【分布式锁】04-使用Redisson实现ReadWriteLock原理
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
<T> RFuture<T> tryLockInnerAsync(long leaseTime, TimeUnit unit, long threadId, RedisStrictCommand<T> 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.<Object>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
<T> RFuture<T> tryLockInnerAsync(long leaseTime, TimeUnit unit, long threadId, RedisStrictCommand<T> 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.<Object>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：
- 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：
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<Boolean> 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.<Object>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<Boolean> 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.<Object>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
本文章首发自本人博客：https://www.cnblogs.com/wang-meng 和公众号：壹枝花算不算浪漫，如若转载请标明来源!
感兴趣的小伙伴可关注个人公众号：壹枝花算不算浪漫
