queued_mutex.rb

def redis_queued_mutex(type, key, options ={})

  # our tests are single threaded so
  if Rails.env.test?
    yield
  else

    # define the maximum number of times we will wait to prevent starvation
    max_waits = options[:max_waits] || 500
    # amount of time we will sleep waiting for the mutex to clear
    sleep_for = (options[:sleep_for] || rand(100..200)/1000.0)

    # In the locks namespace
    with_redis('queued_locks') do |redis|

      id = SecureRandom.urlsafe_base64(12)
      # key for this lock
      key = "#{type}##{key}"

      waits = 0

      redis.lpush(key, id)

      until redis.lindex(0) == id
        sleep sleep_for
        waits += 1

        if waits == max_waits
          raise "Exceeded the maximum number of waits for lock #{type}:#{key}"
        end
      end

      begin

        # this is where the mutex code is actually run
        yield

      ensure
        # no matter what happens lets pop out our entry
        redis.lpop(key, id)

        # sleep just incase we are in a loop or something, allowing other processes to
        # claim the lock
        sleep sleep_for
      end
    end
  end
end