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Sculpting Monoliths: The Art of Packaging Rails with Components, Gems, and Engines


This content originally appeared on DEV Community and was authored by Alex Aslam

Every seasoned Rails developer knows the feeling. You start with a clean app/ directory, a canvas of pure potential. The controllers are lean, the models are eloquent, and the views are pristine. Fast forward two years. You have a app/models directory that scrolls for days, concerns that concern everyone and no one, and a test suite that groans under the weight of a thousand implicit dependencies.

Your masterpiece has become a marble block you can no longer lift, let alone reshape.

This is the journey every growing application and every maturing developer must take: the path from a single, unified codebase to a composed architecture of distinct, purposeful parts. It’s not just engineering; it’s an art form. It’s the art of packaging.

Let’s explore our three primary chisels: the focused Component, the sharable Gem, and the integrated Engine.

The Journey Begins: Recognizing the Cracks in the Marble

You don’t start by breaking things. You start by noticing the strain.

  • The Domain Blur: Is your User model responsible for authentication, billing, content creation, and logging? It’s a god object, and it’s tired.
  • The Test Molasses: To run a unit test for a Post, do you need to boot the entire universe, including the payment gateway and email service? Your tests are telling you something.
  • The Team Treadmill: Four teams are committing to the same repository, and every merge is a high-stakes negotiation. The friction is a drag on velocity.

When you see these patterns, it’s time to pick up your tools.

Chisel #1: The Component – The Art of Internal Encapsulation

The Philosophy: A Component isn’t a packaged artifact you gem install. It’s a pattern, a discipline of internal organization. It’s the first and most crucial step in defining boundaries within your monolith.

In the old days, we might have used a lib/ or app/concerns folder. The modern, Rails-way is to leverage Zeitwerk and autoloading to create pristine namespaces.

The Artistry in Practice:

Imagine your bloated app/models folder. You have a User model that’s deeply entangled with a Subscription, Plan, and Invoice. This is your Billing domain.

You don’t need a gem or an engine yet. You need a boundary.

You create a new home:

app/
  components/
    billing/
      app/
        models/
          billing/subscription.rb
          billing/plan.rb
          billing/invoice.rb
        controllers/
          billing/subscriptions_controller.rb
        jobs/
          billing/charge_customer_job.rb
      lib/
        billing/version.rb
      component.rb

The magic file is component.rb, which acts as the main require point.

# app/components/billing/component.rb
module Billing
  # This file bootstraps the component for the main application.
  # It can require all necessary files and configure the component.
  require_relative "app/models/billing/subscription"
  require_relative "app/models/billing/plan"
  # ... and so on

  # Perhaps some component-wide configuration
  class << self
    attr_accessor :default_currency
  end
  @default_currency = :usd
end

When to Wield This Chisel:

  • Logical Separation: You want to isolate a business domain (Billing, Content, Analytics) without the overhead of a separate package.
  • Team Scalability: You can assign a team to “own” the billing component. The public interface is clear, the internals are private.
  • The First Step: This is often the perfect precursor to extracting a gem or engine. You prove the boundary works before you cut it loose.

The Payoff: You’ve introduced structure and clarity. Your main app/models is lighter. Your Billing domain is no longer a scattered set of files but a cohesive, conceptual unit.

Chisel #2: The Gem – The Art of the Pure Abstraction

The Philosophy: A Gem is a packaged unit of reusable code. Its essence is independence. A well-crafted gem doesn’t know or care if it’s being used in a Rails app, a Sinatra app, or a plain Ruby script. It solves one problem elegantly.

The Artistry in Practice:

You look at your Billing component and realize the core logic—creating plans, calculating prorations, applying taxes—is pure business logic. It has no direct dependency on Rails.

This is a candidate for a gem.

You run bundle gem mycorp-billing and you begin the careful surgery. You extract the models, but you don’t call them ActiveRecord::Base anymore. They are Plain Old Ruby Objects (POROs). You extract the ChargeCustomerJob, but it becomes a class that expects a logger and a payment_gateway object to be injected.

Your gem’s structure is classic and clean:

mycorp-billing/
  lib/
    mycorp/
      billing/
        subscription.rb
        plan.rb
        invoice_calculator.rb
    mycorp-billing.rb
  mycorp-billing.gemspec

The gem’s entry point sets up the namespace and requires the files.

