dan.thoughts

Sometimes its necessary to have a code revision tag somewhere on the website. Use cases are usually different and vary from just checking against the current revision, automatic upgrades and such.

If you’re rolling deployments with capistrano, it will insert the REVISION file under the app’s root dir with git sha or svn revision or whatever tag based on SCM of your choice.

Here is the simple rack middleware that injects that revision as a ‘X-REVISION’ header in responses.

module Rack
  class Revision
    @@revision = nil
    File = ::File
 
    def initialize(app, &block)
      @app   = app
      @block = block
      @file  = File.join(Dir.pwd, 'REVISION')
    end
 
    def call(env)
      status, headers, body = @app.call(env)
      headers['X-Revision'] = revision
      [status, headers, body]
    end
 
    protected
 
    def revision
      @@revision ||= read_revision
    end
 
    def read_revision
      File.exists?(@file) ? File.read(@file).strip : 'UNDEFINED'
    end
  end
end

For instance, just put that file as lib/rack/revision.rb and change your config.ru file:

require ::File.expand_path('../config/environment',  __FILE__)
use Rack::Revision
run YOUR_APP::Application

You’ll need to restart the app to apply the changes.

To test if it works just run:

curl -i -X HEAD "http://YOUR_WEBSITE/"

Sample output would be:

ETag: "8d6228d86642025c31e3d54e9a67b14b"
Cache-Control: max-age=0, private, must-revalidate
X-Runtime: 0.001491
X-Rack-Cache: miss
X-Revision: f8c51630843898e88848261dd5ebac3fdebc5e48