In Rails (3.2.8), I needed to allow different browser tabs to behave as if they were different apps (my rails app will be embedded in an iframe in various apps). For a variety of reasons, I couldn't track apps via sub-domains. Cookies failed, because browser tabs all share the same cookies, so there were race-conditions and other buggery. My sol…

gistfile1.rb

require 'addressable/uri'

module MyApp
  # This rack-filter gets run before any urls are processed by the routing 
  # subsystem, enabling us to extract the app_id query parameter and cache it for
  # the duration of the current request thread.
  class ClientIdManager
    def initialize(app)
      @app = app   
    end

    def call(env)
      url = Addressable::URI.parse(env['REQUEST_URI'])
      if (url.query_values && url.query_values.has_key?('app_id'))
          ClientIdProvider.app_id = url.query_values['app_id']
      else
          ClientIdProvider.app_id = '-1'
      end
      @app.call(env)
    end  
  end
end

module MyApp
  # This provides convenient access to the current app_id
  # It also (via to_s) is able to provide said app_id to 
  # external classes which are configured in application
  # initialization
  class ClientIdProvider
    def self.app_id
      Thread.current['app_id']
    end

    def self.app_id=(val)
      Thread.current['app_id'] = val
    end

    def to_s
      ClientIdProvider.app_id
    end
  end
end

# In your application.rb
	# Ensure that app_id is persisted in all urls generated from this app
  Rails.application.routes.default_url_options = { 'app_id' => MyApp::ClientIdProvider.new }
	# Add our filter to the rack middleware
  config.middleware.use 'MyApp::ClientIdManager'
	
# In your omniauth configuration:
	# Pass the app_id to external providers (for Google, this is in the 'state' parameter)
	# Be sure to set provider_ignores_state: true
	# This prevents an annoying issue where specifying state makes
	# calls to Google fail.
	config.omniauth :google_oauth2, ENV['GOOGLE_KEY'], ENV['GOOGLE_SECRET'], { state: MyApp::ClientIdProvider.new, provider_ignores_state: true, scope: 'userinfo.email,userinfo.profile', access_type: 'online', approval_prompt: ''}

# In your omniauth controller processing methods
  # Be sure to set the app_id (or whatever param you want to persist)
  # When it comes back from external omniauth calls.
	# Example Omniauth callback controller Google processing function:

	def process_google_response
	  MyApp::ClientIdProvider.app_id = params['state']
	  # ...