Shulou(Shulou.com)05/31 Report--

In this issue, the editor will bring you about how to achieve a distributed lock in Redis. The article is rich in content and analyzes and describes it from a professional point of view. I hope you can get something after reading this article.

Scheme

Use the SETNX and EXPIRE commands

SETNX key value EXPIRE key seconds DEL key if (setnx ("item_1_lock", 1)) {expire ("item_1_lock", 30); try {. Logic} catch {...} finally {del ("item_1_lock");}}

This method seems to solve the problem, but there is a certain risk, because the wave of operations of SETNX and EXPIRE are non-atomic, and if an error occurs after the success of the SETNX, the EXPIRE is not executed, resulting in a deadlock when the lock does not set a timeout.

In this case, we can use lua scripts to keep the operation atomic, ensuring that both SETNX and EXPIRE operations are either successful or unsuccessful.

If (redis.call ('setnx', KEYS [1], ARGV [1])

< 1) then return 0; end; redis.call('expire', KEYS[1], tonumber(ARGV[2])); return 1; 通过这样的方法，我们初步解决了竞争锁的原子性问题，虽然其他功能还未实现，但是应该不会造成死锁???。 Redis 2.6.12 以上可灵活使用 SET 命令 SET key value NX EX 30 DEL key if (set("item_1_lock", 1, "NX", "EX", 30)) { try { ... 逻辑 } catch { ... } finally { del("item_1_lock"); } } 改进后的方法不需要借助 lua 脚本就解决了 SETNX 和 EXPIRE 的原子性问题。现在我们再仔细琢磨琢磨，如果 A 拿到了锁顺利进入代码块执行逻辑，但是由于各种原因导致超时自动释放锁。在这之后 B 成功拿到了锁进入代码块执行逻辑，但此时如果 A 执行逻辑完毕再来释放锁，就会把 B 刚获得的锁释放了。就好比用自己家的钥匙开了别家的门，这是不可接受的。 为了解决这个问题我们可以尝试在 SET 的时候设置一个锁标识，然后在 DEL 的时候验证当前锁是否为自己的锁。 String value = UUID.randomUUID().toString().replaceAll("-", ""); if (set("item_1_lock", value, "NX", "EX", 30)) { try { ... 逻辑 } catch { ... } finally { ... lua 脚本保证原子性 } } if (redis.call('get', KEYS[1]) == ARGV[1]) then return redis.call('del', KEYS[1]) else return 0 end 到这里，我们终于解决了竞争锁的原子性问题和误删锁问题。但是锁一般还需要支持可重入、循环等待和超时自动续约等功能点。下面我们学习使用一个非常好用的包来解决这些问题。 入门 Redisson Redission 的锁，实现了可重入和超时自动续约功能，它都帮我们封装好了，我们只要按照自己的需求调用它的 API 就可以轻松实现上面所提到的几个功能点。详细功能可以查看 Redisson 文档 在项目中安装 Redisson org.redisson redisson 3.13.2 implementation 'org.redisson:redisson:3.13.2' 用 Maven 或者 Gradle 构建，目前最新版本为 3.13.2，也可以在这里 Redisson 找到你需要的版本。 简单尝试 RedissonClient redissonClient = Redisson.create(); RLock lock = redissonClient.getLock("lock"); boolean res = lock.lock(); if (res) { try { ... 逻辑 } finally { lock.unlock(); } } Redisson 将底层逻辑全部做了一个封装 ?，我们无需关心具体实现，几行代码就能使用一把完美的锁。下面我们简单折腾折腾源码 ???。 加锁 private void lock(long leaseTime, TimeUnit unit, boolean interruptibly) throws InterruptedException { long threadId = Thread.currentThread().getId(); Long ttl = tryAcquire(leaseTime, unit, threadId); if (ttl == null) { return; } RFuture future = subscribe(threadId); if (interruptibly) { commandExecutor.syncSubscriptionInterrupted(future); } else { commandExecutor.syncSubscription(future); } try { while (true) { ttl = tryAcquire(leaseTime, unit, threadId); if (ttl == null) { break; } if (ttl >

= 0) {try {future.getNow () .getLatch () .tryAcquire (ttl, TimeUnit.MILLISECONDS);} catch (InterruptedException e) {if (interruptibly) {throw e } future.getNow () .getLatch () .tryAcquire (ttl, TimeUnit.MILLISECONDS);}} else {if (interruptibly) {future.getNow () .getLatch () .acquire () } else {future.getNow () .getLatch () .acquireUninterruptibly ();} finally {unsubscribe (future, threadId);}}

Acquire lock

Private RFuture tryAcquireAsync (long leaseTime, TimeUnit unit, long threadId) {if (leaseTime! =-1) {return tryLockInnerAsync (leaseTime, unit, threadId, RedisCommands.EVAL_LONG);} RFuture ttlRemainingFuture = tryLockInnerAsync (commandExecutor.getConnectionManager (). GetCfg (). GetLockWatchdogTimeout (), TimeUnit.MILLISECONDS, threadId, RedisCommands.EVAL_LONG); ttlRemainingFuture.onComplete ((ttlRemaining, e)-> {if (e! = null) {return) } if (ttlRemaining = = null) {scheduleExpirationRenewal (threadId);}}); return ttlRemainingFuture;} RFuture tryLockInnerAsync (long leaseTime, TimeUnit unit, long threadId, RedisStrictCommand command) {internalLockLeaseTime = unit.toMillis (leaseTime) Return evalWriteAsync (getName (), 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 (getName ()), internalLockLeaseTime, getLockName (threadId);}

Delete lock

Public RFuture unlockAsync (long threadId) {RPromise result = new RedissonPromise (); RFuture future = unlockInnerAsync (threadId); future.onComplete ((opStatus, e)-> {cancelExpirationRenewal (threadId); if (e! = null) {result.tryFailure (e); return } if (opStatus = = null) {IllegalMonitorStateException cause = new IllegalMonitorStateException ("attempt to unlock lock, not locked by current thread by node id:" + id + "thread-id:" + threadId); result.tryFailure (cause); return;} result.trySuccess (null);}; return result } protected RFuture unlockInnerAsync (long threadId) {return evalWriteAsync (getName (), 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 (getName (), getChannelName ()), LockPubSub.UNLOCK_MESSAGE, internalLockLeaseTime, getLockName (threadId));} this is how to implement a distributed lock in the Redis shared by the editor. If you happen to have similar doubts, please refer to the above analysis to understand. If you want to know more about it, you are welcome to follow the industry information channel.

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