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
release lock: DEL lock.foo
acquire lock when time expired: GETSET lock.foo
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.
SOAP、WSDL(WebServicesDescriptionLanguage)、UDDI(UniversalDescriptionDiscovery andIntegration)之一， soap用来描述传递信息的格式， WSDL 用来描述如何访问具体的接口， uddi用来管理，分发，查询webService 。具体实现可以搜索 Web Services简单实例 ; SOAP 可以和现存的许多因特网协议和格式结合使用，包括超文本传输协议（HTTP），简单邮件传输协议（SMTP），多用途网际邮件扩充协议
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.
