gaspar.rb

require 'rufus/scheduler'
class CronManager
  def self.schedule(redis, &block)
    @singleton ||= new(redis, &block)
  end

  def every(timing, options = {}, &block)
    # In order to make sure that jobs are executed at the same time regardless of who runs them
    # we quantitize the start time to the next-nearest time slice. This more closely emulates
    # cron-style behavior.
    #
    # This does assume that your system dates are in sync.
    seconds = Rufus.parse_duration_string(timing)
    now = Time.now.to_i
    start_at = Time.at( now + (seconds - (now % seconds)) )
    options = options.merge(:first_at => start_at)

    schedule :every, timing, options, &block
  end

  def cron(timing, options = {}, &block)
    schedule :cron, timing, options, &block
  end

  private

  def schedule(method, timing, options, &block)
    name = options.delete :name
    key = "cron:%s-%s" % [timing, name || Digest::SHA1.hexdigest(block.to_ruby)]

    @scheduler.send method, timing, options do
      # If we can acquire a lock...
      if @redis.setnx key
        # ...set the lock to expire in 95% of the duration's time, to account for system time drift
        @redis.expire key, (seconds * 0.95).floor
        # ...and then run the job
        block.call
      end
    end
  end

  def initialize(redis, &block)
    @redis = redis
    @scheduler = Rufus::Scheduler.start_new
    instance_eval &block
  end
end