目的：无法阻止宕机的缓存被使用，但可以尽可能保证正常的服务器一切如常。[取余的方式一旦有机器增减，数据位置100%失效，缓存雪崩]
场景：缓存服务器集群
原理：节点在0-2^32圜环上占位，每次使用HASH（KEY）无限趋近的 节点作为落地。【虚拟节点的目的是为了尽可能保持数据平衡，3台机器很容易失衡导致某一台机器数据过载。】

<?php

class consistentHash
{
private $nodes;

private $virtualNodes;

const VIRTUAL_NODES_NUM = 2;

public function commonHash($key)
{
    return sprintf("%u", crc32($key));
}

public function addNode($node)
{
    if (isset($this->nodes[$node])) {
        return;
    }
    for ($i = 0; $i <= self::VIRTUAL_NODES_NUM; $i++) {
        $hash = $this->commonHash($node . '-' . $i);
        $this->virtualNodes[$hash] = $node;
        $this->nodes[$node][] = $hash;
    }
    ksort($this->virtualNodes, SORT_REGULAR);
}

public function delNode($node)
{
    if (!isset($this->nodes[$node])) {
        return;
    }
    foreach ($this->nodes[$node] as $k => $v) {
        unset($this->virtualNodes[$v]);
    }
    unset($this->nodes[$node]);
}

public function getNode($key)
{
    $hash = $this->commonHash($key);
    $res = current($this->virtualNodes);
    foreach ($this->virtualNodes as $k => $v) {
        if ($hash <= $k) {
            $res = $v;
            break;
        }
    }
    reset($this->virtualNodes);
    return $res;
}

}

$obj = new consistentHash();
$obj->addNode("192.168.1.1");
$obj->addNode("192.168.1.2");
$obj->addNode("192.168.1.3");
$obj->addNode("192.168.1.4");
$obj->addNode("192.168.1.5");

echo $obj->getNode(111) . PHP_EOL;
echo $obj->getNode(222) . PHP_EOL;
echo $obj->getNode(222) . PHP_EOL;

?>