本文学习Redisson分布式锁的原理以及优缺点

1. Redisson分布式锁原理

lua脚本是原子操作，redis会将整个脚本作为一个整体执行，中间不会被其他命令打断

# RedissonLock.tryLockInnerAsync方法内lua脚本加锁
<T> RFuture<T> tryLockInnerAsync(long waitTime, long leaseTime, TimeUnit unit, long threadId, RedisStrictCommand<T> command) {
        return evalWriteAsync(getRawName(), LongCodec.INSTANCE, command,
                "if (redis.call('exists', KEYS[1]) == 0) then " +
                        "redis.call('hincrby', KEYS[1], ARGV[2], 1); " +
                        "redis.call('pexpire', KEYS[1], ARGV[1]); " +
                        "return nil; " +
                        "end; " +
                        "if (redis.call('hexists', KEYS[1], ARGV[2]) == 1) then " +
                        "redis.call('hincrby', KEYS[1], ARGV[2], 1); " +
                        "redis.call('pexpire', KEYS[1], ARGV[1]); " +
                        "return nil; " +
                        "end; " +
                        "return redis.call('pttl', KEYS[1]);",
                Collections.singletonList(getRawName()), unit.toMillis(leaseTime), getLockName(threadId));
    }

1.1 加锁原理

如图与代码所示，sk_10001_stock_lock不存在时，加锁时会创建hash类型键sk_10001_stock_lock，并新增field为客户端id，value设置为1(hincrby key field 1不存在key则会创建它，并且设置field为1)，并设置过期时间为60s

1.2 可重入原理

如lua脚本所示，当sk_10001_stock_lock存在客户端id时，会将客户端id的值+1,并且重新设置过期时间，所以保存客户端id就是为了可重入

1.3 锁互斥原理

如lua脚本所示，对于其他的客户端返回锁pttl过期时间，后续流程如代码所示

// RedissonLock.tryLock方法
    public boolean tryLock(long waitTime, long leaseTime, TimeUnit unit) throws InterruptedException {
        Long ttl = tryAcquire(waitTime, leaseTime, unit, threadId);
        // ttl为空获取到锁
        if (ttl == null) {
            return true;
        }
        // 如果时间超过waitTime则获取锁失败
        time -= System.currentTimeMillis() - current;
        if (time <= 0) {
            acquireFailed(waitTime, unit, threadId);
            return false;
        }
	// 订阅锁释放事件
        CompletableFuture<RedissonLockEntry> subscribeFuture = subscribe(threadId);
        try {
            subscribeFuture.get(time, TimeUnit.MILLISECONDS);
        } catch (TimeoutException e) {
        } catch (ExecutionException e) {   
        }

        try {
            // 在最大等待时间内，循环获取锁，直到成功或失败
            while (true) {
                long currentTime = System.currentTimeMillis();
                ttl = tryAcquire(waitTime, leaseTime, unit, threadId);
                // lock acquired
                if (ttl == null) {
                    return true;
                }

                time -= System.currentTimeMillis() - currentTime;
                if (time <= 0) {
                    acquireFailed(waitTime, unit, threadId);
                    return false;
                }

                // 通过信号量semaphore共享锁阻塞等待
                currentTime = System.currentTimeMillis();
                if (ttl >= 0 && ttl < time) {
                   commandExecutor.getNow(subscribeFuture).getLatch().tryAcquire(ttl, TimeUnit.MILLISECONDS);
                } else {
                    commandExecutor.getNow(subscribeFuture).getLatch().tryAcquire(time, TimeUnit.MILLISECONDS);
                }
            }
        } finally {
            // 取消订阅
            unsubscribe(commandExecutor.getNow(subscribeFuture), threadId);
        }
    }

1.4 锁续期原理

// RedissonLock.tryAcquireAsync方法
private <T> RFuture<Long> tryAcquireAsync(long waitTime, long leaseTime, TimeUnit unit, long threadId) {
        CompletionStage<Long> f = ttlRemainingFuture.thenApply(ttlRemaining -> {
            // 获取到锁
            if (ttlRemaining == null) {
                if (leaseTime > 0) {
                    internalLockLeaseTime = unit.toMillis(leaseTime);
                } else {
                    // 当leaseTime<=0时锁会自动续期
                    scheduleExpirationRenewal(threadId);
                }
            }
            return ttlRemaining;
        });
        return new CompletableFutureWrapper<>(f);
    }

