some_middleware.rb

class SomeMiddleware
  def initialize(app, session_key)
    @app = app
  end
  def call(env)
    env['myapp.myvar'] = :something
    @app.call(env)
  end
end

users_controller.rb

class UsersController < ApplicationController
  def show
    if request.env['myapp.myvar'] == :something
      # ...
    end
  end
end