FEATURE: Adding Recurring Job Support to Delayed_Job

Delayed_job provides and ideal platform for providing support for distributed job scheduling, however it currently lacks a way for repeat or recurring jobs to run. Adding support for recurring jobs will provide developers with several benefits:

• Move ‘cron’ type operational configs into application code
• Ensure redundancy by allowing recurring jobs to run anywhere workers are running
• Ensure recurring jobs don’t run in multiple locations simultaneously (overlap)

CRON SPOF (Single Point of Failure)
While I’m sure some cron hackers have solved this problem in a variety of ways, it’s a non trivial challenge to ensure that regular, scheduled jobs in a redundant cluster run only once (on schedule), and still run if the cron server(s) is down. Recurring job support in delayed_job can solve both of these issues (redundancy and Singleton execution) in clustered environments cleanly.

Other distributed scheduling implementations

• Quartz Scheduler (Java): http://www.quartz-scheduler.org/

There are at least two forks that add recurring job scheduling to delayed_job:

• http://github.com/briandemant/delayed_job
• http://github.com/terrbear/delayed_job

Requirements:

• easy to understand API for usage
• continued support for run_at semantics
• addition of recurring semantics: 5.minutes, 1.day, 1.week, etc.
• ability to ensure that a set of scheduled, recurring jobs are properly configured and running in the queue
• error handling and recovery, compatible with the delayed_job design

(Proposed)Design

Add :run_every to migrations.

Any job that has a non-nil :run_every entry is by default a recurring job. :run_every should be applied when jobs are run to ensure there is no time drift. For example:
:run_at => Time(8.AM), :run_every => 1.day
on execution, would ensure that the next run_at time is set for 8AM the next day, not 8AM+job_execution_time – that would be considered drift

Raise exception if attempt to enqueue a redundant recurring job

Implement recurring functionality as a Mixin/Extension, so code can be clearly delinated from the mainline delayed_job code