引子

redis作为一个强大的key/value数据库。事实上还能够用来实现轻量级的分布式锁。

1.实现方案1

最早官方在命令页给了一个实现：

acquire lock: SETNX lock.foo 
     

release lock: DEL lock.foo

acquire lock when time expired: GETSET lock.foo 
     

只是这个方法有漏洞。就是release lock用的DEL命令不支持cas删除（delete if current value equals old value）。这样忽略race condition将会出现故障：

A client will try to release the lock after the expire time deleting the key created by another client that acquired the lock later.

2.实现方案2

官方在命令页介绍了新的方案：SET command + Lua script：

Starting with Redis 2.6.12 it is possible to create a much simpler locking primitive using the  command to acquire the lock, and a simple Lua script to release the lock. The pattern is documented in the  command page.

The old  based pattern is documented below for historical reasons.

该方案有2个优化：

（1）SET 命令能够设置key过期时间：SET key value [EX seconds] [PX milliseconds] [NX|XX]

The lock will be auto-released after the expire time is reached.

（2）使用Lua脚本实现cas删除（详见命令页）

It is possible to make this system more robust modifying the unlock schema as follows:

• Instead of setting a fixed string, set a non-guessable large random string, called token.
• Instead of releasing the lock with , send a script that only removes the key if the value matches.