【分布式锁】04-使用Redisson实现ReadWriteLock原理

2012 年 10 月 24 日

前言

关于读写锁，大家应该都了解JDK中的 ReadWriteLock ，当然Redisson也有读写锁的实现。

所谓读写锁，就是多个客户端同时加读锁，是不会互斥的，多个客户端可以同时加这个读锁，读锁和读锁是不互斥的

Redisson中使用 RedissonReadWriteLock 来实现读写锁，它是 RReadWriteLock 的子类，具体实现读写锁的类分别是： RedissonReadLock 和 RedissonWriteLock

Redisson读写锁使用例子

还是从官方文档中找的使用案例：

RReadWriteLock rwlock = redisson.getReadWriteLock("tryLock");

RLock lock = rwlock.readLock();
// or
RLock lock = rwlock.writeLock();

// traditional lock method
lock.lock();

// or acquire lock and automatically unlock it after 10 seconds
lock.lock(10, TimeUnit.SECONDS);

// or wait for lock aquisition up to 100 seconds 
// and automatically unlock it after 10 seconds
boolean res = lock.tryLock(100, 10, TimeUnit.SECONDS);
if (res) {
   try {
     ...
   } finally {
       lock.unlock();
   }
}

Redisson加读锁逻辑原理

public class RedissonReadLock extends RedissonLock implements RLock {
    @Override
     RFuture tryLockInnerAsync(long leaseTime, TimeUnit unit, long threadId, RedisStrictCommand command) {
        internalLockLeaseTime = unit.toMillis(leaseTime);

        return commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, command,
                                "local mode = redis.call('hget', KEYS[1], 'mode'); " +
                                "if (mode == false) then " +
                                  "redis.call('hset', KEYS[1], 'mode', 'read'); " +
                                  "redis.call('hset', KEYS[1], ARGV[2], 1); " +
                                  "redis.call('set', KEYS[2] .. ':1', 1); " +
                                  "redis.call('pexpire', KEYS[2] .. ':1', ARGV[1]); " +
                                  "redis.call('pexpire', KEYS[1], ARGV[1]); " +
                                  "return nil; " +
                                "end; " +
                                "if (mode == 'read') or (mode == 'write' and redis.call('hexists', KEYS[1], ARGV[3]) == 1) then " +
                                  "local ind = redis.call('hincrby', KEYS[1], ARGV[2], 1); " + 
                                  "local key = KEYS[2] .. ':' .. ind;" +
                                  "redis.call('set', key, 1); " +
                                  "redis.call('pexpire', key, ARGV[1]); " +
                                  "redis.call('pexpire', KEYS[1], ARGV[1]); " +
                                  "return nil; " +
                                "end;" +
                                "return redis.call('pttl', KEYS[1]);",
                        Arrays.asList(getName(), getReadWriteTimeoutNamePrefix(threadId)), 
                        internalLockLeaseTime, getLockName(threadId), getWriteLockName(threadId));
    }
}

客户端A（UUID_01:threadId_01）来加读锁

注：

以下文章中客户端A用： UUID_01:threadId_01标识

客户端B用： UUID_02:threadId_02标识

KEYS：

KEYS1: getName() = tryLock
KEYS[2]: getReadWriteTimeoutNamePrefix(threadId) = {anyLock}:UUID_01:threadId_01:rwlock_timeout

ARGV：

ARGV1: internalLockLeaseTime = 30000毫秒
ARGV[2]: getLockName(threadId) = UUID_01:threadId_01
ARGV[3]: getWriteLockName(threadId) = UUID_01:threadId_01:write

接着对代码中lua脚本一行行解读:

hget anyLock mode 第一次加锁时是空的
mode = false，进入if逻辑
hset anyLock UUID_01:threadId_01 1
anyLock是hash结构，设置hash的key、value
set {anyLock}:UUID_01:threadId_01:rwlock_timeout:1 1
设置一个string类型的key value数据
pexpire {anyLock}:UUID_01:threadId_01:rwlock_timeout:1 30000
设置key value的过期时间
pexpire anyLock 30000
设置anyLock的过期时间

