How does Delayed Job work in Ruby on Rails ?

Question

I am new to this and is little confused about how Delayed Job works ?

I know it creates a table and puts the jobs in the table and then I need to run

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Answer 1

Does DJ script checks the table every minute and when the time matches job_at time, it runs that job ?

yes. It checks the database every 5 seconds.

How it is different than cron (whenever gem) if the script is just checking the table every min ?

In the context of background jobs, they are not that different. Their main difference is how they usually run the jobs.

          DJ                  |            Crontab
 uses additional database     | you should either set up a rake task
 table but that's it. easier  | or a runner which can be called on the
 to code compared to crontab  | crontab
------------------------------|------------------------------------------
 requires you to run a worker | requires you to setup your cron which
 that will poll the database  | you can easily do using the whenever gem
------------------------------|------------------------------------------
 since this uses a table, it  | you have to setup some sort of logging so
 is easier to debug errors    | that you have an idea what caused the error
 when they happen             |
------------------------------|------------------------------------------
 the worker should always be  | as long as your crontab is set up properly,
 running to perform the job   | you should have no issues
------------------------------|------------------------------------------
 harder to setup recurring    | easy to setup recurring tasks
 tasks                        |
------------------------------|------------------------------------------