Are class variables and class instance variables shared among different requests?

rubyonrails-talk
1

Hi, I just wanted some clarification on this.

In production mode, while I know that object instance variables are local to the object and so to the request, but is this same with class variables and class instance variables or are they shared between requests?

I have this question, because I know that classes are cached in production mode. But what does this mean? Like, classes are initialised and cached at the start of the server (and so are class variables and class level instance variables) and are persistent in the memory throughout, or it's like, just class definitions are cached (so they are not read every time from the disk), and they are re- initialised for every request, so that every request has its own set?

Say, in file `lib/example.rb` I have defined something like -

  #lib/example.rb   module Example   class << self

    def assign value       @@class_variable = value       @class_instance_variable = value     end

    def read       [@@class_variable, @class_instance_variable]     end

  end   end

  #app/controllers/application_controller.rb   class ApplicationController < ActionController::Base     before_filter :initialise

    def initialise       Example.assign params[:name]     end   end

1. Request "A" comes with params[:name] as "a". 2. While request "A" executes, request "B" comes with params[:name] as "b". 3. Now after arrival of request "B", if code in request "A" accesses Example.read(), what would it return? ['b', 'b'], ['a', 'a'] or (maybe, just maybe) ['a', 'b']?

I'd also like to write one more example -

  #lib/example.rb   module Example   class << self

    @@class_variable = 'default'     @class_instance_variable = 'default'

    def assign value       @@class_variable =         @class_instance_variable = value     end

   def read      [@@class_variable, @class_instance_variable]    end   end   end

1. Request "A" comes, executes Example.assign('a') and finishes. 2. Request "B" comes and tries to execute Example.read(), without modifying the variables. What would it get? "default" or "a"?

PS: While writing, I felt like this is a silly question. I'm a novice, anyway.

- thanks.

2

I believe that the classes actually do stay around, but trying to use them to pass messages is a Bad Idea. There's too many ways for any changes to get lost - a subsequent request might go to a different worker, the worker with the value might get killed (by hitting memory limits, or just things like Passenger's idle timeout, etc). And they *definitely* go away in development mode, of course.

Note that the typical use of class variables in most of the Rails source is as a cache of values that should stay constant throughout execution, which avoids the issue entirely by only setting values once.

--Matt Jones