redis的分布式锁

引子

redis是一个很强大的数据结构存储的nosql数据库，很方便针对业务模型进行效率的优化。最近我的工作是负责对现有Java服务器框架进行整理，并将网络层与逻辑层脱离，以便于逻辑层和网络层的横向扩展。尽管我在逻辑层上使用了AKKA作为核心框架，尽可能lockfree，但是还是免不了需要跨jvm的锁。所以我需要实现一个分布式锁。

官方的实现

官方在SETNX 这一页给了一个实现。

C4 sends SETNX lock.foo in order to acquire the lock
The crashed client C3 still holds it, so Redis will reply with 0 to C4.
C4 sends GET lock.foo to check if the lock expired. If it is not, it will sleep for some time and retry from the start.
Instead, if the lock is expired because the Unix time at lock.foo is older than the current Unix time, C4 tries to perform: GETSET lock.foo (current Unix timestamp + lock timeout + 1)
Because of the GETSET semantic, C4 can check if the old value stored at key is still an expired timestamp. If it is, the lock was acquired.
If another client, for instance C5, was faster than C4 and acquired the lock with the GETSET operation, the C4 GETSET operation will return a non expired timestamp. C4 will simply restart from the first step. Note that even if C4 set the key a bit a few seconds in the future this is not a problem.

但是使用官方推荐的getset实现的话，未竞争到锁的一方确实可以判断到自己未能竞争到锁，但却将持有锁一方的时间修改了，这样的直接后果就是，持有锁的一方无法解锁！！！

基于lua的实现

其实官方实现出现的问题，是因为使用redis独立的命令不能将get-check-set这个过程进行原子化，所以我决定引入redis-lua，将get-check-set这个过程使用lua脚本来实现。

加锁：

script params: lock_key, current_timestamp, lock_timeout
setnx lock_key (current_timestamp + lock_timeout). if not success, set lock_key (current_timestamp + lock_timeout) if current_timestamp > value
client save current_timestamp(lock_create_timestamp)

解锁：

script params: lock_key, lock_create_timestamp, lock_timeout
delete if lock_create_timestamp + lock_timeout == value

具体的实现:

---lock

local now = tonumber(ARGV[1])
local timeout = tonumber(ARGV[2])
local to = now + timeout
local locked = redis.call('SETNX', KEYS[1], to)
if (locked == 1) then
    return 0
end
local kt = redis.call('type', KEYS[1]);
if (kt['ok'] ~= 'string') then
    return 2
end
local keyValue = tonumber(redis.call('get', KEYS[1]))
if (now > keyValue) then
    redis.call('set', KEYS[1], to)
    return 0
end
return 1

---unlock

local begin = tonumber(ARGV[1])
local timeout = tonumber(ARGV[2])
local kt = redis.call('type', KEYS[1]);
if (kt['ok'] == 'string') then
    local keyValue = tonumber(redis.call('get', KEYS[1]))
    if ((keyValue - begin) == timeout) then
        redis.call('del', KEYS[1])
        return 0
    end
end
return 1