This is what I have learned from trying to expire fragment cache from background scripts in Ruby om Rails. It simply is not possible. If a controller is not instantiated when you hit the expire_fragment method
(or any other cache sweeper method) you get a nil reply.

I understand why this decision has been made: Fragments are in views. Only the controller should deal with views. The controller is instantiated when it is requested by the web-server. Otherwise you break the MVC paradigm. Except that this mental model does not take into account the fact that
every web-application (except the simplest ones) have stuff going on in the background. Long calculations are made by script so the user doesn’t have to wait while they are being finished. Stuff is done at a given time of day, like expiring old subscriptions. This is why we have BackgroundDRB and BJ. Background jobs need access to the fragment cache. If anyone who is really into rails could find a way to let background jobs do fragment cache things, I’d be very grateful.

For now, I have to periodically expire or reset all fragments because I know that background jobs periodically change the data used to generate them. Most inelegant and inefficient.