前言
分布式锁一般有三种实现方式：1. 数据库乐观锁；2. 基于Redis的分布式锁；3. 基于ZooKeeper的分布式锁。本篇博客将介绍第二种方式，基于Redis实现分布式锁。虽然网上已经有各种介绍Redis分布式锁实现的博客，然而他们的实现却有着各种各样的问题，为了避免误人子弟，本篇博客将详细介绍如何正确地实现Redis分布式锁。
可靠性
首先，为了确保分布式锁可用，我们至少要确保锁的实现同时满足以下四个条件：
- 互斥性。在任意时刻，只有一个客户端能持有锁。
- 不会发生死锁。即使有一个客户端在持有锁的期间崩溃而没有主动解锁，也能保证后续其他客户端能加锁。
- 具有容错性。只要大部分的Redis节点正常运行，客户端就可以加锁和解锁。
- 解铃还须系铃人。加锁和解锁必须是同一个客户端，客户端自己不能把别人加的锁给解了
代码实现
<?php 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); } }
使用示例
<?php require_once __DIR__ . \'/../src/RedLock.php\'; $servers = [ [\'127.0.0.1\', 6379, 0.01], [\'127.0.0.1\', 6389, 0.01], [\'127.0.0.1\', 6399, 0.01], ]; $redLock = new RedLock($servers); while (true) { $lock = $redLock->lock(\'test\', 10000); if ($lock) { print_r($lock); } else { print "Lock not acquired\n"; } }
参考文档
https://www.cnblogs.com/linjiqin/p/8003838.html
https://github.com/ronnylt/redlock-php
