How to create a request scope for your rails application

Some time ago I missed one of the cool features from the time I worked with JavaEE, that was a request scope for some variables.

I know the instance variables on controllers do a great job simulating it, but I needed to access context in a completely different part of the application, that wasn’t always called from controllers, variables that should be scoped to that request.

So I thought, I could create something like that for my rails app.

To start, I created a file called ‘request_scope.rb’ in the lib folder, with something similar to this (I’ll not post the actual file here because it is from a client’s project)

class RequestScope
  def self.start_scope  
        raise "request_context already set" if Thread.current[:request_context]
    Thread.current[:request_context] = {}
  end
 
  def self.end_scope
    raise "request_context already nil" unless Thread.current[:request_context]
    Thread.current[:request_context] = nil
  end 
 
  def self.scoped_hash
    raise "request_context not initialized" unless Thread.current[:request_context]
    Thread.current[:request_context]
  end
 
  def self.[](name)
    self.scoped_hash[name]
  end
 
  def self.[]=(name,val)
    self.scoped_hash[name] = val
  end
end

That will allow me to do things like:

RequestScope['alpha'] = 1
RequestScope['alpha']

And for that to work, as a real request scope, I just needed to add one bit of code to the application_controller.rb of my application:

class ApplicationController < ActionController::Base
  around_action :define_request_scope
 
  def define_request_scope
    EnvConfig.start_scope
    begin
      yield
    ensure
      EnvConfig.end_scope
    end
  end
end

There is one trick thou, if you need to read or write to the request context in any other filters, remember that they need to be defined after the “define_context_scope”, because the filters are executed in the order they are defined.

So, what do you think about this approach to create a request scope? would you do different? do you see any problems in this implementation?