How to structure a large Rails application?

Structuring a large Rails application is essential for maintainability, scalability, and collaboration. As an application grows, adhering to Rails’ conventional structure might not suffice. Here’s a roadmap for structuring a large-scale Rails application:

1. Modular Design with Engines: Consider using Rails engines to break the application into smaller, modular components. Each engine can encapsulate related functionality, making the application more manageable and modular.

2. Service Objects: Instead of bloating models with business logic, use service objects. They promote single responsibility and encapsulate specific business actions, making code clearer and more reusable.

3. Interactors: To manage complex business logic involving multiple steps or validations, use the interactor pattern. Interactors represent a single-use case for the application.

4. Decorators/Presenters: Separate the display logic from models using decorators or presenters. This keeps models focused on business logic and relationships.

5. Domain-driven Design (DDD): Organize the application around business domains. Separate models, services, and other components based on domain boundaries.

6. Concerns: Utilize Rails’ concerns to share code between models. However, be cautious: concerns can lead to tangled code if overused.

7. Namespacing: Namespace related classes and modules, especially if you’re organizing code based on domains or features.

8. Front-end Structure: For advanced front-end interactions, consider integrating with JavaScript frameworks like React or Vue.js. Use Rails’ webpacker gem for managing front-end dependencies.

9. Configuration and Initializers: Keep configuration settings centralized in the `config` directory. Use initializers for setup processes that need to run at startup.

10. Database: Use database schemas to group related tables, especially in multi-tenant applications. This makes the database layout clearer and more scalable.

11. Background Jobs: For asynchronous operations, use background processing tools like Sidekiq or Resque. Structure background tasks in a way that they are retryable and idempotent.

12. API Versioning: If the Rails app serves as an API, ensure endpoints are versioned. This allows for smoother transitions and changes without breaking existing integrations.

As a Rails application grows, it’s imperative to re-evaluate and adjust its structure, ensuring it remains maintainable and comprehensible. The techniques and patterns mentioned above can significantly aid in achieving this.