# Rails Engines & Modular Apps (Ruby on Rails)

> Mountable engines, isolated engines, namespacing, gem-based architecture

- 18 interview questions
- Senior
- [Interview Questions: Ruby on Rails](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions.md)

## 1. What is a Rails Engine?

**Answer**

A Rails Engine is a miniature Rails application that can be mounted within a host application. It allows sharing code (models, controllers, views, routes) between multiple applications or creating isolated and reusable features. Gems like Devise, Sidekiq Web UI and ActiveAdmin are examples of popular engines used in the Rails ecosystem.

## 2. Which command is used to generate a new mountable Rails Engine?

**Answer**

The rails plugin new command with the --mountable option generates a complete engine with its own namespace, isolated routes and folder structure. The --mountable option creates an engine fully isolated from the host application, unlike --full which generates a non-isolated engine sharing the main application's namespace.

## 3. What is the main difference between a --mountable engine and a --full engine?

**Answer**

A --mountable engine is fully isolated with its own namespace (module), its own helpers and routes isolated under a configurable prefix. A --full engine shares the host application's namespace, which can create naming conflicts. The mountable engine's isolation ensures that the engine's classes and routes don't pollute the main application's namespace.

## 15 more questions available

- How to mount a Rails Engine in the host application's routes.rb file?
- What is the purpose of the isolate_namespace directive in a Rails Engine?

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## Other Ruby on Rails interview topics

- [Ruby Basics](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/ruby-basics.md): 25 questions, Junior
- [Ruby Object-Oriented Programming](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/ruby-oop.md): 20 questions, Junior
- [Rails Fundamentals](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/rails-fundamentals.md): 18 questions, Junior
- [Routing & Controllers](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/routing-controllers.md): 22 questions, Junior
- [ActiveRecord Basics](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/active-record-basics.md): 25 questions, Junior
- [Views & ERB Templates](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/views-erb-templates.md): 20 questions, Junior
- [ActiveRecord Associations](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/active-record-associations.md): 24 questions, Mid-Level
- [Advanced ActiveRecord Queries](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/active-record-queries.md): 28 questions, Mid-Level
- [Rails Forms](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/rails-forms.md): 20 questions, Mid-Level
- [Authentication & Authorization](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/authentication-authorization.md): 22 questions, Mid-Level
- [Modern Asset Pipeline & Frontend](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/rails-asset-pipeline.md): 18 questions, Mid-Level
- [Rails API Mode](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/rails-api-mode.md): 20 questions, Mid-Level
- [Testing with RSpec](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/rails-testing-rspec.md): 24 questions, Mid-Level
- [ActiveJob & Background Jobs](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/active-job-background-jobs.md): 20 questions, Mid-Level
- [ActionCable & WebSockets](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/action-cable-websockets.md): 18 questions, Mid-Level
- [ActionMailer](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/action-mailer.md): 18 questions, Mid-Level
- [ActiveStorage](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/active-storage.md): 20 questions, Mid-Level
- [Caching Strategies](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/caching-strategies.md): 20 questions, Mid-Level
- [Advanced Migrations](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/database-migrations-advanced.md): 20 questions, Mid-Level
- [Performance Optimization](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/performance-optimization.md): 26 questions, Senior
- [Rails Design Patterns](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/rails-design-patterns.md): 22 questions, Senior
- [Ruby Metaprogramming](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/ruby-metaprogramming.md): 20 questions, Senior
- [Rails Security](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/rails-security.md): 22 questions, Senior
- [GraphQL with Rails](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/graphql-rails.md): 20 questions, Senior
- [Deployment & Production](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/rails-deployment-production.md): 20 questions, Senior
- [Monitoring & Logging](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/monitoring-logging.md): 20 questions, Senior
- [Rails Upgrade Strategies](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions/rails-upgrade-strategies.md): 18 questions, Senior

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