此时redis中存在的数据结构为：

anyLock: {
  "mode": "read",
  "UUID_01:threadId_01": 1
}

{anyLock}:UUID_01:threadId_01:rwlock_timeout:1  1

客户端A 第二次来加读锁

继续分析，客户端A已经加过读锁，此时如果继续加读锁会怎样处理呢？

hget anyLock mode 此时mode=read，会进入第二个if判断
hincrby anyLock UUID_01:threadId_01 1 此时hash中的value会加1，变成2
set {anyLock}:UUID_01:threadId_01:rwlock_timeout:2 1
ind 为hincrby结果，hincrby返回是2
pexpire anyLock 30000
pexpire {anyLock}:UUID_01:threadId_01:rwlock_timeout:2 30000

此时redis中存在的数据结构为：

anyLock: {
  “mode”: “read”,
  “UUID_01:threadId_01”: 2
}

{anyLock}:UUID_01:threadId_01:rwlock_timeout:1  1
{anyLock}:UUID_01:threadId_01:rwlock_timeout:2  1

客户端B （UUID_02:threadId_02）第一次来加读锁

基本步骤和上面一直，加锁后redis中数据为：

anyLock: {
  "mode": "read",
  "UUID_01:threadId_01": 2,
  "UUID_02:threadId_02": 1
}

{anyLock}:UUID_01:threadId_01:rwlock_timeout:1  1
{anyLock}:UUID_01:threadId_01:rwlock_timeout:2  1
{anyLock}:UUID_02:threadId_02:rwlock_timeout:1  1

这里需要注意一下：

为哈希表 key 中的域 field 的值加上增量 increment，如果 key 不存在，一个新的哈希表被创建并执行 HINCRBY 命令。

Redisson加写锁逻辑原理

Redisson中由 RedissonWriteLock 来实现写锁，我们看下写锁的核心逻辑：

public class RedissonWriteLock extends RedissonLock implements RLock {
    @Override
     RFuture tryLockInnerAsync(long leaseTime, TimeUnit unit, long threadId, RedisStrictCommand command) {
        internalLockLeaseTime = unit.toMillis(leaseTime);

        return commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, command,
                            "local mode = redis.call('hget', KEYS[1], 'mode'); " +
                            "if (mode == false) then " +
                                  "redis.call('hset', KEYS[1], 'mode', 'write'); " +
                                  "redis.call('hset', KEYS[1], ARGV[2], 1); " +
                                  "redis.call('pexpire', KEYS[1], ARGV[1]); " +
                                  "return nil; " +
                              "end; " +
                              "if (mode == 'write') then " +
                                  "if (redis.call('hexists', KEYS[1], ARGV[2]) == 1) then " +
                                      "redis.call('hincrby', KEYS[1], ARGV[2], 1); " + 
                                      "local currentExpire = redis.call('pttl', KEYS[1]); " +
                                      "redis.call('pexpire', KEYS[1], currentExpire + ARGV[1]); " +
                                      "return nil; " +
                                  "end; " +
                                "end;" +
                                "return redis.call('pttl', KEYS[1]);",
                        Arrays.asList(getName()), 
                        internalLockLeaseTime, getLockName(threadId));
    }
}

还是像上面一样，一行行来分析每句lua脚本执行语义。

客户端A先加读写、再加写锁

KEYS和ARGV参数：

KEYS1 = anyLock
ARGV1 = 30000
ARGV[2] = UUID_01:threadId_01:write

hget anyLock mode，此时没人加锁，mode=false
hset anyLock mode write
hset anyLock UUID_01:threadId_01:write 1
pexpire anyLock 30000

此时redis中数据格式为：

anyLock: {
    "mode": "write",
    "UUID_01:threadId_01:write": 1
}

此时再次来加写锁，直接到另一个if语句中：

hexists anyLock UUID_01:threadId_01:write
hincrby anyLock UUID_01:threadId_01:write 1
pexpire anyLock pttl + 30000

此时redis中数据格式为：

anyLock: {
    "mode": "write",
    "UUID_01:threadId_01:write": 2
}

客户端A和客户端B，先后加读锁，客户端C来加写锁

读锁加完后，此时redis数据格式为：

anyLock: {
  "mode": "read",
  "UUID_01:threadId_01": 1,
  "UUID_02:threadId_02": 1
}

{anyLock}:UUID_01:threadId_01:rwlock_timeout:1    1
{anyLock}:UUID_02:threadId_02:rwlock_timeout:1    1