 private void renewExpiration() {
        // Watch Dog机制，每10秒检查是否仍然持有锁，是则续期
        Timeout task = commandExecutor.getConnectionManager().newTimeout(new TimerTask() {
            @Override
            public void run(Timeout timeout) throws Exception {
	    }
        }, internalLockLeaseTime / 3, TimeUnit.MILLISECONDS);
        
        ee.setTimeout(task);
    }

    // 依次进入方法，最终找到RedissonBaseLock.renewExpirationAsync实现
    protected CompletionStage<Boolean> renewExpirationAsync(long threadId) {
        return evalWriteAsync(getRawName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,
                "if (redis.call('hexists', KEYS[1], ARGV[2]) == 1) then " +
                        "redis.call('pexpire', KEYS[1], ARGV[1]); " +
                        "return 1; " +
                        "end; " +
                        "return 0;",
                Collections.singletonList(getRawName()),
                internalLockLeaseTime, getLockName(threadId));
    }

通过代码可知，当leasetime<=0时，锁通过Watch Dog机制[其实就是一个后台定时任务线程]每10秒检查是否持有锁，持有则自动续期30s，也就是重新设置了sk_10001_stock_lock中field为客户端id的过期时间为30s

1.5 锁释放原理

public RFuture<Void> unlockAsync(long threadId) {
        // 释放锁
        RFuture<Boolean> future = unlockInnerAsync(threadId);
    
        CompletionStage<Void> f = future.handle((opStatus, e) -> {
            // 取消Watch Dog机制
            cancelExpirationRenewal(threadId);
        });

        return new CompletableFutureWrapper<>(f);
    }

protected RFuture<Boolean> unlockInnerAsync(long threadId) {
        return evalWriteAsync(getRawName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,
                "if (redis.call('hexists', KEYS[1], ARGV[3]) == 0) then " +
                        "return nil;" +
                        "end; " +
                        "local counter = redis.call('hincrby', KEYS[1], ARGV[3], -1); " +
                        "if (counter > 0) then " +
                        "redis.call('pexpire', KEYS[1], ARGV[2]); " +
                        "return 0; " +
                        "else " +
                        "redis.call('del', KEYS[1]); " +
                        "redis.call('publish', KEYS[2], ARGV[1]); " +
                        "return 1; " +
                        "end; " +
                        "return nil;",
                Arrays.asList(getRawName(), getChannelName()), LockPubSub.UNLOCK_MESSAGE, internalLockLeaseTime, getLockName(threadId));
    }

由代码可知，释放锁时将sk_10001_stock_lock中field为客户端id的值依次减1，直到为0后在进行删除，删除后会向redisson_lock__channel通道中发送UNLOCK_MESSAGE消息也就是0L，通知阻塞等待的客户端

2. Redisson优缺点

优点

• Redisson通过Watch Dog机制解决锁的续期问题
• 与Zookeeper相比较，Redisson基于Redis性能更高
• 通过Redisson实现分布式可重入锁，比原生的 SET mylock userId NX PX milliseconds + lua更加简洁，让我们更多关注业务逻辑
• 在等待申请锁的实现上进行了优化，减少了无效锁申请，提升了资源利用率

缺点

在redisson3.12.5前，无法解决在master节点加锁后，master异步复制给slave时宕机，slave变为了master,其他客户端在新的master上重复加锁问题，需要使用redlock算法获取集群大多数锁时才算获取锁成功额外编码

RLock rLock = redissonClient.getLock(PRODUCT_STOCK_KEY + "_lock");
RLock rLock = redissonClient1.getLock(PRODUCT_STOCK_KEY + "_lock");
RLock rLock = redissonClient2.getLock(PRODUCT_STOCK_KEY + "_lock");
RedissonRedLock redissonRedLock = new RedissonRedLock(rLock, rLock1, rLock2);
redissonRedLock.tryLock(30, 60, TimeUnit.SECONDS);

https://github.com/redisson/redisson/blob/master/CHANGELOG.md
16-Apr-2020 - 3.12.5 released
Improvement - increased RLock reliability during failover. RedLock was deprecated

但是对于redisson3.12.5及之后就不存在该问题会等到master异步复制给slave完成后才会进行加锁

 /**
     * Returns Lock instance by name.
     * <p>
     * Implements a <b>non-fair</b> locking so doesn't guarantees an acquire order by threads.
     * <p>
     * To increase reliability during failover, all operations wait for propagation to all Redis slaves.
     *
     * @param name - name of object
     * @return Lock object
     */
    RLock getLock(String name);

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