分布式锁（2) ----- 基于redis的分布式锁

Posted 2021-11-22 wuweishuo

Redis单机版实现

set和lua实现

获取锁

SET resource_name my_random_value NX PX 30000

NX key不存在时才set
PX 设置过期时间
my_random_value 要保证每台客户端的每个锁请求唯一，可以使用UUID+ThreadID
该命令在Redis 2.6.12才有，网上有基于setnx、epire的实现和基于setnx、get、getset的实现，这些多多少少都有点瑕疵，大概率是旧版本的redis实现，建议高版本的redis还是使用这个实现。

释放锁

if redis.call("get",KEYS[1]) == ARGV[1] then
    return redis.call("del",KEYS[1])
else
    return 0
end

通过上述lua脚本实现，先判断锁是否该请求拥有，防止误解锁。

java代码

public boolean tryLock(long waitTime, long leaseTime) {
    long end = Calendar.getInstance().getTimeInMillis() + waitTime;
    Jedis jedis = jedisPool.getResource();
    try {
        do {
            String result = jedis.set(lockName, getClientId(), "NX", "EX", leaseTime);
            if ("OK".equals(result)) {
                return true;
            }
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                return false;
            }
        } while (Calendar.getInstance().getTimeInMillis() < end);
    }finally {
        if(jedis != null) {
            jedis.close();
        }
    }
    return false;
}

public boolean unlock() {
    String lua = "if redis.call(\"get\",KEYS[1]) == ARGV[1] then\n" +
                "    return redis.call(\"del\",KEYS[1])\n" +
                "else\n" +
                "    return 0\n" +
                "end";
    Jedis jedis = jedisPool.getResource();
    try {
        Object obj = jedis.eval(lua, Collections.singletonList(lockName), Collections.singletonList(getClientId()));
        if (obj.equals(1)) {
            return true;
        }
    }finally {
        if(jedis != null){
            jedis.close();
        }
    }
    return false;
}

测试代码

public void lockTest() {
    //用线程模拟进程
    ExecutorService threadPool = Executors.newFixedThreadPool(20);
    CyclicBarrier barrier = new CyclicBarrier(20);
    for (int i = 0; i < 20; i++) {
        threadPool.execute(new RedisLockTest(barrier));
    }
    threadPool.shutdown();
    while (!threadPool.isTerminated()) {
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}
private static class RedisLockTest implements Runnable{

    private CyclicBarrier barrier;
    RedisLockTest(CyclicBarrier barrier){
        this.barrier = barrier;
    }

    @Override
    public void run() {
        //模拟一台客户端一个jedisPool
        JedisPool jedisPool = new JedisPool("192.168.9.150", 6379);
        try {
            DistributeLock lock = new SingletonRedisLock(jedisPool, "lock");
            barrier.await();
            boolean flag = lock.tryLock(Integer.MAX_VALUE, 300);
            try {
                System.out.println(Thread.currentThread().getId() + "get lock:flag=" + flag);
                Thread.sleep(100);
                System.out.println(Thread.currentThread().getId() + "get unlock");
            } finally {
                lock.unlock();
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            jedisPool.close();
        }
    }
}

存在的问题

该实现简单，但存在以下问题：
1.没有实现可重入
2.没有锁续租，如果代码在锁的租期内没有执行完成，那么锁过期会导致另一个客户端获取锁

Redisson实现

private static class RedissonLockTest implements Runnable{

    private CyclicBarrier barrier;
    RedissonLockTest(CyclicBarrier barrier){
        this.barrier = barrier;
    }

    @Override
    public void run() {
        Config config = new Config();
        config.useSingleServer().setAddress("redis://192.168.9.150:6379");
        RedissonClient client = Redisson.create(config);
        try {
            RLock lock = client.getLock("lock");
            barrier.await();
            lock.lock();
            try {
                System.out.println(Thread.currentThread().getName() + "get lock");
                Thread.sleep(100);
                System.out.println(Thread.currentThread().getName() + "get unlock");
            } finally {
                lock.unlock();
            }
        }catch (Exception e){
            e.printStackTrace();
        }finally {
            client.shutdown();
        }
    }
}

Redisson的分布式锁实现可重入，同时有LockWatchDogTimeout来实现锁续约
github：https://github.com/redisson/redisson

Redis多实例版实现

单机实现在多实例下问题

如果Redis实现了集群，由于主从之间时通过异步复制的，假设客户端A在主机上获得锁，这时在未将锁数据复制到从机时，主机挂了，从机切换为主机，那么从机没有这条数据，客户端B同样可以获得锁。

RedLock算法

使用多个独立（非集群）的实例来实现分布式锁，由于实例独立不需复制同步，所以没有上述问题；而保证可用性的是靠数据冗余，将数据多存放几份在不同的实例上。算法如下：

1. 使用相同的key和value从N个实例上获取锁；
2. 当从大于（N/2+1）个实例获取锁的时间<锁的过期时间，才获取锁成功
3. 如果获取失败，解锁所有实例

Redisson实现

public void redLockTest() {
    Config config = new Config();
    config.useSingleServer().setAddress("redis://192.168.9.150:7000");
    RedissonClient client = Redisson.create(config);
    Config config1 = new Config();
    config1.useSingleServer().setAddress("redis://192.168.9.150:7001");
    RedissonClient client1 = Redisson.create(config1);
    Config config2 = new Config();
    config2.useSingleServer().setAddress("redis://192.168.9.150:7002");
    RedissonClient client2 = Redisson.create(config2);
    try{
        RLock lock = client.getLock("lock");
        RLock lock1 = client1.getLock("lock");
        RLock lock2 = client2.getLock("lock");
        RedissonRedLock redLock = new RedissonRedLock(lock, lock1, lock2);
        redLock.lock();
        try{
            System.out.println(Thread.currentThread().getName() + "get lock");
            Thread.sleep(100);
            System.out.println(Thread.currentThread().getName() + "get unlock");
        } finally {
            redLock.unlock();
        }
    }catch (Exception e){
        e.printStackTrace();
    }finally {
        client.shutdown();
        client1.shutdown();
        client2.shutdown();
    }
}

参考资料

https://redis.io/topics/distlock
https://github.com/redisson/redisson/wiki