Ruby Academy · Lesson

Middleware and Rack

The Rack stack.

Lesson 4 of 413 steps

Middleware and Rack is a free Ruby Academy lesson on CoddyKit. This is lesson 4 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Ruby Academy learning path, and your progress syncs across the web and the CoddyKit app. The Ruby Academy course includes 4 lessons in total.

What Is Rack?

Rack is the minimal interface between Ruby web servers and frameworks. Sinatra, Rails, and others all run on Rack.

Standardizes how requests and responses flow
Enables reusable middleware
A Rack app is anything that responds to call(env)

app = lambda do |env|
  [200, { 'Content-Type' => 'text/plain' }, ['Hello Rack']]
end

puts app.call({}).inspect

The Rack Contract

A Rack app's call must return a three element array:

Status integer (e.g. 200)
Headers hash
Body that responds to each (usually an array of strings)

status, headers, body = 200, { 'Content-Type' => 'text/html' }, ['<h1>Hi</h1>']
puts status
puts headers.inspect
body.each { |chunk| puts chunk }

The env Hash

The env argument is a hash describing the request.

REQUEST_METHOD like GET or POST
PATH_INFO the requested path
QUERY_STRING raw query parameters

env = { 'REQUEST_METHOD' => 'GET', 'PATH_INFO' => '/home' }
puts "#{env['REQUEST_METHOD']} #{env['PATH_INFO']}"

What Is Middleware?

Middleware wraps an app to add behavior before or after the request reaches it.

Logging, authentication, compression
Each middleware also responds to call(env)
It calls the next app in the chain

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

  def call(env)
    puts "Request: #{env['PATH_INFO']}"
    @app.call(env)
  end
end

Building a Middleware Class

Middleware stores the wrapped app and delegates to it in call.

Constructor receives the inner app
call can modify env, then forward
It can also alter the response on the way back

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

  def call(env)
    status, headers, body = @app.call(env)
    new_body = body.map(&:upcase)
    [status, headers, new_body]
  end
end

puts 'Middleware defined'

The Middleware Stack

Middleware forms a stack. The request flows down to the app, the response flows back up.

Outer middleware runs first on the way in
Outer middleware runs last on the way out

inner = lambda { |env| [200, {}, ['core']] }

class Wrap
  def initialize(app); @app = app; end
  def call(env)
    s, h, b = @app.call(env)
    [s, h, b.map { |x| "[#{x}]" }]
  end
end

puts Wrap.new(inner).call({}).last.inspect

Using Middleware in Sinatra

Add middleware to a Sinatra app with use.

use Rack::Logger
Order of use calls defines the stack order

require 'sinatra'

use Rack::Logger

get '/' do
  'Logged request'
end

config.ru and rackup

A config.ru file boots a Rack app with rackup or Puma.

run sets the final app
use adds middleware before it

# config.ru
require './app'
use Rack::CommonLogger
run Sinatra::Application

Common Built-in Middleware

Rack ships with useful middleware.

Rack::CommonLogger request logs
Rack::Deflater gzip compression
Rack::Static serve files
Rack::Session::Cookie sessions

# In config.ru
# use Rack::Deflater
# use Rack::Static, urls: ['/assets'], root: 'public'
puts 'See comments for usage'

Modifying the Response

Middleware often adds response headers after the inner app runs.

Capture the response triple
Mutate headers, then return it

class AddHeader
  def initialize(app); @app = app; end
  def call(env)
    status, headers, body = @app.call(env)
    headers['X-Powered-By'] = 'Ruby'
    [status, headers, body]
  end
end

app = AddHeader.new(lambda { |e| [200, {}, ['ok']] })
puts app.call({})[1].inspect

Short Circuiting Requests

Middleware can halt a request without calling the inner app.

Useful for auth gates or rate limits
Return a response triple directly

class Auth
  def initialize(app); @app = app; end
  def call(env)
    return [401, {}, ['Unauthorized']] unless env['token']
    @app.call(env)
  end
end

app = Auth.new(lambda { |e| [200, {}, ['ok']] })
puts app.call({})[0]

Quick Check

Verify your Rack understanding.

Answer it in the interactive lesson

Recap

You learned the Rack stack:

Rack is the common interface beneath Ruby web frameworks
A Rack app responds to call(env) and returns [status, headers, body]
Middleware wraps an app to add cross cutting behavior
Use use in Sinatra or config.ru to compose the stack
Middleware can modify responses or short circuit requests

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Yes — any 3 lessons of the Ruby Academy learning path, including “Middleware and Rack”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Ruby Academy course includes 4 lessons in total.

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The Rack stack. You practise Ruby Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

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