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<?php
/*
* This file is part of the Predis package.
*
* (c) Daniele Alessandri <suppakilla@gmail.com>
*
* For the full copyright and license information, please view the LICENSE
* file that was distributed with this source code.
*/
namespace Predis\Cluster\Distributor;
use Predis\Cluster\Hash\HashGeneratorInterface;
/**
* This class implements an hashring-based distributor that uses the same
* algorithm of memcache to distribute keys in a cluster using client-side
* sharding.
*
* @author Daniele Alessandri <suppakilla@gmail.com>
* @author Lorenzo Castelli <lcastelli@gmail.com>
*/
class HashRing implements DistributorInterface, HashGeneratorInterface
{
const DEFAULT_REPLICAS = 128;
const DEFAULT_WEIGHT = 100;
private $ring;
private $ringKeys;
private $ringKeysCount;
private $replicas;
private $nodeHashCallback;
private $nodes = array();
/**
* @param int $replicas Number of replicas in the ring.
* @param mixed $nodeHashCallback Callback returning a string used to calculate the hash of nodes.
*/
public function __construct($replicas = self::DEFAULT_REPLICAS, $nodeHashCallback = null)
{
$this->replicas = $replicas;
$this->nodeHashCallback = $nodeHashCallback;
}
/**
* Adds a node to the ring with an optional weight.
*
* @param mixed $node Node object.
* @param int $weight Weight for the node.
*/
public function add($node, $weight = null)
{
// In case of collisions in the hashes of the nodes, the node added
// last wins, thus the order in which nodes are added is significant.
$this->nodes[] = array(
'object' => $node,
'weight' => (int) $weight ?: $this::DEFAULT_WEIGHT,
);
$this->reset();
}
/**
* {@inheritdoc}
*/
public function remove($node)
{
// A node is removed by resetting the ring so that it's recreated from
// scratch, in order to reassign possible hashes with collisions to the
// right node according to the order in which they were added in the
// first place.
for ($i = 0; $i < count($this->nodes); ++$i) {
if ($this->nodes[$i]['object'] === $node) {
array_splice($this->nodes, $i, 1);
$this->reset();
break;
}
}
}
/**
* Resets the distributor.
*/
private function reset()
{
unset(
$this->ring,
$this->ringKeys,
$this->ringKeysCount
);
}
/**
* Returns the initialization status of the distributor.
*
* @return bool
*/
private function isInitialized()
{
return isset($this->ringKeys);
}
/**
* Calculates the total weight of all the nodes in the distributor.
*
* @return int
*/
private function computeTotalWeight()
{
$totalWeight = 0;
foreach ($this->nodes as $node) {
$totalWeight += $node['weight'];
}
return $totalWeight;
}
/**
* Initializes the distributor.
*/
private function initialize()
{
if ($this->isInitialized()) {
return;
}
if (!$this->nodes) {
throw new EmptyRingException('Cannot initialize an empty hashring.');
}
$this->ring = array();
$totalWeight = $this->computeTotalWeight();
$nodesCount = count($this->nodes);
foreach ($this->nodes as $node) {
$weightRatio = $node['weight'] / $totalWeight;
$this->addNodeToRing($this->ring, $node, $nodesCount, $this->replicas, $weightRatio);
}
ksort($this->ring, SORT_NUMERIC);
$this->ringKeys = array_keys($this->ring);
$this->ringKeysCount = count($this->ringKeys);
}
/**
* Implements the logic needed to add a node to the hashring.
*
* @param array $ring Source hashring.
* @param mixed $node Node object to be added.
* @param int $totalNodes Total number of nodes.
* @param int $replicas Number of replicas in the ring.
* @param float $weightRatio Weight ratio for the node.
*/
protected function addNodeToRing(&$ring, $node, $totalNodes, $replicas, $weightRatio)
{
$nodeObject = $node['object'];
$nodeHash = $this->getNodeHash($nodeObject);
$replicas = (int) round($weightRatio * $totalNodes * $replicas);
for ($i = 0; $i < $replicas; ++$i) {
$key = $this->hash("$nodeHash:$i");
$ring[$key] = $nodeObject;
}
}
/**
* {@inheritdoc}
*/
protected function getNodeHash($nodeObject)
{
if (!isset($this->nodeHashCallback)) {
return (string) $nodeObject;
}
return call_user_func($this->nodeHashCallback, $nodeObject);
}
/**
* {@inheritdoc}
*/
public function hash($value)
{
return crc32($value);
}
/**
* {@inheritdoc}
*/
public function getByHash($hash)
{
return $this->ring[$this->getSlot($hash)];
}
/**
* {@inheritdoc}
*/
public function getBySlot($slot)
{
$this->initialize();
if (isset($this->ring[$slot])) {
return $this->ring[$slot];
}
}
/**
* {@inheritdoc}
*/
public function getSlot($hash)
{
$this->initialize();
$ringKeys = $this->ringKeys;
$upper = $this->ringKeysCount - 1;
$lower = 0;
while ($lower <= $upper) {
$index = ($lower + $upper) >> 1;
$item = $ringKeys[$index];
if ($item > $hash) {
$upper = $index - 1;
} elseif ($item < $hash) {
$lower = $index + 1;
} else {
return $item;
}
}
return $ringKeys[$this->wrapAroundStrategy($upper, $lower, $this->ringKeysCount)];
}
/**
* {@inheritdoc}
*/
public function get($value)
{
$hash = $this->hash($value);
$node = $this->getByHash($hash);
return $node;
}
/**
* Implements a strategy to deal with wrap-around errors during binary searches.
*
* @param int $upper
* @param int $lower
* @param int $ringKeysCount
*
* @return int
*/
protected function wrapAroundStrategy($upper, $lower, $ringKeysCount)
{
// Binary search for the last item in ringkeys with a value less or
// equal to the key. If no such item exists, return the last item.
return $upper >= 0 ? $upper : $ringKeysCount - 1;
}
/**
* {@inheritdoc}
*/
public function getHashGenerator()
{
return $this;
}
}