php 实现Redis分布式锁

简介

多线程情况下访问一些共享资源需要加锁，否则就会导致数据错乱的问题
分布式锁可以通过DB，Redis，Zk等方式实现，本节主要介绍php使用Redis实现分布式锁

set命令

setnx key value 设置一个值，当key已经存在时，返回flase，代表失败
使用setnx实现分布锁有个缺陷，setnx操作无法设置key的ttl，需要配合exprie key ttl 一起使用
好在set命令就集成了nx和ex操作set key name NX PX 10000

$redis = new Redis();
$redis->connect('127.0.0.1', 6380);
$rs = $redis->set('testnx', 123, ['nx', 'ex' => 10]);
var_dump($rs);//返回true代表加锁成功，返回false代表加锁失败

Redlock

set命令还有一个问题，当你要提前释放这个锁的时候，使用expire key 0或者使用del key
如果expire或者del命令发送了阻塞，锁自动失效，这时候B获取了锁，expire/del命令到达，导致B获取的锁失效
Redlock在加锁的时候value值要保证唯一性，在释放锁的时候要验证value是否和申请锁时value是否一致

class RedLock
{
    private $retryDelay;
    private $retryCount;
    private $clockDriftFactor = 0.01;
    private $quorum;
    private $servers = array();
    private $instances = array();
    function __construct(array $servers, $retryDelay = 200, $retryCount = 3)
    {
        $this->servers = $servers;
        $this->retryDelay = $retryDelay;
        $this->retryCount = $retryCount;
        $this->quorum  = min(count($servers), (count($servers) / 2 + 1));
    }
    public function lock($resource, $ttl)
    {
        $this->initInstances();
        $token = uniqid();
        $retry = $this->retryCount;
        do {
            $n = 0;
            $startTime = microtime(true) * 1000;
            foreach ($this->instances as $instance) {
                if ($this->lockInstance($instance, $resource, $token, $ttl)) {
                    $n++;
                }
            }
            # Add 2 milliseconds to the drift to account for Redis expires
            # precision, which is 1 millisecond, plus 1 millisecond min drift
            # for small TTLs.
            $drift = ($ttl * $this->clockDriftFactor) + 2;
            $validityTime = $ttl - (microtime(true) * 1000 - $startTime) - $drift;
            if ($n >= $this->quorum && $validityTime > 0) {
                return [
                    'validity' => $validityTime,
                    'resource' => $resource,
                    'token'    => $token,
                ];
            } else {
                foreach ($this->instances as $instance) {
                    $this->unlockInstance($instance, $resource, $token);
                }
            }
            // Wait a random delay before to retry
            $delay = mt_rand(floor($this->retryDelay / 2), $this->retryDelay);
            usleep($delay * 1000);
            $retry--;
        } while ($retry > 0);
        return false;
    }
    public function unlock(array $lock)
    {
        $this->initInstances();
        $resource = $lock['resource'];
        $token    = $lock['token'];
        foreach ($this->instances as $instance) {
            $this->unlockInstance($instance, $resource, $token);
        }
    }
    private function initInstances()
    {
        if (empty($this->instances)) {
            foreach ($this->servers as $server) {
                list($host, $port, $timeout) = $server;
                $redis = new \Redis();
                $redis->connect($host, $port, $timeout);
                $this->instances[] = $redis;
            }
        }
    }
    private function lockInstance($instance, $resource, $token, $ttl)
    {
        return $instance->set($resource, $token, ['NX', 'PX' => $ttl]);
    }
    private function unlockInstance($instance, $resource, $token)
    {
        $script = '
            if redis.call("GET", KEYS[1]) == ARGV[1] then
                return redis.call("DEL", KEYS[1])
            else
                return 0
            end
        ';
        return $instance->eval($script, [$resource, $token], 1);
    }
}

$servers = [
    ['127.0.0.1', 6379, 0.01],
];
$redLock = new RedLock($servers);
while (true) {
    $lock = $redLock->lock('test', 10000);
    if ($lock) {
        print_r($lock);
        $redLock->unlock(['resource' => 'test', 'token' => '5d1c123121538']);
    } else {
        print "Lock not acquired\n";
    }
}

后续

Redis分布式锁还有没有问题？

解决方法：引入版本号

参考：https://time.geekbang.org/col...