Class variables and Rails ... how global? When initialized? Thread safety?

rubyonrails-talk
1

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

This is a follow-up to Matt Jones’ answer in Redirecting to Google Groups

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 class User < ActiveRecord::Base

def self.any_users?

@any_users ||= exists?

end

end

``

So at some point in the code, I call User.any_users;

``

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 How do I know whether my Rails app is thread-safe or not? - by Jarkko of Bear Metal but it really isn’t helping me understand what is and isn’t common between each thread/user/session.

2

It depends on how you serve your rails app. For some setups you have many processes, each process handles a request at a time. Each process holds a completely separate copy of your rails application. Others use one process with many threads. This is one process, so there is only one copy of your rails app in memory. Some setups are a hybrid (many processes, each of which serving multiple concurrent requests).

As for the scope of you data, first off there isn’t really such a thing as session scope. global / class variables are shared across all threads. If the data is an instance variable of a User, then it’s specific to that user (note though that in the code you post @any_users is a instance variable of the User class (not an individual user), so it is shared.

Fred