Understanding Clean Architecture

Clean Architecture is a software design philosophy originally introduced by Robert C. Martin (Uncle Bob). It emphasizes organizing your codebase into distinct layers that have clear responsibilities, making the application easier to maintain, scale, and test. The key idea is simple: keep different concerns separate, so changes in one part of your application don't unexpectedly affect other parts.

Since theres too much about Clean Architecture online everywhere I keep this brief and focused on basic understanding of the idea.

Core Principles of Clean Architecture

• Separation of Concerns - Different parts of your application (business logic, UI, data storage) should not be mixed together. Each part should have its clear, dedicated responsibility.

• Dependency Rule - Inner layers, like business logic, should never depend on outer layers, like frameworks or UI components, more about it below. Instead, outer layers depend on inner layers through clear abstractions.

• Framework Independence - Clean architecture isn't tied to a specific technology or framework.

Basic Structure of Clean Architecture (in NestJS)

In this section we go over each layer of Clean Architecture and I will provide some examples using NestJS framework. This exploration is meant to serve as an example only and does not imply that Clean Architecture is limited to the NestJS framework.

The Clean Architecture layers

1. Entities (Business Logic) – Define your core business rules and data structures. These would be your domain entities (not tied directly to databases or frameworks).
   

2. Use Cases (Application Logic) – Contain your application-specific logic and orchestrate interactions between entities and external components. In NestJS, this would typically be your services.
   

3. Interface Adapters (Presentation Layer & Data Transformation) – This layer translates data between the application’s core logic and external systems. It includes:

   • Controllers – Handle HTTP requests and responses, forwarding requests to the appropriate use cases.

   • DTOs & Mappers – Ensure data is properly structured when passed between layers.
     

4. Frameworks and Drivers (External Dependencies) – This layer contains the actual implementations of external dependencies, such as:

   • Database Repositories – Implement the interfaces defined in the use case layer (e.g. using TypeORM in NestJS).

   • External APIs & Authentication – Communication with third-party services, cloud storage, or authentication providers.

   • UI Frameworks – Any frontend-related frameworks if the application includes a UI component.

Why should you use it?

• Maintainability - Since each layer has a clear responsibility, modifying or extending functionality is easier without unintended side effects.

• Scalability - Clean separation of concerns allows the system to grow without becoming a tangled mess of dependencies. See my other articles (Spaghetti, Stateful Services) for examples.

• Testability - Business logic remains isolated from frameworks, making it easier to write unit tests and ensure correctness.

• Technology Agnostic - Applications built with Clean Architecture are not tied to any specific framework, database, or UI, allowing flexibility in technology choices over time.

• Easier Refactoring – Since infrastructure details (like databases or APIs) are at the outer layers, swapping technologies or updating dependencies does not impact the core business logic.

Following this structure helps your code remain resilient to change, easier to test, and straightforward to scale and maintain.

And that's it ...

Clean Architecture provides a structured approach to building software that is modular, maintainable, and adaptable to change. By following its core principles, separation of concerns, dependency inversion, and framework independence, you create a system that is easier to scale and refactor while keeping business logic isolated from external dependencies. While it may introduce initial complexity, the long-term benefits in testability, scalability, and flexibility make it a strong architectural choice for modern applications.

Image source and read more: https://blog.cleancoder.com/uncle-bob/2012/08/13/the-clean-architecture.html