Introduction

Ruby on Rails Engines provide a way to build modular applications by encapsulating functionality into separate components. They act as mini Rails applications that can be embedded into larger projects, making them useful for code reusability, modularity, and scalability.

In this guide, we explore real-world use cases of Rails Engines, highlighting how they are leveraged in e-commerce, SaaS platforms, enterprise applications, and multi-tenant architectures.

Why Use Rails Engines?

Rails Engines help solve common software development challenges:

  • Code Reusability – Develop features as standalone modules that can be used across multiple applications.
  • Modular Development – Keep applications well-structured by separating concerns into engines.
  • Faster Development – Build once, reuse multiple times to reduce development effort.
  • Scalability – Easily integrate and scale components without affecting the core application.

Now, let’s explore how different industries use Rails Engines effectively.

1. E-Commerce Platforms

Use Case: Payment Processing Module

In e-commerce applications, payment gateways are crucial. Instead of tightly coupling payment logic within the main application, developers use a Rails Engine to handle transactions separately.

Example: Spree Commerce & Solidus

  • Spree and Solidus are open-source Rails e-commerce platforms that use engines to separate cart, checkout, payments, and shipping into independent modules.
  • A PaymentsEngine can support multiple payment gateways like Stripe, PayPal, and Razorpay without modifying the core application.

Example folder structure for a Payments Engine:

my_store/  
app/  
engines/  
payments/  
app/  
config/  
lib/

Benefits

  • Decoupled architecture for integrating various payment providers.
  • Easier maintenance and upgrades for payment systems.

2. SaaS Multi-Tenant Applications

Use Case: Tenant Management Engine

SaaS platforms often serve multiple clients (tenants) within a single application. A Rails Engine can manage tenant authentication, billing, and user roles separately.

Example: Apartment & ActsAsTenant

  • Apartment uses a Rails Engine to provide multi-tenancy with database schema separation.
  • Each tenant (customer) gets an isolated environment while sharing the core application.

Example multi-tenant structure:

my_saas_app/  
engines/  
tenants/  
app/models/tenant.rb  
config/routes.rb  
lib/tasks/tenants.rake

Benefits

  • Scalable architecture supporting thousands of tenants.
  • Easier onboarding of new clients without affecting existing users.

3. Enterprise Applications

Use Case: Internal Tools and Dashboards

Large enterprises often require internal tools like admin dashboards, analytics, and reporting. Instead of embedding these directly into the main app, they can be developed as separate Rails Engines.

Example: ActiveAdmin

  • ActiveAdmin is a Rails Engine that provides a fully-featured admin dashboard with authentication and role-based access control.
  • Enterprises use this for managing customer data, generating reports, and handling system configurations.

Example structure for an internal dashboard engine:

company_app/  
engines/  
admin_dashboard/  
app/controllers/admin/  
app/views/admin/  
config/routes.rb

Benefits

  • Isolated business logic that can be used across different applications.
  • Secure and controlled access to internal tools.

4. Content Management Systems (CMS)

Use Case: Modular CMS for Blogs & Pages

Companies and startups often need a CMS to manage dynamic content like blogs, landing pages, and FAQs. Rails Engines help create a reusable CMS without modifying the core app.

Example: Refinery CMS

  • Refinery CMS is a Rails Engine that provides a fully-featured CMS for managing pages, posts, and assets.
  • It is commonly used in marketing sites, SaaS products, and corporate websites.

Example CMS structure:

my_cms_app/  
engines/  
cms/  
app/models/page.rb  
app/controllers/pages_controller.rb  
app/views/pages/

Benefits

  • Centralized CMS that can be plugged into multiple applications.
  • Easy content updates without redeploying the application.

5. API-First Applications

Use Case: Versioned API Modules

Modern applications expose public APIs for third-party integrations. Instead of managing all API versions within a single application, developers use Rails Engines to separate API logic.

Example: Grape API Engine

  • grape is a Rails Engine that enables developers to build RESTful APIs independently.
  • Companies use it to provide versioned APIs, ensuring backward compatibility.

Example API structure:

my_api/  
engines/  
v1/  
app/controllers/api/v1/  
v2/  
app/controllers/api/v2/

Benefits

  • Seamless API versioning without affecting existing consumers.
  • Scalable architecture that can be extended with new API modules.

Implementing a Custom Rails Engine

To create a custom Rails Engine, use the following command:

rails plugin new engines/my_engine --mountable

This generates a mountable engine, which can be added to routes.rb:

mount MyEngine::Engine, at: "/my_engine"

Steps to Integrate

  1. Develop the engine’s models, controllers, and views separately.
  2. Mount the engine inside the main application.
  3. Reuse it across multiple projects.

Conclusion

Rails Engines offer a powerful way to modularize applications, improving code reusability and maintainability. They are widely used in e-commerce, SaaS platforms, enterprise solutions, CMS, and API-driven applications.

By leveraging engines, developers can create scalable and efficient architectures, reducing technical debt and accelerating development.

Have you used Rails Engines in your projects? Let us know in the comments!