Rails%203%20In%20Action

518 CHAPTER 18 Rack-based applications
18.1.1 A basic Rack application
To build a basic Rack application, you only need to have an object in Ruby that
responds to the call method. That call method needs to take one argument (the
request) and also needs to return a three-element Array object. This array represents
the response that will be given back to Rack, and looks something like this:
[200, { "Content-Type" => "text/plain"}, ["Hello World"]]
The first element in this response array is the status code for your response. In this
case, it’s 200, which represents a successful response. You had a bit of a play with status
codes back in chapter 13 when you were building your API, so these should be no mystery
at this point.
The second element in this array are the headers that will be sent back. These
headers are used by the browser to determine how to deal with the response. In this
case, the response will be rendered as-is to the page because the Content-Type header
is text/plain, indicating normal text with no formatting applied. Usually your Rack
application would set this to text/html to indicate an HTML response.
Finally, the third element represents the response body, which is sent back along
with the status code and headers to Rack. Rack then compiles it all into an HTTP
response, which is sent back to where the request came from.
Let’s see this in action now. You’re going to create a light Rack application that
responds with “Hello World” whenever it receives a request. This kind of application is
often used to check and see if a server is still up and responding to HTTP calls. You’ll
create a new file inside your Ticketee’s application’s lib called lib/heartbeat.ru (you’re
checking the “heartbeat” of the server) and fill it with this content:
run lambda { |env| [200, {'Content-Type' => 'text/plain'}, ['OK']] }
The .ru extension for this file represents a Rack configuration file, also known as a
Rackup file. In it, you call the run method, which needs an object that responds to
You already have a Rackup file
Your Rails application also has one of these .ru files, called config.ru, which is used
by Rack-based servers to run your application. You can see this in action by running
the rackup config.ru command, which will start up your application using the
config.ru file’s configuration.
If you look in this file, you’ll see these lines:
# This file is used by Rack-based servers to start the application.
require ::File.expand_path('../config/environment', __FILE__)
run Ticketee::Application
The first line requires config/environment.rb for the application, which is responsible
for setting up the environment of the application. Then it uses the run method—just
as you are—except it’s passing Ticketee::Application, which actually responds
to call.
Cool stuff.