middleware.md

Middleware

• HTTP Request/Response Cycle
• MVC

How does the application know how to map HTTP requests to Controller actions?

Rack::Middleware

Rails routes are a middleware that map the HTTP Verb and Path (within a HTTP request) to a Controller action.

Wobbe::Application.routes.draw do
  get '/profile', to: 'users#show'  # UsersController.action(:show).call
end

Middleware in Use

A chain of classes that are initialized with a Rack Application, and respond to #call with [status_code, headers, body].

$ rake middleware
use Rails::Rack::Logger  # responsible for the files in ./log/
use Rollbar::Middleware::Rails::RollbarMiddleware  # responsible for reporting 400/500 status codes.
use ActionDispatch::Cookies
use ActionDispatch::Session::CookieStore
use ActionDispatch::Flash
...
run Wobbe::Application.routes  # the HTTP Verbs and Paths that Wobbe can respond to

Rails Middleware

Types of Middleware

• Development/Debugging
• Logging/Error reporting
• Session management
• Routing

Two common types: When given a HTTP request, perform a side effect, or return some information (respond).

Examples

• ActiveRecord::Migration::CheckPending
• Rack::CommonLogger
• ActionDispatch::Flash
• Fun with Rack
• Rollbar::Middleware

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

  def call(env)
    # before HTTP response
    status, headers, body = @app.call(env)  # call the next middleware application
    # after building HTTP response
    [status, headers, body]
  end
end

Questions:

• What is stored in env?
• Where is @app.call(env) located in the middleware?
• What happens in the #call method?
• What does the #call method return?
• What could we do with this?

Other Reading

• Introducing Rack
• rack.github.io
• Add Middleware to Rails