Rescue Scheduler is an extension to Resque that provides both scheduled and delayed jobs. This service can replace cron and bring the ability to schedule task to execute at a certain time.

This tutorial is based off the previous tutorial on using Resque with Redis To Go. We will be using the same cookie_monster application.

Before we begin you should be aware of the difference between a scheduled job and a delayed job. A scheduled job is a task that is specified to be run at a certain time. For example you may want to update any indexes at midnight everyday; this is a scheduled job. A delayed job is a job that gets put on the queue with a specified time to run at. For example you may want to schedule a followup email to go out a few days after a user has signed up.

Now lets get started.

Setup

First check out the cookie_monster app from github.

$ git clone git://github.com/waratuman/cookie-monster.git
$ cd cookie-monster

Install Resque Scheduler

Modify the Gemfile to include resque-scheduler.

gem 'resque-scheduler'

Install dependencies.

$ bundle install

Configure Resque Scheduler

Resque Scheduler run as a process. To run it we will end up calling rake resque:scheduler. Before that can happen the lib/tasks/resque.rake file will need to be updated to load these task. Added the following line to the file:

require 'resque_scheduler/tasks'

Now you should be able to run rake resque:scheduler. Since the crontab for Resque Scheduler has not been set the process will schedule any jobs. It will however look for delayed jobs that need to be run.

Lets go ahead and create the cron tab for Resque Scheduler. Create the file config/resque_schedule.yml and insert the following text:

breakfast:
  cron: 0 8 * * *
  class: Eat
  args: scrambled egg
  description: Breakfast
lunch:
  cron: 0 12 * * *
  class: Eat
  args: penut-butter and jelly sandwich
  description: Lunch
dinner:
  cron: 0 6 * * *
  class: Eat
  args: Steak
  description: Dinner
midnight_snack:
  cron: 0 0 * * *
  class: Eat
  args: Milk & Cookies
  description: Midnight Snack

We still need to load the schedule when we initialize Resque. To do this open config/initializers/resque.rb and add the following lines.

require 'resque_scheduler'
Resque.schedule = YAML.load_file("#{Rails.root}/config/resque_schedule.yml")

Start the Scheduler Process

All that is left is to run the scheduler process.

$ rake resque:scheduler

Note that if the scheduler process is not running when a scheduled job needs to be enqueued, it will not be put on the queue once the scheduler process is running again. In some cases this may not be an issue, it may be fine for cookie-monster to miss breakfast on occasion. FlightCaster needed jobs to be run every minute. The system was build so that if one job was missed the next one would pick up the work. An alternative solution would have been to have a schedule job put delayed jobs on the queue to run at the specified times. Then if the scheduler process died the jobs would be performed once the process was up again.

Delayed Jobs

To schedule a job to take place after a certain amount of time is almost as easy as enqueuing a job. Simple specify the time you want to have the Job run at.

Resque.enqueue_at(2.days.from_now, Eat, 'leftovers')

Web Additions

Resque scheduler also extends the Resque web UI, so you get a great looking interface from which you can see delayed jobs and scheduled jobs.

Conclusion

Rescue Scheduler offers a simple and easy way to added scheduling abilities to any queueing system and is well worth the move from cron.