Can you explain the concept of multi-layer architecture in software system development?

Multilayer architecture is an approach to designing software systems where the application is divided into several logical layers, each responsible for its own area of concern. This increases modularity, simplifies maintenance and testing.

Main layers:

1. Presentation Layer — responsible for user interaction (UI, API). Requests are processed here, and information is displayed.

2. Business Logic Layer — contains the rules and logic of the application, data processing, validation.

3. Data Access Layer — responsible for interaction with the database or other data sources.

4. Data Model Layer — structures and objects representing data.

Example in C#:

// Business logic layer
public class OrderService {
    private readonly IOrderRepository _orderRepository;

    public OrderService(IOrderRepository orderRepository) {
        _orderRepository = orderRepository;
    }

    public void PlaceOrder(Order order) {
        // business rules
        if(order.Items.Count == 0) throw new Exception("Empty order");
        _orderRepository.Save(order);
    }
}

Such separation allows changing one layer without affecting others, improves readability and scalability of the system.