Imagine your app like an onion with several layers. The most important rules stay protected in the center while external tools sit on the outer rings.

The Onion Structure

Dependencies only point inward toward the center. This means the core of your app never has to worry about what is happening in the outside world.

The One-Way Rule

These are the heart of your application and represent basic concepts like 'Customer' or 'Product.' They contain the most fundamental rules that rarely ever change.

Core Business Entities

These are specific instructions for tasks the app performs, such as 'Create a New Account.' They coordinate how data moves between the core and the user.

Application Use Cases

These act as bridges that reformat information between the app's internal logic and external systems. They ensure everyone is speaking the same language.

The Data Translators

The outermost ring is reserved for things like databases, websites, and third-party services. These are treated as replaceable parts rather than the main focus.

External Tools Layer

This principle means keeping different types of tasks in separate folders or files. It prevents a small update in the visual design from breaking the calculation logic.

Separation of Concerns

You can check if the app's brain works correctly without needing the internet or a real database. This makes finding mistakes much faster and easier.

Testing in Isolation

Because the core logic is separate, you can switch from a website to a mobile app easily. The way the app 'thinks' stays exactly the same.

Interchangeable User Interfaces

The app is designed so that the specific brand of database doesn't matter. You can swap one for another without rewriting your main business rules.

Plug-and-Play Databases

This is a way to ask for information without knowing exactly where it is stored. It keeps the messy details of data retrieval hidden from the main logic.

The Data Storage Bridge

Every small piece of your software should have exactly one job to do. This makes it much simpler to fix, understand, and improve over time.

Single Purpose Code

Instead of your app following the rules of external tools, the tools must follow the rules of your app. This keeps your business logic in charge.

Keeping Logic in Control

Your main app logic should not depend on popular coding libraries to work. This prevents your software from becoming outdated when a library is no longer supported.

The Software Onion