# lib/mycorp-billing.rb
require_relative "mycorp/billing/subscription"
require_relative "mycorp/billing/plan"
# ...

module Mycorp
  module Billing
    # Your elegant, framework-agnostic code lives here.
  end
end

When to Wield This Chisel:

  • Reusable Logic: You have code that is used across multiple, disparate applications.
  • Algorithm Isolation: The core of your domain is a complex algorithm (e.g., a search index, a billing calculator) that deserves a life of its own.
  • Dependency Minimization: You want to keep this unit of code lean and free from the “Rails Way” if it doesn’t need it.

The Payoff: You now have a distributable, versioned, and tested library. It’s a sharp, focused tool in your organization’s utility belt.

Chisel #3: The Engine – The Art of the Integrated Mini-Application

The Philosophy: An Engine is a mini-Rails application that can be mounted inside another. It’s the full-stack package. If a Gem is a library, an Engine is a plugin. It expects to be inside a Rails context and can contain models, controllers, views, routes, and assets.

The Artistry in Practice:

Your Billing component has views for a subscription management panel. It has routes like /billing/subscriptions. It has mailers for sending invoices. This isn’t just logic; it’s a full-fledged feature.

This is the perfect candidate for a Rails Engine.

You create it with rails plugin new Mycorp::Billing --mountable --full.

The --mountable flag creates the isolated namespace and a dedicated Engine class. The --full flag ensures it gets the full treatment with ActiveRecord, ActionMailer, etc.

Your structure now mirrors a full Rails app:

mycorp-billing/
  app/
    controllers/
      mycorp/billing/subscriptions_controller.rb
    models/
      mycorp/billing/subscription.rb
    views/
      mycorp/billing/subscriptions/index.html.erb
  config/
    routes.rb
  lib/
    mycorp/billing/engine.rb # The heart of the engine
    mycorp/billing.rb

The magic is in the engine.rb and the main file.

# lib/mycorp/billing.rb
module Mycorp
  module Billing
  end
end

require "mycorp/billing/engine" # This is critical!

# lib/mycorp/billing/engine.rb
module Mycorp
  module Billing
    class Engine < ::Rails::Engine
      isolate_namespace Mycorp::Billing # This prevents conflicts with the host app

      # Optional: Auto-load migrations from our path
      initializer "mycorp-billing.load_migrations" do |app|
        app.config.paths["db/migrate"] += paths["db/migrate"].existent
      end
    end
  end
end

In your host application’s config/routes.rb:

Rails.application.routes.draw do
  mount Mycorp::Billing::Engine, at: "/billing"
  # ... your other routes
end

And just like that, all engine routes are available under /billing.

When to Wield This Chisel:

  • Full-Feature Extraction: You are packaging a complete vertical slice of your application (e.g., an admin panel, a blog, a helpdesk).
  • Shared UI & Behavior: Multiple applications need the same set of controllers, views, and routes.
  • Product Modularity: You want to be able to optionally include features in different deployments of your main product.

The Payoff: You have created a self-contained, reusable feature that integrates seamlessly into a host Rails application. It’s a masterpiece of modularity.

The Master’s Touch: Knowing Which Tool to Reach For

The artistry isn’t in knowing how to use the tools; it’s in knowing when.

  1. Start with a Component. Feel the boundaries of your domain within the monolith. Live with it. Refine the public API.
  2. Does it need to be framework-agnostic? Is the core logic valuable outside of a web context? Extract it as a Gem. Your Engine can then depend on this gem, layering the web interface on top of the pure logic.
  3. Is it a full-stack feature? Does it need routes, views, and a UI? Package it as an Engine.

The most elegant solutions often use a combination: a pure logic Gem, wrapped by a full-stack Engine, both developed and tested using the disciplined structure of a Component inside a host application.

This journey from a tangled monolith to a symphony of well-defined parts is the highest calling of a senior engineer. It’s not just about making code work; it’s about sculpting it into something maintainable, scalable, and beautiful.

Now go forth and sculpt. Your marble block awaits.


This content originally appeared on DEV Community and was authored by Alex Aslam