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'
require 'active_support/time' # Allow numeric durations (eg: 1.minutes)

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

  # handler receives the time when job is prepared to run in the 2nd argument
  # handler do |job, time|
  #   puts "Running #{job}, at #{time}"
  # 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

Run it with the clockwork executable:

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

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 using 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/Stalker:

require 'stalker'
require 'active_support/time'

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

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

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 environment, 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) }

Use with database events

In addition to managing static events in your clock.rb, you can configure clockwork to synchronise with dynamic events from a database. Like static events, these database-backed events say when they should be run, and how frequently; the difference being that if you change those settings in the database, they will be reflected in clockwork.

To keep the database events in sync with clockwork, a special manager class DatabaseEvents::Manager is used. You tell it to sync a database-backed model using the sync_database_events method, and then, at the frequency you specify, it will fetch all the events from the database, and ensure clockwork is using the latest settings.

Example `clock.rb` file

Here we're using an ActiveRecord model called ClockworkDatabaseEvent to store events in the database:

require 'clockwork'
require 'clockwork/database_events'
require_relative './config/boot'
require_relative './config/environment'

module Clockwork

  # required to enable database syncing support
  Clockwork.manager = DatabaseEvents::Manager.new

  sync_database_events model: ClockworkDatabaseEvent, every: 1.minute do |model_instance|

    # do some work e.g...

    # running a DelayedJob task, where #some_action is a method
    # you've defined on the model, which does the work you need
    model_instance.delay.some_action

    # performing some work with Sidekiq
    YourSidekiqWorkerClass.perform_async
  end

  [other events if you have]

end

This tells clockwork to fetch all ClockworkDatabaseEvent instances from the database, and create an internal clockwork event for each one. Each clockwork event will be configured based on the instance's frequency and, optionally, its at, if?, ignored_attributes, name, and, tz methods. The code above also says to reload the events from the database every 1.minute; we need pick up any changes in the database frequently (choose a sensible reload frequency by changing the every: option).

When one of the events is ready to be run (based on it's frequency, and possible at, if?, ignored attributes, and tz methods), clockwork arranges for the block passed to sync_database_events to be run. The above example shows how you could use either DelayedJob or Sidekiq to kick off a worker job. This approach is good because the ideal is to use clockwork as a simple scheduler, and avoid making it carry out any long-running tasks.

Your Model Classes

ActiveRecord models are a perfect candidate for the model class. Having said that, the only requirements are:

the class responds to all returning an array of instances from the database
the instances returned respond to:
- id returning a unique identifier (this is needed to track changes to event settings)
- frequency returning the how frequently (in seconds) the database event should be run

attributes returning a hash of [attribute name] => [attribute value] values (or really anything that we can use store on registering the event, and then compare again to see if the state has changed later)
at (optional) return any acceptable clockwork :at string
name (optional) returning the name for the event (used to identify it in the Clockwork output)
if?(optional) returning either true or false, depending on whether the database event should run at the given time (this method will be passed the time as a parameter, much like the standard clockwork :if)
ignored_attributes (optional) returning an array of model attributes (as symbols) to ignore when determining whether the database event has been modified since our last run
tz (optional) returning the timezone to use (default is the local timezone)
skip_first_run (optional) self explanatory, see dedicated section

Example Setup

Here's an example of one way of setting up your ActiveRecord models:

# db/migrate/20140302220659_create_frequency_periods.rb
class CreateFrequencyPeriods < ActiveRecord::Migration
  def change
    create_table :frequency_periods do |t|
      t.string :name

      t.timestamps
    end
  end
end

# 20140302221102_create_clockwork_database_events.rb
class CreateClockworkDatabaseEvents < ActiveRecord::Migration
  def change
    create_table :clockwork_database_events do |t|
      t.integer :frequency_quantity
      t.references :frequency_period
      t.string :at

      t.timestamps
    end
    add_index :clockwork_database_events, :frequency_period_id
  end
end

# app/models/clockwork_database_event.rb
class ClockworkDatabaseEvent < ActiveRecord::Base
  belongs_to :frequency_period
  attr_accessible :frequency_quantity, :frequency_period_id, :at

  # Used by clockwork to schedule how frequently this event should be run
  # Should be the intended number of seconds between executions
  def frequency
    frequency_quantity.send(frequency_period.name.pluralize)
  end
end

# app/models/frequency_period.rb
class FrequencyPeriod < ActiveRecord::Base
  attr_accessible :name
end

# db/seeds.rb
...
# creating the FrequencyPeriods
[:second, :minute, :hour, :day, :week, :month].each do |period|
  FrequencyPeriod.create(name: period)
end
...

Example use of `if?`

Database events support the ability to run events if certain conditions are met. This can be used to only run events on a given day, week, or month, or really any criteria you could conceive. Best of all, these criteria e.g. which day to run it on can be attributes on your Model, and therefore change dynamically as you change the Model in the database.

So for example, if you had a Model that had a day and month integer attribute, you could specify that the Database event should only run on a particular day of a particular month as follows:

# app/models/clockwork_database_event.rb
class ClockworkDatabaseEvent < ActiveRecord::Base

  ...

  def if?(time)
    time.day == day && time.month == month
  end

  ...
end

Example use of `ignored_attributes`

Clockwork compares all attributes of the model between runs to determine if the model has changed, and if it has, it runs the event if all other conditions are met.

However, in certain cases, you may want to store additional attributes in your model that you don't want to affect whether a database event is executed prior to its next interval.

