Journey Into Rails Routing -- an Under the Hood Look at How Routing Works

Following code was tested with edge rails (rails4) .

When a Rails application boots then it reads the config/routes.rb file. In your routes you might have code like this

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Rails4demo::Application.routes.draw do
  root 'users#index'
  resources :users
  get 'photos/:id' => 'photos#show', :defaults => { :format => 'jpg' }
  get '/logout' => 'sessions#destroy', :as => :logout
  get "/stories" => redirect("/photos")
end

In the above case there are five different routing statements. Rails needs to store all those routes in a manner such that later when url is ‘/photos/5’ then it should be able to find the right route statement that should handle the request.

In this article we are going to take a peek at how Rails handles the whole routing business.

Normalization in action

In order to compare various routing statements first all the routing statements need to be normalized to a standard format so that one can easily compare one route statement with another route statement.

Before we take a deep dive into how the normalization works lets first see some normalizations in action.

get call with defaults

Here we have following route

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Rails4demo::Application.routes.draw do
  get 'photos/:id' => 'photos#show', :defaults => { :format => 'jpg' }
end

After the normalization process the above routing statement is transformeed into five different variables. The values for all those five varibles is shown below.

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app: #<ActionDispatch::Routing::RouteSet::Dispatcher:0x007fd05e0cf7e8 
           @defaults={:format=>"jpg", :controller=>"photos", :action=>"show"},
           @glob_param=nil,
           @controller_class_names=#<ThreadSafe::Cache:0x007fd05e0cf7c0 
           @backend={},
           @default_proc=nil>>
conditions: {:path_info=>"/photos/:id(.:format)", :required_defaults=>[:controller, :action], :request_method=>["GET"]}
requirements: {}
defaults: {:format=>"jpg", :controller=>"photos", :action=>"show"}
as: nil
anchor: true

app is the application that will be executed if conditions are met. conditions are the conditions. Pay attention to :path_info in conditions. This is used by Rails to determine the right route statement. defaults are defaults and requirements are the constraints.

get call with as

Here we have following route

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Rails4demo::Application.routes.draw do
  get '/logout' => 'sessions#destroy', :as => :logout
end

After normalization above code gets following values

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app: #<ActionDispatch::Routing::RouteSet::Dispatcher:0x007f8ded87e740 
           @defaults={:controller=>"sessions", :action=>"destroy"},
           @glob_param=nil,
           @controller_class_names=#<ThreadSafe::Cache:0x007f8ded87e718 @backend={}, 
           @default_proc=nil>>
conditions: {:path_info=>"/logout(.:format)", :required_defaults=>[:controller, :action], :request_method=>["GET"]}
requirements: {}
defaults: {:controller=>"sessions", :action=>"destroy"}
as: "logout"
anchor: true

Notice that in the above case as is populate with logout .

root call

Here we have following route

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Rails4demo::Application.routes.draw do
  root 'users#index'
end

After normalization above code gets following values

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app: #<ActionDispatch::Routing::RouteSet::Dispatcher:0x007fe91507f278 
           @defaults={:controller=>"users", :action=>"index"},
           @glob_param=nil,
           @controller_class_names=#<ThreadSafe::Cache:0x007fe91507f250 @backend={}, 
           @default_proc=nil>>
conditions: {:path_info=>"/", :required_defaults=>[:controller, :action], :request_method=>["GET"]}
requirements: {}
defaults: {:controller=>"users", :action=>"index"}
as: "root"
anchor: true

Notice that in the above case as is populated. And the path_info is / since this is the root url .

get call with constraints

Here we have following route

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Rails4demo::Application.routes.draw do
  #get 'pictures/:id' => 'pictures#show', :constraints => { :id => /[A-Z]\d{5}/ }
end

After normalization above code gets following values

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app: #<ActionDispatch::Routing::RouteSet::Dispatcher:0x007f8158e052c8 
           @defaults={:controller=>"pictures", :action=>"show"},
           @glob_param=nil,
           @controller_class_names=#<ThreadSafe::Cache:0x007f8158e05278 @backend={}, 
           @default_proc=nil>>
conditions: {:path_info=>"/pictures/:id(.:format)", :required_defaults=>[:controller, :action], :request_method=>["GET"]}
requirements: {:id=>/[A-Z]\d{5}/}
defaults: {:controller=>"pictures", :action=>"show"}
as: nil
anchor: true

Notice that in the above case requirements is populated with constraints mentioned in the route definition .

