Ideas for improvements to Cron

[bensheldon]

Some ideas for improving Cron:

• Add a cron_at column, along with a uniqueness validation on cron_key, cron_at. This would push down to the database the largest source of enqueue concurrency conflicts, and provide a better guarantee (I don't know of anyone who is using intending Cron to enqueue multiple identical jobs from multiple workers at the same point in time)
• Turn the Cron configuration into an ActiveModel object. E.g. a CronEntry class that has .all, .find(cron_key) methods, and instance accessors for the cron configuration values, and encapsulate the next_at and enqueue logic in methods too. I think that might dry up the code (or at least avoid primitive abuse as we're doing more and more with the cron configuration hashes), as well as allow us to use some niceties like dom_id instead of calling to_param on the strings in the Dashboard.
• The args parameter is not bulletproof across different combinations of positional args and kwargs and Ruby versions. (I would love some help on this one).

[bensheldon]

Noting that there is a way for the adapter to pass back errors on enqueue:

https://github.com/rails/rails/blob/826f947ce7078a66e93276c8102dd235bb629911/activejob/lib/active_job/enqueuing.rb#L74

Thinking about that in the context of adding unique index and handling failed uniqueness exceptions.

[ssendev]

Another idea is to have a Last run at column.

An option to disable Cron jobs from the web interface is also useful since it allows quick intervention when a job is misbehaving without taking down everything.

These two and being able to run Cron tasks immediately (as mentioned in #425) are available in sidekiq-cron

A minor nitpick is, at least for me Cron Job Name and Class is always the same and pretty long which means the table is needlessly cramped.

[bensheldon]

@ssendev strong agree! Thank you for suggesting.

Disabling cron globally isn't easily done (yet... GoodJob doesn't have any global configuration in the database right now).

Adding the option to run a cron task immediately is very doable.

And thanks for the UI feedback. I'm thinking of just showing the Name, schedule, and then putting all the configuration params in an expandable toggle. That should free up some space.