Breaking down a Rails 2 monolith using plugins

Going SOA is no easy feat, and as usual when something is complex and daunting, the best course of action is to break down the problem and take it one step at a time. With a big application, a good first step would be to separate the application into it’s apparent domain concepts.

If you, like many other successful startups, have a large monolithic Rails 2 application and want to break it down into multiple more manageable parts, using plugins might be a way forward.

I will use an admin area to illustrate my point, but this process can be applied to any area of an application.

Rails plugins to the rescue

Rails 2 comes with support for putting application logic into plugins. This concept was evolved and extended into engines in Rails 3. A plugin can contain routes, controllers, models, rubygems and tests, so they act as self-contained applications.

By moving the admin area into a plugin, it becomes cleanly separated from the rest of the application and it can be worked on and tested in isolation. It could even eventually be moved outside the application.

The following describes the various aspects of moving domain logic into a plugin.

Using an alternative plugin folder

The default location for plugins in Rails 2 is vendor/plugins, but given this is not vendor logic, we’d like to put it elsewhere. We’ve decided on adding a services folder to contain extracted plugins. You can tell Rails to look here for plugins by adding this to your `config/environment.rb: do |config|
  config.plugin_paths << 'services'

Bootstrapping a plugin

You can scaffold a new plugin via script/generate plugin <name> and move the created folder (vendor/plugins/<name>) into the services folder.

The scaffold creates the appropriate hook files you need to integrate the plugin with the main application. This includes hooks into how the plugin is installed and removed, which you propbably don’t need. A file named init.rb is also created, and this is where you add any initialization the plugin might need when the main application is booted.

Directory structure

Below is an example of how a plugin can be structured.

▾ services/
  ▾ admin/
    ▾ app/
      ▸ controllers/
      ▸ helpers/
      ▸ mailers/
      ▸ models/
      ▸ views/
    ▸ config/
    ▸ lib/

Rails will automatically add app/controllers, app/models, app/views and config to the load path. If you wish to add other folders like app/mailers and app/helpers, this can be done by adding the following to init.rb:

# add plugin folders to ruby and rails load-paths
%w{ helpers mailers }.each do |dir|
  path = File.join(File.dirname(__FILE__), 'app', dir)
  $LOAD_PATH << path
  ActiveSupport::Dependencies.autoload_paths << path

# include helpers so they are accessible in views
ActionView::Base.class_eval do
  include Admin::Helper


Routes can easily be extracted to a plugin by adding a routes.rb in the config folder:

ActionController::Routing::Routes.draw do |map|
  map.namespace :admin do |admin|
    admin.resources :permissions


Rails 2 doesn’t support mailers in plugins out of the box, but you can easily make it work by setting the template_root in your extracted mailers:

# file located in: services/admin/app/mailers/admin/mailer.rb
module Admin
  class Mailer < ActionMailer::Base

    self.template_root = "#{RAILS_ROOT}/services/admin/app/views"

    def admin_welcome_message
      # ...

This will allow you to put you mailer views in services/admin/app/views/admin/mailer.

Are plugins future proof?

If you’re moving towards Rails 3/4, then going with a plugin-based solution might seem like going backwards. Plugins can easily be changed to work with both Rails 3 and 4. One method of doing that is covered here.

Another solution is to reorganise the plugin into a Rails engine structure and make it work more like a full-fledged Rails app on it’s own.


While Rails 2 plugins are not as neat and full-featured as Engines in Rails 3, they can be used to break down a large application into separate parts. The extraction is pretty straightforward and leaves you with a common place for domain logic, as opposed to having to search through the app folder.

Splitting your test suite is another neat benefit and can save development and testing time by only needing to run the tests relevant for your changes.

If you hit any issues with this approach, please do get in touch. Go forth and refactor!


photo of Jesper Kjeldgaard

Software Engineer
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