get with a redirect

Here we have following route

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Rails4demo::Application.routes.draw do
  get "/stories" => redirect("/posts")
end

After normalization above code gets following values

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app: redirect(301, /posts)
conditions: {:path_info=>"/stories(.:format)", :required_defaults=>[], :request_method=>["GET"]}
requirements: {}
defaults: {}
as: "stories"
anchor: true

Notice that in the above case app is a simple redirect .

resources

Here we have following route

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Rails4demo::Application.routes.draw do
  resources :users
end

After normalization above code gets following values

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app: #<ActionDispatch::Routing::RouteSet::Dispatcher:0x007f9d41a315c0 
           @defaults={:action=>"index", :controller=>"users"}, @glob_param=nil, @controller_class_names=#<ThreadSafe::Cache:0x007f9d41a31598 @backend={}, @default_proc=nil>>
conditions: {:path_info=>"/users(.:format)", :required_defaults=>[:action, :controller], :request_method=>["GET"]}
defaults: {:action=>"index", :controller=>"users"}
as: "users"

app: #<ActionDispatch::Routing::RouteSet::Dispatcher:0x007f9d41a4ef80 
           @defaults={:action=>"create", :controller=>"users"}, @glob_param=nil, @controller_class_names=#<ThreadSafe::Cache:0x007f9d41a4ef58 @backend={}, @default_proc=nil>>
conditions: {:path_info=>"/users(.:format)", :required_defaults=>[:action, :controller], :request_method=>["POST"]}
defaults: {:action=>"create", :controller=>"users"}
as: nil

app: #<ActionDispatch::Routing::RouteSet::Dispatcher:0x007f9d41b63790 
           @defaults={:action=>"new", :controller=>"users"}, @glob_param=nil, @controller_class_names=#<ThreadSafe::Cache:0x007f9d41b63768 @backend={}, @default_proc=nil>>
conditions: {:path_info=>"/users/new(.:format)", :required_defaults=>[:action, :controller], :request_method=>["GET"]}
defaults: {:action=>"new", :controller=>"users"}
as: "new_user"

app: #<ActionDispatch::Routing::RouteSet::Dispatcher:0x007f9d41a10550 
           @defaults={:action=>"edit", :controller=>"users"}, @glob_param=nil, @controller_class_names=#<ThreadSafe::Cache:0x007f9d41a10528 @backend={}, @default_proc=nil>>
conditions: {:path_info=>"/users/:id/edit(.:format)", :required_defaults=>[:action, :controller], :request_method=>["GET"]}
defaults: {:action=>"edit", :controller=>"users"}
as: "edit_user"

app: #<ActionDispatch::Routing::RouteSet::Dispatcher:0x007f9d41f31818 
           @defaults={:action=>"show", :controller=>"users"}, @glob_param=nil, @controller_class_names=#<ThreadSafe::Cache:0x007f9d41f317f0 @backend={}, @default_proc=nil>>
conditions: {:path_info=>"/users/:id(.:format)", :required_defaults=>[:action, :controller], :request_method=>["GET"]}
defaults: {:action=>"show", :controller=>"users"}
as: "user"

app: #<ActionDispatch::Routing::RouteSet::Dispatcher:0x007f9d44a9bb70 
           @defaults={:action=>"update", :controller=>"users"}, @glob_param=nil, @controller_class_names=#<ThreadSafe::Cache:0x007f9d44a9bb48 @backend={}, @default_proc=nil>>
conditions: {:path_info=>"/users/:id(.:format)", :required_defaults=>[:action, :controller], :request_method=>["PATCH"]}
defaults: {:action=>"update", :controller=>"users"}
as: nil

app: #<ActionDispatch::Routing::RouteSet::Dispatcher:0x007f9d41b17480 
           @defaults={:action=>"update", :controller=>"users"}, @glob_param=nil, @controller_class_names=#<ThreadSafe::Cache:0x007f9d41b17458 @backend={}, @default_proc=nil>>
conditions: {:path_info=>"/users/:id(.:format)", :required_defaults=>[:action, :controller], :request_method=>["PUT"]}
defaults: {:action=>"update", :controller=>"users"}
as: nil

app: #<ActionDispatch::Routing::RouteSet::Dispatcher:0x007f9d439ddf68 
           @defaults={:action=>"destroy", :controller=>"users"}, @glob_param=nil, @controller_class_names=#<ThreadSafe::Cache:0x007f9d439ddf40 @backend={}, @default_proc=nil>>
conditions: {:path_info=>"/users/:id(.:format)", :required_defaults=>[:action, :controller], :request_method=>["DELETE"]}
defaults: {:action=>"destroy", :controller=>"users"}
as: nil

In this case I omitted requirements and anchor for brevity .

