cron_swanson

CronSwanson

CronSwanson helps distribute schedule cron jobs.

If you've ever had load spikes when many applications all starting the same cron job at the same time, CronSwanson can help you.

The library generates crontab schedule strings which are consistent (they aren't random) but which are fuzzed/shifted depending on some input.

Build Status

Installation

Add this line to your application's Gemfile:

gem 'cron_swanson'

And then execute:

$ bundle

Or install it yourself as:

$ gem install cron_swanson

Usage

build_schedule

Use build_schedule to build a cron schedule string. The supplied string is hashed to determine the run time of the job.

CronSwanson.build_schedule 'whiskey'
#=> "33 18 * * *"

To keep two applications running the same job from executing at once, make the application name part of the build_schedule key.

CronSwanson.build_schedule 'application-a whiskey'
#=> "4 19 * * *"

CronSwanson.build_schedule 'application-b whiskey'
#=> "11 7 * * *"

An interval (in seconds) can be supplied if you want a job to be run more than once/day. This interval must be a factor of 24 hours.

CronSwanson.build_schedule 'bacon', interval: 60 * 60 * 4
#=> "26 2,6,10,14,18,22 * * *"

You can also use ActiveSupport::Duration instances.

CronSwanson.build_schedule 'bacon', interval: 4.hours
#=> "26 2,6,10,14,18,22 * * *"

Whenever Integration

CronSwanson is built to integrate with the fantastic whenever gem.

#schedule will calculate a run time for jobs by hashing the text of the job definitions in the given block. (Plus an optional seed string if one is supplied.)

NOTE: This means that if you change the jobs in the block, or the seed, you will also change the schedule time for these jobs.

Daily

# in config/build_schedule.rb
swanson = CronSwanson::Whenever.new(self, seed: 'application-name')
swanson.schedule do
  rake 'sample:job'
end

This will result in rake sample:job being scheduled once per day, at a time determined by CronSwanson.

Multiple times/day

# in config/build_schedule.rb

# with ActiveSupport
swanson = CronSwanson::Whenever.new(self, seed: 'application-name')
swanson.schedule(interval: 4.hours) do
  rake 'sample:job'
end

# without ActiveSupport
swanson = CronSwanson::Whenever.new(self, seed: 'application-name')
swanson.schedule(interval: 60 * 60 * 4) do
  rake 'sample:job'
end

job types

Any custom job types which have been defined will work.

# in config/build_schedule.rb
job_type :ron, '/usr/bin/ron :task'

swanson = CronSwanson::Whenever.new(self, seed: 'application-name')
swanson.schedule do
  ron 'bacon whiskey'
end

roles

Roles are supported. See the whenever documentation for more information on this.

swanson = CronSwanson::Whenever.new(self, seed: 'application-name')
swanson.schedule do
  rake 'will_run_on_all_roles'
end

# Added to servers with the :restricted role
swanson.schedule(roles: [:restricted]) do
  rake 'restricted_only'
end

seeds

Varying the seed string affects all jobs scheduled by that instance.

swanson = CronSwanson::Whenever.new(self, seed: 'application-a')
swanson.schedule do
  rake 'job'
end

swanson = CronSwanson::Whenever.new(self, seed: 'application-b')
swanson.schedule do
  rake 'job'
end

In this example, the job being scheduled is the same, but the schedule times will be different because the seed string is different.

Development

After checking out the repo, run bin/setup to install dependencies. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/alexdean/cron_swanson.