What are Cacher & GarbageCollector for?

In the default environment.rb that is created with a new Rails app,
the following line is commented out:

# config.active_record.observers = :cacher, :garbage_collector

What are these two observers for? I can't find documentation for
these anywhere.

Thanks,
-Jason

Those aren't real. That line is used as an example. If anything,
they're real observers for someone's app. I mean, if you were going
to make them up, why not use something like:

# config.active_record.observers = :flux_capacitor