Solution Architecture

Overview

Solution architecture defines how code is organized into layers and components. This skill covers Domain-Driven Design (DDD) layered architecture and essential coding principles like DRY and SOLID.

Core principle: The domain is the heart of your application. All other layers exist to support it.

When to Use

• Starting a new project or module
• Refactoring monolithic code into layers
• Deciding where business logic belongs
• Implementing repositories, services, or domain models
• Reviewing code for architectural violations

Not for: Simple CRUD apps, scripts, or prototypes where layering adds unnecessary complexity.

The Four Layers

┌─────────────────────────────────────┐
│         PRESENTATION LAYER          │  ← UI, Controllers, ViewModels
├─────────────────────────────────────┤
│         APPLICATION LAYER           │  ← Use Cases, Orchestration, CQRS
├─────────────────────────────────────┤
│           DOMAIN LAYER              │  ← Entities, Value Objects, Domain Services
├─────────────────────────────────────┤
│        INFRASTRUCTURE LAYER         │  ← Database, External APIs, Messaging
└─────────────────────────────────────┘

Dependency Rule: Each layer only depends on the layer directly inside it. Domain is innermost and has NO external dependencies.

Presentation Layer

Responsibility: User interaction and display

Contains:

• UI components (pages, views, components)
• Controllers / API endpoints
• ViewModels / DTOs for display
• Input validation (format only, not business rules)

Rules:

• No business logic
• Thin controllers - delegate to Application layer
• Platform-specific concerns only

// ✅ GOOD: Thin controller
[HttpPost]
public async Task<IActionResult> CreateOrder(CreateOrderRequest request)
{
    var result = await _orderService.CreateOrderAsync(request.ToCommand());
    return result.IsSuccess ? Ok(result.Value) : BadRequest(result.Error);
}

// ❌ BAD: Business logic in controller
[HttpPost]
public async Task<IActionResult> CreateOrder(CreateOrderRequest request)
{
    if (request.Items.Sum(i => i.Price) < 10)
        return BadRequest("Minimum order is $10");
    // ... more business logic
}

Application Layer

Responsibility: Use case orchestration and conditional business rules

Contains:

• Application Services (use case handlers)
• Commands and Queries (CQRS)
• DTOs for input/output
• Conditional business rules (context-dependent, policy-based)
• Cross-cutting concerns (authorization, validation, logging)
• Domain event handlers

Rules:

• Implements conditional/use-case specific business rules
• One use case = one unit of work (atomic)
• Publishes domain events
• Uses repository interfaces (not implementations)

// ✅ GOOD: Conditional rules in Application Service
public async Task<Result<OrderDto>> CreateOrderAsync(CreateOrderCommand command)
{
    var customer = await _customerRepository.GetByIdAsync(command.CustomerId);
    if (customer is null) return Result.Fail("Customer not found");
    
    // Conditional rule: Check credit limit (depends on external data)
    var pendingTotal = await _orderRepository.GetPendingTotalAsync(customer.Id);
    if (pendingTotal + command.Total > customer.CreditLimit)
        return Result.Fail("Order exceeds credit limit");
    
    // Conditional rule: Discount policy (may vary by context)
    var discount = customer.IsLoyal ? 0.1m : 0m;
    
    // Conditional rule: Minimum order for online channel
    if (command.IsOnlineOrder && command.Total < 25)
        return Result.Fail("Minimum online order is $25");
    
    var order = Order.Create(customer, command.Items, discount);
    
    await _orderRepository.AddAsync(order);
    await _unitOfWork.SaveChangesAsync();
    
    return Result.Ok(order.ToDto());
}

Domain Layer

Responsibility: Core business rules and invariants

Contains:

• Entities (identity + behavior)
• Value Objects (no identity, immutable)
• Aggregates (consistency boundaries)
• Domain Services (logic spanning entities within same aggregate)
• Domain Events
• Repository Interfaces (not implementations)
• Specifications
• Core business rules (invariants that must ALWAYS hold)

Rules:

• No dependencies on other layers
• No infrastructure concerns (no ORM attributes, no HTTP)
• Entities encapsulate behavior, not just data
• Aggregates enforce invariants

Core vs Conditional Rules

Rule Type	Layer	Characteristics	Examples
Core Rules	Domain	Always true, no exceptions, entity protects itself	"Quantity > 0", "Email must be valid format", "Order must have lines"
Conditional Rules	Application	Context-dependent, policy-based, may need external data	"Min $25 for online orders", "Credit limit check", "Loyalty discount"

How to decide:

• Can the entity enforce this alone? → Domain
• Does it need database queries? → Application
• Does it vary by use case/channel? → Application
• Would violation create impossible state? → Domain
• Is it a business policy that might change? → Application

// ✅ GOOD: Entity with behavior
public class Order : Entity<OrderId>
{
    private readonly List<OrderLine> _lines = new();
    public IReadOnlyList<OrderLine> Lines => _lines.AsReadOnly();
    public Money Total => _lines.Sum(l => l.Subtotal);
    
    public static Order Create(Customer customer, IEnumerable<OrderLineDto> items)
    {
        var order = new Order(OrderId.New(), customer.Id);
        foreach (var item in items)
            order.AddLine(item.ProductId, item.Quantity, item.Price);
        return order;
    }
    
    public void AddLine(ProductId productId, int quantity, Money price)
    {
        if (quantity <= 0) throw new DomainException("Quantity must be positive");
        _lines.Add(new OrderLine(productId, quantity, price));
        AddDomainEvent(new OrderLineAddedEvent(Id, productId));
    }
}

