Ruby on Rails

I'm trying to understand something basic about Rails (and Apache).

This is a follow-up to Matt Jones' answer in https://groups.google.com/forum/#!topic/rubyonrails-talk/y6ktv7rL6IA

I repeat his answer here

I suspect `User.exists?` is going to be the lightest possible option. `User.count == 0` will also work, but I know there are some DBs where that operation can be expensive on large tables.

This is still going to do a query every time it checks, so if that's too much load you could cache the result:

class User < ActiveRecord::Base

  def self.any_users?

    @any_users ||= exists?

  end

end

Then your role-defaulting code can check `self.class.any_users?`, which will only run one query (per server) that returns true.

NOTE NOTE NOTE: the above is not 100% thread-safe. It assumes that there is exactly one user sending requests trying to be the first. If you're worried about somebody racing to sign up for that first account, you'll want to come up with a more secure approach.

--Matt Jones

In

So at some point in the code, I call
So Ruby says "Ok, @any_users isn't defined so I'm going to call exists?"  That makes perfect sense.

Let's say I have several people banging on my website.  Do each of them get a completely different copy of Rails?  How many copies of @any_users are there?  Does @any_users get initialized for each user?  What data is common for all
threads? What's different for all threads?

Where can I learn more about this?  I've skimmed https://bearmetal.eu/theden/how-do-i-know-whether-my-rails-app-is-thread-safe-or-not/ but it really isn't helping me understand what is and isn't common between each thread/user/session.