12factor/content/config-in-practice.md

Example: Canonical host config

rack-canonical-host is of Rack middleware for Ruby web apps which redirects the user to the canonical hostname for the app. For example, if your production deploy can be reached at both hostnames www.example.com and example.com, but you wish for www.example.com to be the canonical host, using rack-canonical-host will automatically redirect users there when they access it on example.com.

Doing it wrong #1: Hostname as a constant in the code

CANONICAL_HOST = 'www.example.com'
use(Rack::CanonicalHost, CANONICAL_HOST)

This violates twelve-factor because it stores config in the codebase.

Doing it wrong #2: Hostname from conditional on "environment"

use Rack::CanonicalHost do
  case ENV['RACK_ENV'].to_sym
    when :staging then 'example.com'
    when :production then 'staging.example.com'
  end
end

This violates twelve-factor because it uses a conditional on the RACK_ENV value which stores an deploy name like staging or production. This stores config in the codebase (the hardcoded hostnames), and uses a non-granular, non-orthogonal deploy name to choose the canonical host. Both of these things make it impossible to add new deploys without changing the code. (As a thought experiment, imagine what it would take to open-source an app that had the above block of code in it.)

Doing it right: Hostname from an env var

use(Rack::CanonicalHost, ENV['CANONICAL_HOST']) if ENV['CANONICAL_HOST']

Doing it really right: Library uses env var directly

A well-written library of the nature of rack-canonical-host will use the enviroment variable directly, simplifying the code even further:

use Rack::CanonicalHost