Ruby why are class instance variables threadsafe

Question

According to this answer they are, but then the poster states that things work differently in JRuby so I am confused?

I am implementing a multi tenancy solution using class instance variables, so it doesn't matter what Ruby implementation or web server I am using, I need to make sure that data cannot be leaked.

Here is my code:

class Tenant < ActiveRecord::Base

  def self.current_tenant=(tenant)
    @tenant = tenant
  end

  def self.current_tenant
    @tenant
  end
end

What do I need to do to make sure that no matter what happens (changing Ruby implementation, changing web server, new Ruby threading capabilities etc) that my code is thread safe?

Answer 1

Since the tenancy attribute's scope is a request, I would suggest you keep it in the scope of the current thread. Since a request is handled on a single thread, and a thread handles a single request at a time - as long as you always set the tenancy at the beginning of the request you will be fine (for extra security, you might want to un-assign the tenant at the end of the request).

To do this you can use thread local attributes:

class Tenant < ActiveRecord::Base

  def self.current_tenant=(tenant)
    Thread.current[:current_tenant] = tenant
  end

  def self.current_tenant
    Thread.current[:current_tenant]
  end

  def self.clear_current_tenant
    Thread.current[:current_tenant] = nil
  end
end

Since this is using a thread store, you are totally thread safe - each thread is responsible for its own data.