domain-driven-design

Domain-Driven Design (DDD)

DDD is a software design approach focusing on modeling software to match a domain according to input from that domain's experts. It is essential for tackling high complexity in the heart of software.

When to Use

• Complex Business Domains: Healthcare, fintech, logistics, and enterprise systems with intricate operational rules and workflows.
• Monolith Decomposition: Establishing well-defined Bounded Contexts as boundaries for microservices or modular monoliths.
• Cross-Functional Team Alignment: Creating a shared Ubiquitous Language between domain experts and software engineers.
• High-Change Core Software: Managing business logic that evolves rapidly without introducing unexpected regressions.

Quick Start

// Aggregate Root
public class Order {
    private OrderId id;
    private Money totalAmount;
    private OrderStatus status;
    private List<OrderItem> items; // Aggregates items

    // Behaviors (Rich Model), not just Getters/Setters
    public void addItem(Product product, int quantity) {
        if (this.status != OrderStatus.DRAFT) {
            throw new DomainException("Cannot modify confirmed order");
        }
        this.items.add(new OrderItem(product, quantity));
        recalculateTotal();
    }

    public void confirm() {
        if (items.isEmpty()) throw new DomainException("Order empty");
        this.status = OrderStatus.CONFIRMED;
        // Raise Domain Event
        DomainEvents.publish(new OrderConfirmed(this.id));
    }
}

Core Concepts

Strategic DDD: Bounded Contexts & Context Mapping

Divides a large organization into autonomous boundaries with distinct terminology and models:

[ Sales Context ] ──(Customer = Buyer)──→ [ Context Map ] ──(Customer = Borrower)──→ [ Underwriting Context ]

Value Objects vs Entities

Entities possess a persistent identity that endures across mutations; Value Objects are immutable and defined entirely by their attributes: