An example of baking our own middleware stack to better understand how Rack middleware works.

middleware-example.rb

class Talk
  def call(env)
    "Can I talk to #{env[:name]}!"
  end
end

class Shout
  def initialize(app)
    @app = app
  end

  def call(env)
    @app.call(env).upcase
  end
end

class Zuul
  def initialize(app)
    @app = app
  end

  def call(env)
    "There is no #{env[:name]}. Only Zuul!"
  end
end

def run_the_stack(middleware, app, env)
  prev_app = app

  middleware.reverse.each do |part|
    part = part.new(prev_app)
    prev_app = part
  end

  prev_app.call(env)
end

puts run_the_stack [Shout, Zuul], Talk.new, name: 'Dana'

I like your post about Rack, especially the run_the_stack method, which helped me a lot to understand the basic concept. And I wonder if I could see the app(which attaches to Rack) as just one of the middlewares? Since its #call(env) method is the last one get executed , so it needs a different structure as the others. I don't know if this is a proper understanding.