Comparison of functional tests vs. integration tests using current implementation and a simple implementation using Rack::Test

comparison.rb

require 'benchmark/ips'
require "action_controller/railtie"

class BlogApplication < Rails::Application
  config.eager_load = false
  config.root = File.dirname(__FILE__)
  config.session_store :cookie_store, key: 'session'
  config.secret_key_base = 'secret'
end

BlogApplication.initialize!

BlogApplication.routes.draw do
  resources :posts, only: %w[index]
end

require 'rails/test_help'

class PostsController < ActionController::Base
  def index
    render text: 'Hello, World!'
  end
end

class FunctionalTest < ActionController::TestCase
  tests PostsController

  def test_posts_index
    get :index
    assert_equal 'Hello, World!', response.body
  end
end

class IntegrationTest < ActionDispatch::IntegrationTest
  def test_posts_index
    get '/posts'
    assert_equal 'Hello, World!', response.body
  end
end

class RackTest < ActiveSupport::TestCase
  include Rack::Test::Methods

  def app
    Rails.application
  end

  def response
    last_response
  end

  def test_posts_index
    get '/posts'
    assert_equal 'Hello, World!', response.body
  end
end

Benchmark.ips do |x|
  runner = MiniTest::Unit.new

  functional = FunctionalTest.new('test_posts_index')
  integration = IntegrationTest.new('test_posts_index')
  rack = RackTest.new('test_posts_index')

  x.report('Functional Test') do
    functional.run(runner)
  end

  x.report('Integration Test') do
    integration.run(runner)
  end

  x.report('Rack Test') do
    rack.run(runner)
  end
end

results.txt

$ ruby test_comparison.rb
Calculating -------------------------------------
     Functional Test       104 i/100ms
    Integration Test        65 i/100ms
           Rack Test        83 i/100ms
-------------------------------------------------
     Functional Test     1056.1 (±5.4%) i/s -       5304 in   5.037137s
    Integration Test      671.0 (±4.5%) i/s -       3380 in   5.047506s
           Rack Test      845.4 (±4.6%) i/s -       4233 in   5.017750s
Run options: --seed 14859

# Running tests:

...

Finished tests in 0.005849s, 512.9082 tests/s, 512.9082 assertions/s.

3 tests, 3 assertions, 0 failures, 0 errors, 0 skips