引子 redis作为一个强大的key/value数据库，其实还可以用来实现轻量级的分布式锁。 1.实现方案1 最早官方在SETNX命令页给了一个实现： acquire lock: SETNX lock.foo current Unix time + lock timeout + 1 release lock: DEL lock.foo acquire lock when ti

引子

redis作为一个强大的key/value数据库，其实还可以用来实现轻量级的分布式锁。

1.实现方案1

最早官方在SETNX命令页给了一个实现：

acquire lock: SETNX lock.foo <current Unix time + lock timeout + 1>

release lock: DEL lock.foo

acquire lock when time expired: GETSET lock.foo <current Unix timestamp + lock timeout + 1>

不过这个方案有漏洞，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将会出现问题：

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SETNX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 SET command to acquire the lock, and a simple Lua script to release the lock. The pattern is documented in the SET command page.

The old SETNX based pattern is documented below for historical reasons.

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该方案有2个优化：

（1）SET 命令可以设置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SET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 DEL, send a script that only removes the key if the value matches.