Notice that a single routing statement resources :users created eight normalized routing statements. It means that resources statement is basically a short cut for defining all those eight routing statements .

resources with only

Here we have following route

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Rails4demo::Application.routes.draw do
  resources :users, only: :new
end

After normalization above code gets following values

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app: #<ActionDispatch::Routing::RouteSet::Dispatcher:0x007fdf55043e40 
           @defaults={:action=>"new", :controller=>"users"}, @glob_param=nil, @controller_class_names=#<ThreadSafe::Cache:0x007fdf55043e18 @backend={}, @default_proc=nil>>
conditions: {:path_info=>"/users/new(.:format)", :required_defaults=>[:action, :controller], :request_method=>["GET"]}
defaults: {:action=>"new", :controller=>"users"}
as: "new_user"

Because of only keyword only one routing statement was produced in this case.

Mapper

In Rails ActionDispatch::Routing::Mapper class is responsible for normalizing all routing statements.

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module ActionDispatch
  module Routing
    class Mapper
      include Base
      include HttpHelpers
      include Redirection
      include Scoping
      include Concerns
      include Resources
    end
  end
end

Now let’s look at what these included modules do

Base

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module Base
  def root (options = {})
  end

  def match
  end

  def mount(app, options = {})
  end

As you can see Base handles root, match and mount calls.

HttpHelpers

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module HttpHelpers
  def get(*args, &block)
  end

  def post(*args, &block)
  end

  def patch(*args, &block)
  end

  def put(*args, &block)
  end

  def delete(*args, &block)
  end
end

HttpHelpers handles get, post, patch, put and delete .

Scoping

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module Scoping
  def scope(*args)
  end

  def namespace(path, options = {})
  end

  def constraints(constraints = {})
  end
end

Resources

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module Resources
  def resource(*resources, &block)
  end

  def resources(*resources, &block)
  end

  def collection
  end

  def member
  end

  def shallow
  end
end

Let’s put all the routes together

So now let’s look at all the routes definition together.

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Rails4demo::Application.routes.draw do
  root 'users#index'
  get 'photos/:id' => 'photos#show', :defaults => { :format => 'jpg' }
  get '/logout' => 'sessions#destroy', :as => :logout
  get 'pictures/:id' => 'pictures#show', :constraints => { :id => /[A-Z]\d{5}/ }
  get "/stories" => redirect("/posts")
  resources :users
end

Above routes definition produces following information. I am going to show info path info.

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{ :path_info=>"/":path_info=>"/photos/:id(.:format)" }

{ :path_info=>"/logout(.:format)" }

{ :path_info=>"/pictures/:id(.:format) }

{ :path_info=>"/stories(.:format)" }

{ :path_info=>"/users(.:format), :request_method=>["GET"]}

{:path_info=>"/users(.:format)", :request_method=>["POST"]}

{:path_info=>"/users/new(.:format)", :request_method=>["GET"]}

{:path_info=>"/users/:id/edit(.:format)", :request_method=>["GET"]}

{:path_info=>"/users/:id(.:format)", :controller], :request_method=>["GET"]}

{:path_info=>"/users/:id(.:format)", :request_method=>["PATCH"]}

{:path_info=>"/users/:id(.:format)", :request_method=>["PUT"]}

{:path_info=>"/users/:id(.:format)", :request_method=>["DELETE"]}

How to find the matching route definition

So now that we have normalized the routing definitions the task at hand is to find the right route definition for the given url along with request_method.

For example if the requested page is /pictures/A12345 then the matching routing definition should be get ‘pictures/:id’ => ‘pictures#show’, :constraints => { :id => /[A-Z]\d{5}/ } .

In order to accomplish that I would do something like this.

I would convert all path info into a regular experssion and I would push that regular expression in an array. So in this case I would have 12 regular expressions in the array and for the given url I would try to match one by one.

This strategy will work and this is how Rails worked all the way upto Rails 3.1 .

Aaron Patterson loves computer science

Aaron Patterson noticed that finding the best matching route definition for a given url is nothing else but pattern matching task. And computer science solved this problem much more elegantly and this happens to run faster also by building an AST and walking over it.

So he decided to make a mini language out of the route definitions . After all the route definitions , we write , follow certain rules.

And thus Journey was born.

In the next blog we will see how to write grammar rules for routing definitions , how to parse and then walk the ast to see the best match .

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