uprinter / ddd

DDD Building Blocks

Based on the book by Vladik Khononov "Learning Domain-Driven Design: Aligning Software Architecture and Business Strategy".

Implementing Simple Business Logic

Transaction Script

Active Record

Tackling Complex Business Logic

Domain Model

Value object

• A value object is an object that can be identified by the composition of its values
• Value objects are implemented as immutable objects
• Since the equality of value objects is based on their values rather than on an id field or reference, it’s important to override and properly implement the equality checks

public class Color
{
    public readonly byte Red;
    public readonly byte Green;
    public readonly byte Blue;
    
    public Color(byte r, byte g, byte b)
    {
        this.Red = r;
        this.Green = g;
        this.Blue = b;
    }
    
    public Color MixWith(Color other)
    {
        return new Color(
            r: (byte) Math.Min(this.Red + other.Red, 255),
            g: (byte) Math.Min(this.Green + other.Green, 255),
            b: (byte) Math.Min(this.Blue + other.Blue, 255)
        );
    }
}

Entities

• An entity is the opposite of a value object
• It requires an explicit identification field to distinguish between the different instances of the entity
• Entities are not immutable and are expected to change
• Value objects describe an entity's properties

class Person
{
    public readonly PersonId Id;
    public Name Name { get; set; }
    
    public Person(PersonId id, Name name)
    {
        this.Id = id;
        this.Name = name;
    }
}

Aggregates

• An aggregate is an entity: it requires an explicit identification field and its state is expected to change during an instance's lifecycle
• The goal of the pattern is to protect the consistency of its data
• The aggregate is a consistency enforcement boundary
• Its state can only be mutated by executing corresponding methods of the aggregate's public interface
• An aggregate's public interface is responsible for validating the input and enforcing all of the relevant business rules and invariants

public class Ticket
{
    private UserId          _customer;
    private List<ProductId> _products;
    private UserId          _assignedAgent;
    private List<Message>   _messages;
}

Event-Sourced Domain Model