redis_lock

Installation

gem install redis_lock
require "redis_lock"

Usage

redis = Redis.new

RedisLock.new(redis, "my-awesome-lock-key").lock_for_update do
  this_will_only_happen_if_the_lock_is_acquired
end

RedisLock defaults to 10 retries; you can override by setting the number of retries:

RedisLock.new(redis, "my-awesome-lock-key").retry(5.times).lock_for_update do
  this_will_only_happen_if_the_lock_is_acquired_in_up_to_5_tries
end

RedisLock will raise a RedisLock::LockNotAcquired exception if the lock can't be acquired; you'll want to handle this case in your application.

begin
  RedisLock.new(redis, "my-awesome-lock-key").lock_for_update do
    # lock was acquired
  end
rescue RedisLock::LockNotAcquired
  # couldn't acquire lock!
end

Additionally, RedisLock will raise a RedisLock::UnlockFailure if the lock could not be removed. This could happen if the locking key got removed somehow. This will still execute the code within the lock_for_update call.

begin
  RedisLock.new(redis, "my-awesome-lock-key").lock_for_update do
    # if the lock was acquired, this will always be run
  end
rescue RedisLock::UnlockFailure
  # the lock key went away
end

If you want to lock and unlock manually outside of the context of a block, you can call lock and unlock explicitly.

class SomethingAwesome
  before_save :lock_it_up
  after_save  :break_the_lock

  private

  def locker
    @locker ||= RedisLock.new($redis, "my-awesome-lock-key")
  end

  def lock_it_up
    begin
      locker.lock
    rescue RedisLock::LockNotAcquired
      false
    end
  end

  def break_the_lock
    begin
      locker.unlock
    rescue RedisLock::UnlockFailure
    end
  end
end

Additional Notes

This gem basically implements the algorithm described here: http://redis.io/commands/setnx

Author

Patrick Tulskie; http://patricktulskie.com