What is Clean Architecture? How is it structured and how does it work?

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IT Interview Practice · Accepted Answer

## What is Clean Architecture?

**Clean Architecture** organizes software around business behavior, separating it from user interfaces, databases and frameworks. Its aim is to reduce the effect of technology changes on important application rules.

For example, “a delivered order cannot be cancelled” is a business rule. It remains the same whether the customer uses a website or mobile application, and regardless of the storage technology.

## Main parts

Clean Architecture is commonly illustrated as circles arranged from the inside out:

- **Entities:** Hold core business data and rules, such as an order and its cancellation conditions.
- **Use cases:** Coordinate a particular application operation, such as retrieving an order, requesting cancellation and saving the result.
- **Interface adapters:** Convert data between business logic and external systems. A controller converts an HTTP request into input for the cancellation operation; a storage adapter turns order data into database queries.
- **Frameworks and tools:** Technologies at the outer edge, such as the web framework or database system.

**The essential rule is that inner code must not depend on outer code.** A use case can work with an order-storage interface defined on the business side; an external adapter implements that interface using a specific technology.

## Example: cancelling an order

When a customer requests cancellation, the controller calls the cancellation use case. The use case retrieves the order through a data-access interface, then asks the order object to check whether cancellation is allowed.

If the order has not been delivered and meets the conditions, its status changes to cancelled. The use case requests that the change be saved, the database adapter performs the write, and the controller returns the result to the customer.

At runtime, business logic can still call an adapter through an interface connected during startup. The rule that dependencies point inward concerns relationships between code components; it does not mean data or calls can only travel in one direction.

## Benefits

- **Easier rule testing:** Allowed and rejected cancellations can be tested with data in memory, without starting a web server or database.
- **A clear home for business behavior:** Rules are not scattered across controllers and queries, making them easier to read and change consistently.
- **Less disruption from technology changes:** Replacing a storage library or request handler mainly affects outer components if business contracts stay unchanged.

## Limitations

- **More components:** Interfaces, use cases and data-conversion code take time to write and maintain.
- **Boundaries need discipline:** If a use case directly uses types specific to a framework or database library, business logic becomes dependent on external technology again.

## When should you use it?

Clean Architecture is useful for systems with complex business behavior, a long development life and frequent testing needs. For a small application that only reads and writes a few tables, a simpler structure may be easier to work with. There is no need to create every circle or folder unless it helps separate real responsibilities.

What is Clean Architecture?

Main parts

Example: cancelling an order

Benefits

Limitations

When should you use it?

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