客户端C参数为：

KEYS1 = anyLock
ARGV1 = 30000
ARGV[2] = UUID_03:threadId_03:write

hget anyLock mode，mode = read，已经有人加了读锁，不是写锁，此时会直接执行：pttl

anyLock，返回一个anyLock的剩余生存时间

hget anyLock mode，mode = read，已经有人加了读锁，不是写锁，所以if语句不会成立
pttl anyLock，返回一个anyLock的剩余生存时间

客户端C加锁失败，就会不断的尝试重试去加锁

客户端A先加写锁、客户端B接着加读锁

加完写锁后此时Redis数据格式为：

anyLock: {
  "mode": "write",
  "UUID_01:threadId_01:write": 1
}

客户端B执行读锁逻辑参数为：

KEYS1 = anyLock
KEYS[2] = {anyLock}:UUID_02:threadId_02:rwlock_timeout
ARGV1 = 30000毫秒
ARGV[2] = UUID_02:threadId_02
ARGV[3] = UUID_02:threadId_02:write

接着看下加锁逻辑：

image.png

如上图，客户端B加读锁会走到红框中的if逻辑：

hget anyLock mode，mode = write
客户端A已经加了一个写锁
hexists anyLock UUID_02:threadId_02:write，存在的话，如果客户端B自己之前加过写锁的话，此时才能进入这个分支
返回pttl anyLock，导致加锁失败

客户端A先加写锁、客户端A接着加读锁

还是接着上面的逻辑，继续分析：

hget anyLock mode，mode = write
客户端A已经加了一个写锁
hexists anyLock UUID_01:threadId_01:write，此时存在这个key，所以可以进入if分支
hincrby anyLock UUID_01:threadId_01 1，也就是说此时，加了一个读锁
set {anyLock}:UUID_01:threadId_01:rwlock_timeout:1 1,
pexpire anyLock 30000
pexpire {anyLock}:UUID_01:threadId_01:rwlock_timeout:1 30000

此时redis中数据格式为：

anyLock: {
  "mode": "write",
  "UUID_01:threadId_01:write": 1,
  "UUID_01:threadId_01": 1
}

{anyLock}:UUID_01:threadId_01:rwlock_timeout:1    1

客户端A先加读锁、客户端A接着加写锁

客户端A加读锁后，redis中数据结构为：

anyLock: {
  "mode": "read",
  "UUID_01:threadId_01": 1
}

{anyLock}:UUID_01:threadId_01:rwlock_timeout:1  1

此时客户端A再来加写锁，逻辑如下：

image.png

此时客户端A先加的读锁，mode=read，所以再次加写锁是不能成功的

如果是同一个客户端同一个线程，先加了一次写锁，然后加读锁，是可以加成功的，默认是在同一个线程写锁的期间，可以多次加读锁

而同一个客户端同一个线程，先加了一次读锁，是不允许再被加写锁的

总结

显然还有写锁与写锁互斥的逻辑就不分析了，通过上面一些场景的分析，我们可以知道：

读锁与读锁非互斥
读锁与写锁互斥
写锁与写锁互斥
读读、写写同个客户端同个线程都可重入
先写锁再加读锁可重入
先读锁再写锁不可重入

Redisson读写锁释放原理

Redission 读锁释放原理

不同客户端加了读锁 / 同一个客户端+线程多次可重入加了读锁

例如客户端A先加读锁，然后再次加读锁

最后客户端B来加读锁

此时Redis中数据格式为：

anyLock: {
  "mode": "read",
  "UUID_01:threadId_01": 2,
  "UUID_02:threadId_02": 1
}

{anyLock}:UUID_01:threadId_01:rwlock_timeout:1        1
{anyLock}:UUID_01:threadId_01:rwlock_timeout:2        1
{anyLock}:UUID_02:threadId_02:rwlock_timeout:1        1

接着我们看下释放锁的核心代码：

public class RedissonReadLock extends RedissonLock implements RLock {
    @Override
    protected RFuture unlockInnerAsync(long threadId) {
        String timeoutPrefix = getReadWriteTimeoutNamePrefix(threadId);
        String keyPrefix = getKeyPrefix(threadId, timeoutPrefix);

        return commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,
                "local mode = redis.call('hget', KEYS[1], 'mode'); " +
                "if (mode == false) then " +
                    "redis.call('publish', KEYS[2], ARGV[1]); " +
                    "return 1; " +
                "end; " +
                "local lockExists = redis.call('hexists', KEYS[1], ARGV[2]); " +
                "if (lockExists == 0) then " +
                    "return nil;" +
                "end; " +

