Layered Architecture ও MVC Pattern

3-Tier Layered Architecture

┌──────────────────────┐
│  Presentation (UI)         │
├──────────────────────┤
│  Business Logic            │
├──────────────────────┤
│  Data Access Layer        │
├──────────────────────┤
│       Database            │
└──────────────────────┘

Upper → Lower only। Lower → Upper আনলে coupling বাড়ে।

উদাহরণ

// Presentation
public class OrderController : ControllerBase {
    IOrderService _service;
    [HttpPost] public async Task<IActionResult> Create(Dto dto) {
        var id = await _service.PlaceAsync(dto);
        return Created(...);
    }
}

// Business
public class OrderService : IOrderService {
    IOrderRepository _repo;
    IPaymentGateway _payment;
    public async Task<int> PlaceAsync(Dto dto) {
        var order = new Order(dto);
        await _payment.ChargeAsync(order.Total);
        return await _repo.AddAsync(order);
    }
}

// Data
public class OrderRepository : IOrderRepository {
    AppDbContext _ctx;
    public Task<int> AddAsync(Order o) {...}
}

MVC ⭐

User input → Controller → Model
                  ↓         ↓
                View ←──────┘

MVP

View passive, Presenter সব logic।

MVVM ⭐ (WPF, Vue, Angular)

Model ←→ ViewModel ←→ View
              two-way binding

Repository Pattern

public interface IUserRepository {
    Task<User?> GetByIdAsync(int id);
    Task AddAsync(User u);
    void Update(User u);
}

Unit of Work

public interface IUnitOfWork {
    IUserRepository Users { get; }
    IOrderRepository Orders { get; }
    Task<int> CommitAsync();
}

Dependency Injection

builder.Services.AddScoped<IUserRepository, UserRepository>();
builder.Services.AddScoped<IOrderService, OrderService>();

public class OrderService {
    public OrderService(IUserRepository u, IOrderRepository o) {}
}

DI Lifetimes

• Singleton — পুরো জীবনে একটি
• Scoped — per request
• Transient — প্রতি resolve-এ নতুন

সুবিধা

Loose coupling, easy unit test (mock), config centralize, SOLID-D principle।