Whenever is a Ruby gem that provides a clear syntax for defining cron jobs. It outputs valid cron syntax and can even write your crontab file for you. It is designed to work well with Rails applications and can be deployed with Capistrano. Whenever works fine independently as well.

Regular (non-Rails) install:

$ gem sources -a http://gems.github.com  #you only need to run this once
$ sudo gem install javan-whenever

In a Rails (2.1 or greater) application:

in your “config/environment.rb” file:

Rails::Initializer.run do |config|
  config.gem 'javan-whenever', :lib => false, :source => 'http://gems.github.com'
end

To install this gem (and all other missing gem dependencies), run rake gems:install (use sudo if necessary).

In older versions of Rails:

$ gem sources -a http://gems.github.com  #you only need to run this once
$ gem install javan-whenever

in your “config/environment.rb” file:

Rails::Initializer.run do |config|
  ...
end

require 'whenever'

NOTE: Requiring the whenever gem inside your Rails application is technically optional. However, if you plan to use something like Capistrano to automatically deploy and write your crontab file, you’ll need to have the gem installed on your servers, and requiring it in your app is one way to ensure this.

$ cd /my/rails/app
$ wheneverize .

This will create an initial “config/schedule.rb” file you.

every 3.hours do
  runner "MyModel.some_process"       
  rake "my:rake:task"                 
  command "/usr/bin/my_great_command"
end

every 1.day, :at => '4:30 am' do 
  runner "MyModel.task_to_run_at_four_thirty_in_the_morning"
end

every :hour do # Many shortcuts available: :hour, :day, :month, :year, :reboot
  runner "SomeModel.ladeeda"
end

every :sunday, :at => '12pm' do # Use any day of the week or :weekend, :weekday
  runner "Task.do_something_great"
end

More examples on the wiki: wiki.github.com/javan/whenever/instructions-and-examples

$ cd /my/rails/app
$ whenever

And you’ll see your schedule.rb converted to cron sytax

In your “config/deploy.rb” file do something like:

after "deploy:symlink", "deploy:update_crontab"

namespace :deploy do
  desc "Update the crontab file"
  task :update_crontab, :roles => :db do
    run "cd #{release_path} && whenever --update-crontab #{application}"
  end
end

This will update your crontab file, leaving any existing entries unharmed. When using the --update-crontab option, Whenever will only update the entries in your crontab file related to the current schedule.rb file. You can replace the #{application} with any identifying string you’d like. You can have any number of apps deploy to the same crontab file peacefully given they each use a different identifier.

If you wish to simply overwrite your crontab file each time you deploy, use the --write-crontab option. This is ideal if you are only working with one app and every crontab entry is contained in a single schedule.rb file.

By mixing and matching the --load-file and --user options with your various :roles in Capistrano it is entirely possible to deploy different crontab schedules under different users to all your various servers. Get creative!

Added a ScheduledJob class (the Job namespace is taken).

Added methods on JobList to: compose ScheduledJob objects, fetch the whenever syntax schedule for a given task.

ScheduledJob is similar to the @jobs hash that JobList holds, with the main difference being that it holds its own time/frequency data.

The overall approach of ADG changes, at least for now, is to add to existing behavior, not modify it.

Whenever was created for use at Inkling (inklingmarkets.com) where I work. Their take on it: blog.inklingmarkets.com/2009/02/whenever-easy-way-to-do-cron-jobs-from.html

While building Whenever, I learned a lot by digging through the source code of Capistrano - github.com/jamis/capistrano

Email me: javan [at] javan (dot) us

Copyright © 2009 Javan Makhmali

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

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