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一致性哈希算法(consistent hashing)PHP实现
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一致性哈希算法在1997年由麻省理工学院提出的一种分布式哈希（DHT）实现算法，设计目标是为了解决因特网中的热点(Hot spot)问题，初衷和CARP十分类似。一致性哈希修正了CARP使用的简单哈希算法带来的问题，使得分布式哈希（DHT）可以在P2P环境中真正得到应用。
一致性hash算法提出了在动态变化的Cache环境中，判定哈希算法好坏的四个定义：
1、平衡性(Balance)：平衡性是指哈希的结果能够尽可能分布到所有的缓冲中去，这样可以使得所有的缓冲空间都得到利用。很多哈希算法都能够满足这一条件。
2、单调性(Monotonicity)：单调性是指如果已经有一些内容通过哈希分派到了相应的缓冲中，又有新的缓冲加入到系统中。哈希的结果应能够保证原有已分配的内容可以被映射到原有的或者新的缓冲中去，而不会被映射到旧的缓冲集合中的其他缓冲区。
3、分散性(Spread)：在分布式环境中，终端有可能看不到所有的缓冲，而是只能看到其中的一部分。当终端希望通过哈希过程将内容映射到缓冲上时，由于不同终端所见的缓冲范围有可能不同，从而导致哈希的结果不一致，最终的结果是相同的内容被不同的终端映射到不同的缓冲区中。这种情况显然是应该避免的，因为它导致相同内容被存储到不同缓冲中去，降低了系统存储的效率。分散性的定义就是上述情况发生的严重程度。好的哈希算法应能够尽量避免不一致的情况发生，也就是尽量降低分散性。
4、负载(Load)：负载问题实际上是从另一个角度看待分散性问题。既然不同的终端可能将相同的内容映射到不同的缓冲区中，那么对于一个特定的缓冲区而言，也可能被不同的用户映射为不同 的内容。与分散性一样，这种情况也是应当避免的，因此好的哈希算法应能够尽量降低缓冲的负荷。
在分布式集群中，对机器的添加删除，或者机器故障后自动脱离集群这些操作是分布式集群管理最基本的功能。如果采用常用的 hash(object)%N 算法，那么在有机器添加或者删除后，很多原有的数据就无法找到了，这样严重的违反了单调性原则。接下来主要讲解一下一致性哈希算法是如何设计的：
环形Hash空间
按照常用的hash算法来将对应的key哈希到一个具有 2^32次方 个桶的空间中，即0~(2^32)-1的数字空间中。现在我们可以将这些数字头尾相连，想象成一个闭合的环形。如下图
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);
}
/**
* A list of all potential targets
* @return array
*/
public function getAll()
{
return array(
"targers"=>$this->_positionToTarget,
"positions"=>$this->_targetToPositions);
}
/**
* Looks up the target for the given resource.
* @param string $resource
* @return string
*/
public function lookup($resource)
{
$targets = $this->lookupList($resource, 1);
print_r($targets);die;
if (empty($targets)) throw new Flexihash_Exception('No targets exist');
return $targets[0]; //0表示返回离资源位置最近的机器节点
}
/**
* 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;
$this->_sortPositionTargets();
print_r($this->_positionToTarget);die;
// 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 []= $value;
//var_dump($results);
}
// return when enough results, or list exhausted
//var_dump(count($results));
//var_dump($requestedCount);
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
{
}
$hash = new Flexihash();
$targets=array(
"192.168.1.1:11011",
"192.168.1.1:11012",
"192.168.1.1:11013",
"192.168.1.1:11014",
"192.168.1.1:11015",
);
$hash->addTargets($targets);
for ($i=0; $i < 25; $i++) {
$resource = sprintf("format %d",$i);
var_dump($resource." --> ".$hash->lookup($resource));
}© 著作权归作者所有
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