如何用redis實現分佈式鎖

引子

redis作爲一個強大的key/value數據庫，其實還可以用來實現輕量級的分佈式鎖。

1.實現方案1

最早官方在SETNX命令頁給了一個實現：

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>

不過這個方案有漏洞，就是release lock用的DEL命令不支持cas刪除（delete if current value equals old value），這樣忽略race condition將會出現問題：

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.實現方案2

官方在SETNX命令頁介紹了新的方案：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.

該方案有2個優化：

（1）SET 命令可以設置key過期時間：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腳本實現cas刪除（詳見SET命令頁）

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.