Inspecting Rails 4 Request Dispatch Using Ruby 2 Dot 0

Ruby 2.0 has a cool new feature that many people talk about: TracePoint.

TracePoint essentially allows you to hook into Ruby’s events and listen for events.

Being curious and since I just started a brand new Rails 4/Ruby 2 app, I decided to write a little middleware and see what Rails is up to when handling incoming requests.

Here is my TracePoint Rack Middleware.

class TracePoint
  class Middleware

    def initialize(app)
      @app = app

    def call(env)
      stats = {}
      trace = do |tp|
        stats[tp.defined_class] ||= {}
        stats[tp.defined_class][tp.method_id] ||= 0
        stats[tp.defined_class][tp.method_id] += 1
      response =

      puts "#{stats.keys.size} classes used"
      puts "#{{|k,v| v.keys}.flatten.size} methods used"
      puts "#{{|k,v| v.values}.flatten.sum} methods dispatched"


(the gist shows a modified version so I could dump to disk the json representation of the calls)

I then inserted the middleware in Rails:

# in application.rb
config.middleware.insert_before(ActionDispatch::Static, TracePoint::Middleware)

I saved the output in json format for the curious: click here

On average, in production mode, using Ruby 2.0 and Puma on my laptop, my hello world index page takes 5ms.

To render my page, Rails uses (more or less):

  • 250 classes
  • 750 methods (not including C functions)
  • and dispatches 2704 methods (not including calls to C functions)

Here is a small selection of some of the methods dispatched:

"String": {
    "underscore": 1,
    "blank?": 14,
    "html_safe": 78
"ActiveSupport::Inflector": {
  "underscore": 2,
  "inflections": 2
"Hash": {
  "with_indifferent_access": 2,
  "except": 1,
  "except!": 1,
  "stringify_keys": 5,
  "transform_keys": 13,
  "stringify_keys!": 1,
  "transform_keys!": 3,
  "extractable_options?": 4,
  "extract!": 2,
  "symbolize_keys": 8,
  "reverse_merge": 1,
  "slice": 2,
  "symbolize_keys!": 2
"ActionView::CompiledTemplates": {
  "_app_views_welcome_index_html_erb__4177595130715791755_70209827438920": 1,
  "_app_views_layouts_application_html_erb___652124533295419796_70209827456500": 1
"ActiveSupport::Notifications::Fanout": {
  "start": 4,
  "listeners_for": 12,
  "listening?": 5,
  "finish": 3

TracePoint is a great new addition and I hope to see some new crazy tools being developed (production dead-code analyzer, deprecation code path finder anyone?)

To end, this short post, here is an interesting quote from Chad Fowler Berliner by adoption:

Abstractions are expensive. The cost increases exponentially as you add them to a codebase. Chad Fowler

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