ... ], ... }
* @comment 節點對應位置,用於刪除節點
*/
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 構造函數,確定要使用的hash方法和需擬節點數,虛擬節點數越多,分佈越均勻,但程序的分佈式運算越慢
*/
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 $target
* @chainable
* @comment 添加節點,根據虛擬節點數,將節點分佈到多個虛擬位置上
*/
public function addTarget($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 $requestedCount The length of the list to return
* @return array List of targets
* @comment 查找當前的資源對應的節點,
* 節點爲空則返回空,節點只有一個則返回該節點,
* 對當前資源進行hash,對所有的位置進行排序,在有序的位置列上尋找當前資源的位置
* 當全部沒有找到的時候,將資源的位置確定爲有序位置的第一個(形成一個環)
* 返回所找到的節點
*/
public function lookupList($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 {
}