Ruby - Redis based mutex with expiration implementation

Question

I'm trying to implement a memory based, multi process shared mutex, which supports timeout, using Redis. I need the mutex to be non-blocking, meaning that I just need to be able to know if I was able to fetch the mutex or not, and if not - simply continue with execution of fallback code. something along these lines: ``` if lock('my_lock_key', timeout: 1.minute) # Do some job else # exit end ``` An un-expiring mutex could be implemented using redis's `setnx mutex 1`: ``` if redis.setnx('#{mutex}', '1') # Do some job redis.delete('#{mutex}') else # exit end ``` But what if I need a mutex with a timeout mechanism (In order to avoid a situation where the ruby code fails before the `redis.delete` command, resulting the mutex being locked forever, for example, but not for this reason only). Doing something like this obviously doesn't work: ``` redis.multi do redis.setnx('#{mutex}', '1') redis.expire('#{mutex}', key_timeout) end ``` since I'm re-setting an expiration to the mutex EVEN if I w…

Accepted Answer

If you're using Redis 2.6+, you can do this much more simply with the Lua scripting engine. The Redis documentation says:

A Redis script is transactional by definition, so everything you can do with a Redis transaction, you can also do with a script, and usually the script will be both simpler and faster.

Implementing it is trivial:

```
LUA_ACQUIRE = "return redis.call('setnx', KEYS[1], 1) == 1 and redis.call('expire', KEYS[1], KEYS[2]) and 1 or 0"
def lock(key, timeout = 3600)
  if redis.eval(LUA_ACQUIRE, key, timeout) == 1
    begin
      yield
    ensure
      redis.del key
    end
  end
end
```

Usage:

```
lock("somejob") { do_exclusive_job }
```

Ruby - Redis based mutex with expiration implementation

Solution

Problem