uncomplicated_mutex/README.md

Uncomplicated Mutex

A transactional mutex based in Redis for Ruby. It works across processes, threads, and machines provided you can all access the same Redis server.

This is subtely different from two related projects here on Github: https://github.com/kenn/redis-mutex and https://github.com/dv/redis-semaphore

RedisMutex uses a polling interval and raises an exception on lock timeout. RedisSemaphore uses blpop to wait until a value is pushed into a list. I have noticed several issues with the two approaches. I have had jobs continue to run for days with RedisSemaphore due to random and unknown failures. RedisMutex raises an exception, and in my situation, I do not need an exception, I would prefer to assume that the previous job has failed and that execution should continue.

Major Features

• Uses your own Redis or Redis::Namespace instance
• Uses Redis SET using Lua transactions based on documentation from http://redis.io/commands/SET for a simple locking pattern.
• It sleeps a random amount less than one second until such time that the timed out time has been met.
• When many processes are struggling to get a single lock, if a different process has taken control of the lock when the timeout is met, the timeout is automatically reset. This allows differentiation between a stuck lock and locks with lots of processes struggling to get the lock.
• It pessimistically fails by raising an exception if the timeout has been met.
• A lock will not overwrite a value in Redis if the value was changed from the lock's "secret" token.
• It requires that objects to be locked respond to the method id.

I took many feathers from the cap of Martin Fowler when I wrote this gem. Once initialized, variables contents never change. Methods are not longer than 10 lines. Method names are very specific yet not too long. Methods are alphabetized in the class definition (except the initializer). Tests are included.

Usage

A number of options are available on initialization:

Option	Default value	Description
redis	Redis.new Redis connection to use
ticks	100	Number of times to wait during the timeout period
timeout	300	Time, in seconds, to wait until lock is considered stale
verbose	false	Prints debugging statements

mutex = UncomplicatedMutex.new(my_obj, opts)

mutex.lock do
  my_obj.long_synchronized_process
end

This pattern works very well in Sidekiq or Resque. Also, if you need to access the name of the lock for your own value checking, you may ask the mutex lock_name to get the actual Redis key of the lock.

Locking Algorithm

• Set value in Redis if it does not exist.
• If it exists, wait until :timeout has been met.
• If a different process still holds the lock, update the secret token and reset the timeout.
• If the original process still holds the lock, throw an exception.
• Run block of code
• Release lock if it contains the same value that it was set to