Why use a consistent HASH
The traditional method for mapping requests to the Cache is hash(object)%N, where object represents the IP address of the requester, and hash represents a hash function of some kind, such as CRC32, MD5, or a custom function.
If you want to add a Cache, N becomes 4.
As can be seen, after the cache is added, all the previous cache is invalid, and all the traffic is sent to DB, causing great pressure to DB. Therefore, we want to add a new Cache without affecting the previously allocated objects, i.e., we need to satisfy the monotonicity:
Monotony means that if something has already been hashed into the corresponding buffer, a new buffer is added to the system. The result of the hash should ensure that the previously allocated content is mapped to the new buffer and not to other buffers in the old buffer set.
Consistent HASH principle
We assign the value after the hash to the range 0-2^32-1. Think of this range as a circle. When an Object1 request is received, we hash it to get an integer mapped to the corresponding position in the ring, such as point A, where there is no Cache machine. At this point, we search the Cache clockwise around the ring and find CacheA. So object1 corresponds to a Cache machine called CacheA. Same with object2 and Object3. Note: The Cahce machine also has an IP address, which is assigned to the corresponding position on the ring using the same hash algorithm as object.
Let’s now see what happens when we add machines:
Add CahcheD to CacheB and CacheC. Object1, object2, object3 are still used in the Cache. The position of the Cache machine on the ring remains the same), affecting only the red area of the graph, which used to correspond to CacheC and now corresponds to CacheD.
What if we delete a Cache machine? Such as CacheB:
As you can see, object2, which was allocated to CacheB, is now allocated to CacheD, affecting only the ring between CacheA and CacheB (the red area). Object1 and Object3 are allocated to the same Cache machines.
Virtual node
If we look at the figure above, we can see that the CacheC is idle and no requests are falling on the CahceC, whereas we expect all requests to fall evenly on all caching machines. To solve this situation, we introduced the concept of virtual nodes:
Virtual nodes are replicas of actual nodes in the Hash space. One actual node corresponds to several virtual nodes, which are also the number of replicas. Virtual nodes are arranged in the Hash space with hash values.
We’ll start by setting the “number of replicates”, if it’s 2, which means that CacheA has two virtual nodes, CacheA1 and CacheA2. The same is true for CacheB.
The red areas in the figure are virtual nodes, and we see that Object2 maps to CacheA1, which is a virtual node of CacheA, so obejct2’s request ends up on CacheA. Similarly, object3’s request falls on CacheB1, which is a virtual node of CacheB, so OBEjct3’s request ends up on CacheB. Note that CacheA1, CacheA2, CacheB1, and CacheB2 are not actual Cache machines, but virtual nodes placed on the ring based on the Hash IP.
The IP address of the node can be suffixed to hash virtual Node. For example, assume that the IP address of cache A is 202.168.14.241. Before importing virtual nodes, calculate the hash value of cache A: hash (202.168.14.241). After virtual Node is imported, calculate the hash value of cache A1 and cache A2 at virtual node: hash (202.168.14.2411). / / cache A1 Hash (” 202.168.14.2412 “); // cache A2
Reprint a consistent hash algorithm implemented in PHP:
<?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方法和虚拟节点数,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
{
}Copy the codeAs above, when adding machines, first calculate the hash value of all virtual nodes according to the number of replicated nodes and put it in an array. The key value of the array is the hash value, and the value is the IP address of the machine (only the IP address of the actual added machine, not the calculated VIRTUAL machine IP address).
// 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] []= $position; // target removal
}Copy the codeWhen the request comes in, compute the hash value of the request and iterate through the _positionToTarget array. When the first node larger than the hash value of the request is found, it is the corresponding Cahce machine node.
public function lookupList($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; }Copy the codeThere are two foreach loops because the requested hash value falls between the last Cache node of the circle and 2^32-1, as shown in point B below:
At this point, the first foreach is not sufficient because the key value in the array is not larger than the hash value requested, so it is necessary to add the first $requestedCount values from the _positionToTarget array to results.