Separation of Concerns (SoC) in context of Clean Architecture

Separation of Concerns (SoC) is a software design principle that encourages dividing a program into distinct sections, each addressing a specific concern or responsibility. This principle aims to improve maintainability, flexibility, and readability of the code by reducing complexity and promoting modularity. Clean Architecture is an architectural pattern that strongly emphasizes SoC. Here's how SoC is applied in the context of Clean Architecture:

1. Clear Layered Structure:

   • Clean Architecture divides the application into concentric layers, each with a specific responsibility. The layers are typically arranged in a hierarchy, with the innermost layer containing the most critical and high-level business logic and the outer layers containing details like databases, frameworks, and UI.

2. Application Layer (High-level Policies):

   • The innermost layer in Clean Architecture is the application layer, also known as the high-level policies layer. This layer contains the core business logic, use cases, and application-specific rules.
   • It is responsible for coordinating the flow of data and controlling the application's behavior.

3. Domain Layer (Business Entities and Rules):

   • Within the application layer, there is a domain layer that holds the business entities, domain logic, and business rules.
   • The domain layer encapsulates the heart of the application and should be independent of external frameworks, databases, and UI.

4. Infrastructure Layer (External Details):

   • The outer layers, such as the infrastructure layer, deal with external concerns like databases, frameworks, and external services.
   • Infrastructure components implement interfaces defined in the application layer, allowing the high-level policies to remain independent of specific technologies.

5. User Interface (UI) Layer:

   • The outermost layer is the user interface layer, which could be a web interface, a desktop application, a command-line interface, or any other form of interaction with the user.
   • The UI layer is responsible for presenting information to the user and capturing user input. It should delegate complex business logic to the application layer.

By adhering to SoC in Clean Architecture:

• Maintenance and Changes: Changes in one concern (e.g., business logic) do not affect other concerns (e.g., data storage or UI). This makes the system more maintainable, and changes can be localized to specific layers.

• Testability: Each layer can be tested independently, facilitating unit testing of the core business logic without needing to involve external details.

• Flexibility: The system is more adaptable to changes in technology or requirements. For example, you can change the database or UI framework without affecting the core business rules.

Here's a simplified illustration of Clean Architecture layers and their concerns:

+--------------------------+
|   High-level Policies   |  <-- Application Layer
|   (Application Services) |
+--------------------------+
           ^
           |
+--------------------------+
|   Business Entities      |  <-- Domain Layer
|   (Business Rules)        |
+--------------------------+
           ^
           |
+--------------------------+
|   External Details       |  <-- Infrastructure Layer
|   (Databases, Frameworks) |
+--------------------------+
           ^
           |
+--------------------------+
|   User Interface          |  <-- UI Layer
|   (Web, Desktop, CLI)     |
+--------------------------+

By maintaining a clear separation of concerns in Clean Architecture, you create a modular and flexible system that is easy to understand, extend, and maintain over time.