So for example, you may update an attribute of your model named last_scheduled_at on each run to track the last time it was successfully scheduled. You can tell Clockwork to ignore that attribute in its comparison as follows:

# app/models/clockwork_database_event.rb
class ClockworkDatabaseEvent < ActiveRecord::Base

  ...

  def ignored_attributes
    [ :last_scheduled_at ]
  end

  ...
end

Event Parameters

:at

:at parameter specifies when to trigger the event:

Valid formats:

HH:MM
 H:MM
**:MM
HH:**
(Mon|mon|Monday|monday) HH:MM

Examples

The simplest example:

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

You can omit the leading 0 of the hour:

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

Wildcards for hour and minute are supported:

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

You can set more than one timing:

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

You can specify the day of week to run:

every(1.week, 'myjob', :at => 'Monday 16:20')

If another task is already running at the specified time, clockwork will skip execution of the task with the :at option. If this is a problem, please use the :thread option to prevent the long running task from blocking clockwork's scheduler.

:tz

:tz parameter lets you 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](https://github.com/tzinfo/tzinfo)
# Using the 'tzinfo' gem, run TZInfo::Timezone.all_identifiers to get a list of acceptable identifiers.

: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

By default, clockwork runs in a single-process and single-thread. If an event handler takes a long time, the main routine of clockwork is blocked until it ends. Clockwork does not misbehave, but the next event is blocked, and runs when the process is returned to the clockwork routine.

The :thread option is to avoid blocking. An event with thread: true runs in a different thread.

Clockwork.every(1.day, 'run.me.in.new.thread', :thread => true)

If a job is long-running or IO-intensive, this option helps keep the clock precise.

:skip_first_run

Normally, a clockwork process that is defined to run in a specified period will run at startup. This is sometimes undesired behaviour, if the action being run relies on other processes booting which may be slower than clock. To avoid this problem, :skip_first_run can be used.

Clockwork.every(5.minutes, 'myjob', :skip_first_run => true)

The above job will not run at initial boot, and instead run every 5 minutes after boot.

Configuration

Clockwork exposes a couple of configuration options:

:logger

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

:sleep_timeout

Clockwork wakes up once a second and performs its duties. To change the number of seconds Clockwork sleeps, set the sleep_timeout configuration option as shown below in the example.

From 1.1.0, Clockwork does not accept sleep_timeout less than 1 seconds.

:tz

This is the default timezone to use for all events. When not specified this defaults to the local timezone. Specifying :tz in 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 (log an error) about missing jobs.

:thread

Boolean true or false. Default is false. If set to true, every event will be run in its own thread. Can be overridden on a per event basis (see the :thread option in the Event Parameters section above)

Configuration example

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

error_handler

You can add error_handler to define your own logging or error rescue.

module Clockwork
  error_handler do |error|
    Airbrake.notify_or_ignore(error)
  end
end

Current specifications are as follows.

defining error_handler does not disable original logging
errors from error_handler itself are not rescued, and stop clockwork

Any suggestion about these specifications is welcome.

Old style

include Clockwork is old style. The old style is still supported, though not recommended, because it pollutes the global namespace.

Anatomy of a clock file

clock.rb is standard Ruby. Since we include the Clockwork module (the clockwork executable does this automatically, or you can do it explicitly), this exposes a small DSL 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, which lists the events, 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 Date.leap?(Time.now.year)
end

In addition, Clockwork also supports :before_tick, :after_tick, :before_run, and :after_run callbacks. All callbacks are optional. All before callbacks must return a truthy value otherwise the job will not run. The tick callbacks run every tick (a tick being whatever your :sleep_timeout is set to, default is 1 second):

on(:before_tick) do
  puts "tick"
end

on(:after_tick) do
  puts "tock"
end

on(:before_run) do |event, t|
  puts "job_started: #{event}"
end

on(:after_run) do |event, t|
  puts "job_finished: #{event}"
end

Finally, you can use tasks synchronised from a database as described in detail above:

sync_database_events model: MyEvent, every: 1.minute do |instance_job_name|
  # what to do with each instance
end

You can use multiple sync_database_events if you wish, so long as you use different model classes for each (ActiveRecord Single Table Inheritance could be a good idea if you're doing this).

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 as a daemon.

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

Then,

clockworkd -c YOUR_CLOCK.rb start

For more details, you can run clockworkd -h.

Integration Testing

You could take a look at:

clockwork-mocks that helps with running scheduled tasks during integration testing.
clockwork-test which ensures that tasks are triggered at the right time

Issues and Pull requests

If you find a bug, please create an issue - Issues · Rykian/clockwork.

For a bug fix or a feature request, please send a pull-request. Do not forget to add tests to show how your feature works, or what bug is fixed. All existing tests and new tests must pass (TravisCI is watching).

We want to provide simple and customizable core, so superficial changes will not be merged (e.g. supporting new event registration style). In most cases, directly operating Manager realizes an idea, without touching the core. If you discover a new way to use Clockwork, please create a gist page or an article on your website, then add it to the following "Use cases" section. This tool is already used in various environment, so backward-incompatible requests will be mostly rejected.

Use cases

Feel free to add your idea or experience and send a pull-request.

Sending errors to Airbrake
Read events from a database

Clockwork - a clock process to replace cron

Quickstart

Quickstart for Heroku

Use with queueing

Use with database events

Example clock.rb file

Your Model Classes

Example Setup

Example use of if?

Example use of ignored_attributes

Event Parameters

:at

Valid formats:

Examples

:tz

:if

:thread

:skip_first_run

Configuration

:logger

:sleep_timeout

:tz

:max_threads

:thread

Configuration example

error_handler

Old style

Anatomy of a clock file

In production

Daemonization

Integration Testing

Issues and Pull requests

Use cases

Meta

Example `clock.rb` file

Example use of `if?`

Example use of `ignored_attributes`