                "local counter = redis.call('hincrby', KEYS[1], ARGV[2], -1); " + 
                "if (counter == 0) then " +
                    "redis.call('hdel', KEYS[1], ARGV[2]); " + 
                "end;" +
                "redis.call('del', KEYS[3] .. ':' .. (counter+1)); " +

                "if (redis.call('hlen', KEYS[1]) > 1) then " +
                    "local maxRemainTime = -3; " + 
                    "local keys = redis.call('hkeys', KEYS[1]); " + 
                    "for n, key in ipairs(keys) do " + 
                        "counter = tonumber(redis.call('hget', KEYS[1], key)); " + 
                        "if type(counter) == 'number' then " + 
                            "for i=counter, 1, -1 do " + 
                                "local remainTime = redis.call('pttl', KEYS[4] .. ':' .. key .. ':rwlock_timeout:' .. i); " + 
                                "maxRemainTime = math.max(remainTime, maxRemainTime);" + 
                            "end; " + 
                        "end; " + 
                    "end; " +

                    "if maxRemainTime > 0 then " +
                        "redis.call('pexpire', KEYS[1], maxRemainTime); " +
                        "return 0; " +
                    "end;" + 

                    "if mode == 'write' then " + 
                        "return 0;" + 
                    "end; " +
                "end; " +

                "redis.call('del', KEYS[1]); " +
                "redis.call('publish', KEYS[2], ARGV[1]); " +
                "return 1; ",
                Arrays.asList(getName(), getChannelName(), timeoutPrefix, keyPrefix), 
                LockPubSub.unlockMessage, getLockName(threadId));
    }
}

客户端A来释放锁：

对应的KEYS和ARGV参数为：

KEYS1 = anyLock
KEYS[2] = redisson_rwlock:{anyLock}
KEYS[3] = {anyLock}:UUID_01:threadId_01:rwlock_timeout
KEYS[4] = {anyLock}
ARGV1 = 0
ARGV[2] = UUID_01:threadId_01

接下来开始执行操作：

hget anyLock mode，mode = read
hexists anyLock UUID_01:threadId_01，肯定是存在的，因为这个客户端A加过读锁
hincrby anyLock UUID_01:threadId_01 -1，将这个客户端对应的加锁次数递减1，现在就是变成1，counter = 1
del {anyLock}:UUID_01:threadId_01:rwlock_timeout:2，删除了一个timeout key

此时Redis中的数据结构为：

anyLock: {
  "mode": "read",
  "UUID_01:threadId_01": 1,
  "UUID_02:threadId_02": 1
}

{anyLock}:UUID_01:threadId_01:rwlock_timeout:1    1
{anyLock}:UUID_02:threadId_02:rwlock_timeout:1    1

此时继续往下，具体逻辑如图：

image.png

hlen anyLock > 1，就是hash里面的元素超过1个
pttl {anyLock}:UUID_01:threadId_01:rwlock_timeout:1，此时获取那个timeout key的剩余生存时间还有多少毫秒，比如说此时这个key的剩余生存时间是20000毫秒

这个for循环的含义是获取到了所有的timeout key的最大的一个剩余生存时间，假设最大的剩余生存时间是25000毫秒

客户端A继续来释放锁：

此时客户端A执行流程还会和上面一直，执行完成后Redis中数据结构为：

anyLock: {
  "mode": "read",
  "UUID_02:threadId_02": 1
}

{anyLock}:UUID_02:threadId_02:rwlock_timeout:1    1

因为这里会走 counter == 0 的逻辑，所以会执行 "redis.call('hdel', KEYS[1], ARGV[2]); "

客户端B继续来释放锁：

客户端B流程也和上面一直，执行完后就会删除anyLock这个key

同一个客户端/线程先加写锁再加读锁

上面已经分析过这种情形，操作过后Redis中数据结构为：

anyLock: {
  "mode": "write",
  "UUID_01:threadId_01:write": 1,
  "UUID_01:threadId_01": 1
}

{anyLock}:UUID_01:threadId_01:rwlock_timeout:1    1

此时客户端A来释放读锁：

hincrby anyLock UUID_01:threadId_01 -1，将这个客户端对应的加锁次数递减1，现在就是变成1，counter = 0
hdel anyLock UUID_01:threadId_01，此时就是从hash数据结构中删除客户端A这个加锁的记录
del {anyLock}:UUID_01:threadId_01:rwlock_timeout:1，删除了一个timeout key

