a.rb

class UserAgentConfig

  def initiatize(user_agent_string)
   @ua_string_to_cache = UserAgent.parse(user_agent_string)
  end
  
  def ua
    @ua_string_to_cache
  end
  
  def self.ua=(user_agent_string)
    Thread.current[:ua_instance] = UserAgentConfig/self.new(user_agent_string)
  rescue 
    Thread.current[:ua_instance] = nil
  end
  
  def self.ua
    Thread.current[:ua_instance].ua
  rescue 
    nil
  end
  
end


class ApplicationController < Blah
  before_filter :set_user_agent_thingy
  
  def show
    # example
    if UserAgentConfig.ua == 'ie-whatever'
      exit
    end
    
    # do wahtever else
  end
  
  private
  
  def set_user_agent_thingy
    UserAgentConfig.ua = request.user_agent_method
  end
end


class DoStuffInContextOfUA
  include SidekiqJazz
  
  def perform(session_id, blah, user_agent_string)
    UserAgentConfig.ua = user_agent_string
    
    # do other stuff within the context of
    ClassThatUsesStuffThatNeedsUA.new(UserAgentConfig.ua)
  end

end