Rails%203%20In%20Action

Building Rack applications
call. When Rack receives a request to this application it will call the call method on
the object passed to run, which will then generate and return a response back to the
server. The object in this case is a lambda (or Proc) object, which automatically
responds to call.
When the call method is called on this lambda, it will respond with the threeelement
array inside it, completely ignoring the env object that is passed through.
Inside this array, you have the three elements Rack needs: the HTTP status, the headers
for the response, and the body to return.
To see your lib/heartbeat.ru in action, you can launch a Rack server by using the
command you saw in the sidebar:
rackup lib/heartbeat.ru
This is now running a server on 9292 (the standard port for Rack) using the built-into-Ruby
WEBrick HTTP server, as indicated by the server output you’ll see:
[timestamp] INFO WEBrick 1.3.1
...
[timestamp] INFO WEBrick::HTTPServer#start: pid=... port=9292
You can now go to your browser and open http://localhost:9292 to make a request to
this application. You’ll get back “Hello World,” and that’s okay. You can also make a
request to any path at the http://localhost:9292 application and it will respond in the
same way, such as http://localhost:9292/status.
What you’ve done here is write one of the simplest Rack applications possible. This
application receives a response to any path using any method, and always responds
with OK. This application will respond very quickly because it hasn’t loaded anything,
but at the cost of being a one-trick pony.
You can make this little application respond differently in a number of ways. The
easiest (and most fun!) would be to program it to change its response depending on
the path it’s given, like a Rails application does. For this, you’ll use the env object. First
up, let’s see what this env object gives you by changing your script to do this:
require 'yaml'
run lambda { |env| [200,
{'Content-Type' => 'text/plain'},
["#{env.to_yaml}"]]
}
The to_yaml method provided by the yaml standard library file will transform your
env object (spoilers: it’s a Hash) into a human-readable YAML output (like that found
in config/database.yml in a Rails application).
To make this new change apply, you can’t refresh the page like you would in a Rails
application; you have to stop the server and start it again. You can press Ctrl+C to stop
it, and rerun that command. This time when you go to your server, you’ll see output
that looks like this:
---
GATEWAY_INTERFACE: CGI/1.1
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