此时Redis中数据变成：

anyLock: {
  "mode": "write",
  "UUID_01:threadId_01:write": 1
}

Redisson写锁释放原理

先看下写锁释放的核心逻辑：

public class RedissonWriteLock extends RedissonLock implements RLock {
    @Override
    protected RFuture unlockInnerAsync(long threadId) {
        return commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,
                "local mode = redis.call('hget', KEYS[1], 'mode'); " +
                "if (mode == false) then " +
                    "redis.call('publish', KEYS[2], ARGV[1]); " +
                    "return 1; " +
                "end;" +
                "if (mode == 'write') then " +
                    "local lockExists = redis.call('hexists', KEYS[1], ARGV[3]); " +
                    "if (lockExists == 0) then " +
                        "return nil;" +
                    "else " +
                        "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('hdel', KEYS[1], ARGV[3]); " +
                            "if (redis.call('hlen', KEYS[1]) == 1) then " +
                                "redis.call('del', KEYS[1]); " +
                                "redis.call('publish', KEYS[2], ARGV[1]); " + 
                            "else " +
                                // has unlocked read-locks
                                "redis.call('hset', KEYS[1], 'mode', 'read'); " +
                            "end; " +
                            "return 1; "+
                        "end; " +
                    "end; " +
                "end; "
                + "return nil;",
        Arrays.asList(getName(), getChannelName()), 
        LockPubSub.unlockMessage, internalLockLeaseTime, getLockName(threadId));
    }
}

同一个客户端多次可重入加写锁 / 同一个客户端先加写锁再加读锁

客户端A加两次写锁释放：

此时Redis中数据为：

anyLock: {
  "mode": "write",
  "UUID_01:threadId_01:write": 2,
  "UUID_01:threadId_01": 1
}

{anyLock}:UUID_01:threadId_01:rwlock_timeout:1    1

客户端A来释放锁KEYS和ARGV参数：

KEYS1 = anyLock
KEYS[2] = redisson_rwlock:{anyLock}
ARGV1 = 0
ARGV[2] = 30000
ARGV[3] = UUID_01:threadId_01:write

直接分析lua代码：

上面mode=write，后面使用hincrby进行-1操作，此时count=1
如果count>0，此时使用pexpire然后返回0
此时客户端A再来释放写锁，count=0
hdel anyLock UUID_01:threadId_01:write

此时Redis中数据：

anyLock: {
  "mode": "write",
  "UUID_01:threadId_01": 1
}

{anyLock}:UUID_01:threadId_01:rwlock_timeout:1    1

后续还会接着判断，如果count=0，代表写锁都已经释放完了，此时hlen如果>1，代表加的还有读锁，所以接着执行： hset anyLock mode read , 将写锁转换为读锁

最终Redis数据为：

anyLock: {
  "mode": "read",
  "UUID_01:threadId_01": 1
}

{anyLock}:UUID_01:threadId_01:rwlock_timeout:1    1

总结

Redisson陆续也更新了好几篇了，疫情期间宅在家里一直学习Redisson相关内容，这篇文章写了2天，从早到晚。

读写锁这块内容真的很多，本篇篇幅很长，如果学习本篇文章最好跟着源码一起读，后续还会继续更新Redisson相关内容，如有不正确的地方，欢迎指正！

申明

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前言

Redisson读写锁使用例子

Redisson加读锁逻辑原理

客户端A（UUID_01:threadId_01）来加读锁

KEYS：

ARGV：

客户端A 第二次来加读锁

客户端B （UUID_02:threadId_02）第一次来加读锁

Redisson加写锁逻辑原理

客户端A先加读写、再加写锁

客户端A和客户端B，先后加读锁，客户端C来加写锁

客户端A先加写锁、客户端B接着加读锁

image.png

客户端A先加写锁、客户端A接着加读锁

客户端A先加读锁、客户端A接着加写锁

image.png

总结

Redisson读写锁释放原理

Redission 读锁释放原理

不同客户端加了读锁 / 同一个客户端+线程多次可重入加了读锁

image.png

客户端A继续来释放锁：

客户端B继续来释放锁：

同一个客户端/线程先加写锁再加读锁

Redisson写锁释放原理

同一个客户端多次可重入加写锁 / 同一个客户端先加写锁再加读锁

客户端A加两次写锁释放：

总结

申明

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