CGI: Ruby’s Bare Metal

2015-01-05

How simple can you make a web request?

Happy 2015 everyone! For 2015, I wanted to spend some time documenting and automating my business as much as possible. Ezra Z’s and James Golick’s recent passing was a reminder to myself about life: hope for the best but plan for the worst.

My biggest technical task was then to automate the onboarding of a new Sidekiq Pro customer. If I pass away next week, I want people to still be able to purchase and renew their subscription so my wife and child have recurring income they can count on for the next few years. Essentially I want to automate day-to-day operations.

My Sidekiq Pro server is as simple as humanly possible: it’s running only Apache. Perfect for serving static files but how do I handle an arbitrary request? That’s when I asked myself: How simple can you make a web request? The requirements are straightforward: Stripe will call my server with a subscription event when someone starts or stops their Sidekiq Pro subscription. I need a script to perform the magic to grant/revoke access and send the customer an email with access details. This call will only happen a few times a day, max.

This is a perfect case for going down to the bare metal and using the oldest web technology: CGI.

Common Gateway Interface

CGI was the first standard for tying Unix and the Web together. The Unix programming model says a process should take input on STDIN and output on STDOUT. CGI allows a webserver like Apache to call an external script with the details of a web request as STDIN. The script then outputs the HTTP response back as STDOUT. Ruby’s cgi library will parse the request coming from STDIN and provides some response output helpers your code can use to generate HTML responses.

In my case, Stripe POSTs a blob of JSON in the request body. Since I’m responding to the Stripe robot, it only needs to see a 200 OK response — no fancy view rendering layer required.

#!/usr/bin/env ruby require 'json' require 'cgi' cgi = CGI . new # CGI tries to parse the request body as form parameters so a # blob of JSON awkwardly ends up as the one and only parameter key. stripe_event = JSON . parse(cgi . params . keys . first) do_the_magic(stripe_event) # magic happens right here cgi . out( "status" => "OK" , "type" => "text/plain" , "connection" => "close" ) do "Success" end

I configured Apache to know to execute my CGI script by adding this inside the vhost configuration:

ScriptAlias /stripe/ /opt/stripe/ <Directory /opt/stripe/> Require all granted </Directory>

Now if I request http://server/stripe/event.rb , Apache will call /opt/stripe/event.rb .

Look at what I’m not running: puma or unicorn, rails or sinatra, redis or memcached, postgres or mysql, bundler, capistrano, etc. The real thing is using 3-4 gems. That’s it. The script runs in a few seconds and then exits. Nothing to keep running 24/7 and nothing to monitor. Deployment means using scp to copy the .rb file to the server. I don’t even have to restart anything upon deploy because nothing was running in the first place!

Reality Check

CGI certainly isn’t the right solution for every problem: each request starts a new Ruby process so there’s a small bit of overhead but for systems which expect little traffic but require maximum reliability, it’s something worth considering. There’s a higher performance variant of CGI called FastCGI which solves the performance overhead by keeping a process running 24/7.

Ultimately plain old CGI solved my requirements: only Apache is running 24/7 and new Sidekiq Pro customers now get their license information within seconds of purchase, making everyone happy!