Clockwork - a clock process to replace cron

Cron is non-ideal for running scheduled application tasks, especially in an app deployed to multiple machines. More details.

Clockwork is a cron replacement. It runs as a lightweight, long-running Ruby process which sits alongside your web processes (Mongrel/Thin) and your worker processes (DJ/Resque/Minion/Stalker) to schedule recurring work at particular times or dates. For example, refreshing feeds on an hourly basis, or send reminder emails on a nightly basis, or generating invoices once a month on the 1st.

Quickstart

Create clock.rb:

require 'clockwork'
include Clockwork

handler do |job|
  puts "Running #{job}"
end

every(10.seconds, 'frequent.job')
every(3.minutes, 'less.frequent.job')
every(1.hour, 'hourly.job')

every(1.day, 'midnight.job', :at => '00:00')

Run it with the clockwork binary:

$ clockwork clock.rb
Starting clock for 4 events: [ frequent.job less.frequent.job hourly.job midnight.job ]
Triggering frequent.job

If you would not like to taint the namespace with include Clockwork, you can use it as the module (thanks to hoverlover).

require 'clockwork'

module Clockwork

  configure do |config|
    config[:tz] = "America/Chicago"
  end

  handler do |job|
    puts "Running #{job}"
  end

  every(10.seconds, 'frequent.job')
  every(3.minutes, 'less.frequent.job')
  every(1.hour, 'hourly.job')

  every(1.day, 'midnight.job', :at => '00:00')
end

If you need to load your entire environment for your jobs, simply add:

require './config/boot'
require './config/environment'

under the require 'clockwork' declaration.

Quickstart for Heroku

Clockwork fits well with heroku's cedar stack.

Consider to use clockwork-init.sh to create a new project for heroku.

Use with queueing

The clock process only makes sense as a place to schedule work to be done, not to do the work. It avoids locking by running as a single process, but this makes it impossible to parallelize. For doing the work, you should be using a job queueing system, such as Delayed Job, Beanstalk/Stalker, RabbitMQ/Minion, Resque, or Sidekiq. This design allows a simple clock process with no locks, but also offers near infinite horizontal scalability.

For example, if you're using Beanstalk/Staker:

require 'stalker'

handler { |job| Stalker.enqueue(job) }

every(1.hour, 'feeds.refresh')
every(1.day, 'reminders.send', :at => '01:30')

Using a queueing system which doesn't require that your full application be loaded is preferable, because the clock process can keep a tiny memory footprint. If you're using DJ or Resque, however, you can go ahead and load your full application enviroment, and use per-event blocks to call DJ or Resque enqueue methods. For example, with DJ/Rails:

require 'config/boot'
require 'config/environment'

every(1.hour, 'feeds.refresh') { Feed.send_later(:refresh) }
every(1.day, 'reminders.send', :at => '01:30') { Reminder.send_later(:send_reminders) }

Parameters

:at

:at parameter the hour and minute specifies when the event occur.

The simplest example:

every(1.day, 'reminders.send', :at => '01:30')

You can omit 0 of the hour:

every(1.day, 'reminders.send', :at => '1:30')

The wildcard for hour and minute is supported:

every(1.hour, 'reminders.send', :at => '**:30')
every(10.seconds, 'frequent.job', :at => '9:**')

You can set more than one timing:

every(1.hour, 'reminders.send', :at => ['12:00', '18:00'])
# send reminders at noon and evening

You can also specify a timezone (default is the local timezone):

every(1.day, 'reminders.send', :at => '00:00', :tz => 'UTC')
# Runs the job each day at midnight, UTC.
# The value for :tz can be anything supported by [TZInfo](http://tzinfo.rubyforge.org/)

:if

:if parameter is invoked every time the task is ready to run, and run if the return value is true.

Run on every first day of month.

Clockwork.every(1.day, 'myjob', :if => lambda { |t| t.day == 1 })

The argument is an instance of ActiveSupport::TimeWithZone if the :tz option is set. Otherwise, it's an instance of Time.

This argument cannot be omitted. Please use _ as placeholder if not needed.

Clockwork.every(1.second, 'myjob', :if => lambda { |_| true })

:thread

A handler with :thread parameter runs in a different thread.

If a job is long running or IO-intensive, this option will be useful to keep the clock precise.

Configuration

Clockwork exposes a couple of configuration options you may change:

:logger

By default Clockwork logs to STDOUT. In case you prefer to make it to use our own logger implementation you have to specify the logger configuration option. See example below.

:sleep_timeout

Clockwork wakes up once a second (by default) and performs its duties. If that is the rare case you need to tweak the number of seconds it sleeps then you have the sleep_timeout configuration option to set like shown below.

:tz

This is the default timezone to use for all events. When not specified this defaults to the local timezone. Specifying :tz in the the parameters for an event overrides anything set here.

:max_threads

Clockwork runs handlers in threads. If it exceeds max_threads, it will warn you about missing jobs.

Configuration example

Clockwork.configure do |config|
  config[:sleep_timeout] = 5
  config[:logger] = Logger.new(log_file_path)
  config[:tz] = 'EST'
  config[:max_threads] = 15
end

Anatomy of a clock file

clock.rb is standard Ruby. Since we include the Clockwork module (the clockwork binary does this automatically, or you can do it explicitly), this exposes a small DSL ("handler" and "every") to define the handler for events, and then the events themselves.

The handler typically looks like this:

handler { |job| enqueue_your_job(job) }

This block will be invoked every time an event is triggered, with the job name passed in. In most cases, you should be able to pass the job name directly through to your queueing system.

The second part of the file are the events, which roughly resembles a crontab:

every(5.minutes, 'thing.do')
every(1.hour, 'otherthing.do')

In the first line of this example, an event will be triggered once every five minutes, passing the job name 'thing.do' into the handler. The handler shown above would thus call enqueue_your_job('thing.do').

You can also pass a custom block to the handler, for job queueing systems that rely on classes rather than job names (i.e. DJ and Resque). In this case, you need not define a general event handler, and instead provide one with each event:

every(5.minutes, 'thing.do') { Thing.send_later(:do) }

If you provide a custom handler for the block, the job name is used only for logging.

You can also use blocks to do more complex checks:

every(1.day, 'check.leap.year') do
  Stalker.enqueue('leap.year.party') if Time.now.year % 4 == 0
end

In production

Only one clock process should ever be running across your whole application deployment. For example, if your app is running on three VPS machines (two app servers and one database), your app machines might have the following process topography:

App server 1: 3 web (thin start), 3 workers (rake jobs:work), 1 clock (clockwork clock.rb)
App server 2: 3 web (thin start), 3 workers (rake jobs:work)

You should use Monit, God, Upstart, or Inittab to keep your clock process running the same way you keep your web and workers running.

Daemonization

Thanks to @fddayan, clockworkd executes clockwork script in as a daemon.

You need daemons gem to use clockworkd. It is not automatically installed, please install by yourself.

Then,

clockworkd -c YOUR_CLOCK.rb start

For more details, see help shown by clockworkd.