// ❌ BAD: Anemic entity (data only)
public class Order
{
    public int Id { get; set; }
    public List<OrderLine> Lines { get; set; } = new();
    public decimal Total { get; set; }
}

Infrastructure Layer

Responsibility: Technical implementations and external systems

Contains:

• Repository implementations
• Database context / ORM configuration
• External API clients
• Message queue implementations
• File system access
• Email/SMS services

Rules:

• Implements interfaces defined in Domain/Application
• Contains all platform-specific code
• "Get in, get out quick" - minimal logic

// ✅ GOOD: Repository implementation
public class OrderRepository : IOrderRepository
{
    private readonly AppDbContext _context;
    
    public async Task<Order?> GetByIdAsync(OrderId id)
        => await _context.Orders
            .Include(o => o.Lines)
            .FirstOrDefaultAsync(o => o.Id == id);
    
    public async Task AddAsync(Order order)
        => await _context.Orders.AddAsync(order);
}

DDD Building Blocks

Quick Reference

Building Block	Has Identity	Mutable	Purpose
Entity	Yes	Yes	Objects tracked by identity
Value Object	No	No	Descriptive, replaceable values
Aggregate	Yes	Yes	Consistency boundary with root entity
Domain Service	N/A	N/A	Logic not belonging to single entity
Domain Event	N/A	N/A	Something that happened in domain
Repository	N/A	N/A	Aggregate persistence abstraction

Entity vs Value Object

// Entity: tracked by identity
public class Customer : Entity<CustomerId>
{
    public string Name { get; private set; }
    public Email Email { get; private set; }
}

// Value Object: tracked by value, immutable
public record Email
{
    public string Value { get; }
    
    public Email(string value)
    {
        if (!IsValid(value)) throw new DomainException("Invalid email");
        Value = value;
    }
    
    private static bool IsValid(string email) => /* validation */;
}

Aggregates

Rules:

• One entity is the Aggregate Root
• External references only via root's ID
• Root enforces all invariants
• One aggregate = one transaction

// Order is aggregate root
public class Order : AggregateRoot<OrderId>
{
    private readonly List<OrderLine> _lines = new();
    
    // Only root can modify children
    public void AddLine(ProductId productId, int quantity, Money price)
    {
        ValidateCanAddLine();
        _lines.Add(new OrderLine(productId, quantity, price));
    }
}

// ❌ BAD: External code modifying aggregate internals
order.Lines.Add(new OrderLine(...)); // Bypasses invariant checks

Coding Principles

DRY (Don't Repeat Yourself)

Every piece of knowledge has a single, unambiguous representation.

// ❌ BAD: Duplicated validation
public void CreateUser(string email) { if (!email.Contains("@")) throw ...; }
public void UpdateEmail(string email) { if (!email.Contains("@")) throw ...; }

// ✅ GOOD: Single source of truth
public record Email(string Value)
{
    public Email(string value) : this(value)
    {
        if (!value.Contains("@")) throw new DomainException("Invalid email");
    }
}

SOLID Principles

Principle	Summary	Architectural Application
Single Responsibility	One reason to change	Each layer has distinct responsibility
Open/Closed	Open for extension, closed for modification	Use interfaces, strategy pattern
Liskov Substitution	Subtypes must be substitutable	Aggregates enforce invariants
Interface Segregation	Small, focused interfaces	Repository per aggregate
Dependency Inversion	Depend on abstractions	Domain defines interfaces, Infra implements

Additional Principles

• YAGNI - Don't build it until you need it
• KISS - Keep solutions simple
• Separation of Concerns - Each module handles one thing
• Explicit Dependencies - No hidden service locators

Common Mistakes

Mistake	Fix
Business logic in controllers	Move to Domain layer
Anemic domain models (data bags)	Add behavior to entities
Repository returning DTOs	Return domain objects
Domain depending on ORM	Use POCO entities, configure in Infra
Aggregate referencing another aggregate	Use IDs only
Application layer with business rules	Move rules to Domain
Fat services, thin entities	Entities should have behavior

Layer Dependency Violations

// ❌ Domain layer importing Infrastructure
using Microsoft.EntityFrameworkCore; // VIOLATION

// ❌ Domain layer importing Application
using MyApp.Application.DTOs; // VIOLATION

// ✅ Domain layer - only domain concerns
using MyApp.Domain.ValueObjects;
using MyApp.Domain.Events;

Project Structure Example

src/
├── MyApp.Presentation/          # UI, Controllers
│   ├── Controllers/
│   └── ViewModels/
├── MyApp.Application/           # Use Cases, CQRS
│   ├── Commands/
│   ├── Queries/
│   └── Services/
├── MyApp.Domain/                # Core Business Logic
│   ├── Entities/
│   ├── ValueObjects/
│   ├── Services/
│   ├── Events/
│   └── Repositories/            # Interfaces only
├── MyApp.Infrastructure/        # External Concerns
│   ├── Persistence/
│   ├── ExternalServices/
│   └── Messaging/
└── MyApp.Domain.Shared/         # Shared enums, constants

Decision Guide

Where does this code belong?

Is it UI/display related?
  → Presentation Layer

Is it orchestrating a use case?
  → Application Layer

Is it a core business rule?
  → Domain Layer

Is it talking to external systems?
  → Infrastructure Layer