What are the performance implications of using require_dependency in Rails 3 applications?

Question

I feel like I understand the difference between require and require_dependency (from How are require, require_dependency and constants reloading related in Rails?).

However, I'm wondering what should happen if I use some of the various methods out there (see http://hemju.com/2010/09/22/rails-3-quicktip-autoload-lib-directory-including-all-subdirectories/ and Best way to load module/class from lib folder in Rails 3?) to get all files loading so we:

1. don't need to use require_dependency all over the place in the application and
2. don't have to restart development servers when files in the lib directory change.

It seems like development performance would be slightly impacted, which is not that big of a deal to me. How would performance be impacted in a production environment? Do all of the files generally get loaded only once if you are in production anyway? Is there a better way that I'm not seeing?

If you could include some resources where I could read more about this, they would be greatly appreciated. Some blog posts said that this behavior changed recently with Rails 3 for autoreloading lib/* files and that it was contentious, but I didn't see any links to these discussions. It would be helpful for considering the pros/cons. Thanks!

Answer 1

The code reloader is disabled by default in production. So if you are calling require_dependency at the top of a file it is going to be executed only once.

The Rails 3 change you mentioned is really small. You can usually call Foo and it will be loaded from app/models/foo.rb automatically. Before it could also be loaded from lib/foo.rb. (These directories app/models and lib are called autoload paths.) Rails team decided to remove lib from autoload paths in the 3rd version. You can still put it back. But it is encouraged to leave in lib less frequenttly-changed and project-specific files. If you have something that does not belong to any of the default app subdirectories like app/models or app/controllers you don't have to put it in lib. You can add your own subdirectory. I have app/presenters, for example. There is a discussion on the old issue tracker if you want more info on that.