4 - Clean Architecture: Interface Adapters

Welcome back to our series on Clean Architecture! In the previous posts, we covered the fundamentals of Entities and Use Cases, emphasizing their roles in maintaining a clean and maintainable codebase. Today, we will explore the Interface Adapters layer and its importance in connecting our application’s core logic to external systems.

What are Interface Adapters?

Interface Adapters serve as the bridge between the core application logic (Entities and Use Cases) and external components like databases, user interfaces, or external services. This layer is crucial because it translates data and commands between the outside world and the application, ensuring that the core logic remains isolated and independent of any external concerns.

Responsibilities of Interface Adapters

1. Data Transformation: Convert data formats between the external world (like JSON or XML) and the internal structures used by your Entities and Use Cases.

2. Communication: Handle communication with external services or databases. This includes making API calls, querying databases, and processing responses.

3. Routing: Direct incoming requests to the appropriate Use Cases or application services.

4. Error Handling: Manage errors that may occur during communication with external systems and translate them into user-friendly messages or exceptions.

Implementing an Interface Adapter for Payroll Processing

Let’s implement a simple Interface Adapter for our payroll system. This adapter will handle HTTP requests to process payroll for an employee. We’ll use ASP.NET Core for our web API.

Example: Payroll Controller

Here’s an example of a basic PayrollController that acts as an Interface Adapter:

[ApiController]
[Route("api/[controller]")]
public class PayrollController : ControllerBase
{
private readonly ProcessPayrollUseCase _processPayrollUseCase;

public PayrollController(ProcessPayrollUseCase processPayrollUseCase)
{ 
_processPayrollUseCase = processPayrollUseCase;
}
[HttpPost("{employeeId}")] public IActionResult ProcessPayroll(int employeeId)
{
try
{
   _processPayrollUseCase.ProcessPayroll(employeeId);
            return Ok("Payroll processed successfully.");
        }
        catch (ArgumentException ex)
        {
            return NotFound(ex.Message);
        }
        catch (Exception ex)
        {
            return StatusCode(500, "An error occurred while processing payroll.");
        }
    }
}

Breakdown of the Code

1. Controller Setup: The PayrollController is decorated with [ApiController] and [Route] attributes to define it as an API controller that handles HTTP requests.

2. Dependency Injection: The ProcessPayrollUseCase is injected into the controller, ensuring that we adhere to the Dependency Injection principle and keeping the controller decoupled from concrete implementations.

3. ProcessPayroll Action: This action method handles HTTP POST requests. It:

   • Calls the ProcessPayroll method from the Use Case.
   • Returns appropriate responses based on the outcome:
     • 200 OK for successful processing.
     • 404 Not Found for invalid employee IDs.
     • 500 Internal Server Error for unexpected issues.

Benefits of Using Interface Adapters

1. Decoupling: Interface Adapters help decouple your core application logic from external systems, making it easier to test and maintain your code.

2. Flexibility: By separating the concerns, you can easily change or replace external components without affecting the core logic. For example, if you decide to switch from a REST API to a message queue, you only need to update the adapter.

3. Testing: Interface Adapters allow you to test your core logic independently by mocking or stubbing external systems during unit testing.

4. Consistency: They ensure a consistent interface for the core application logic, regardless of how data is sourced or delivered.

Conclusion

In this post, we explored the Interface Adapters layer of Clean Architecture and its essential role in bridging the core application logic with external systems. We implemented a simple API controller for our payroll system, demonstrating how to handle requests and translate them into use case operations.

Next, we’ll focus on how to integrate all these layers together in a complete application, ensuring that our design adheres to Clean Architecture principles.

Stay tuned for part 5!