domain-driven-design

Domain-Driven Design (DDD)

Overview

DDD is a software design approach that models complex business domains through a shared ubiquitous language, bounded contexts, and tactical patterns like aggregates and domain events.

Strategic Design

Bounded Contexts

┌─────────────────┐    ┌──────────────────┐    ┌───────────────┐
│   Identity &    │    │    Ordering       │    │   Shipping    │
│   Access        │    │                   │    │               │
│                 │    │  Order            │    │  Shipment     │
│  User           │◄──►│  LineItem         │◄──►│  Tracking     │
│  Role           │    │  Cart             │    │  Address      │
│  Permission     │    │  Payment          │    │  Carrier      │
└─────────────────┘    └──────────────────┘    └───────────────┘
     Context Map: Published Language / Anti-Corruption Layer

Each bounded context has its own:

• Ubiquitous language — "User" in Identity vs "Customer" in Ordering
• Models — same real-world concept, different representations
• Data store — ideally its own database/schema

Context Mapping

Pattern	When to Use
Shared Kernel	Two teams co-own a small shared model
Customer-Supplier	Upstream provides what downstream needs
Anti-Corruption Layer	Translate between contexts to prevent leaking
Published Language	Well-defined API/events for integration

Tactical Patterns

Aggregate

// Aggregate Root — consistency boundary
class Order {
  private items: OrderItem[] = [];
  private status: OrderStatus = 'draft';

  addItem(product: ProductRef, quantity: number, price: Money): void {
    if (this.status !== 'draft') throw new DomainError('Cannot modify submitted order');
    const existing = this.items.find(i => i.productId === product.id);
    if (existing) {
      existing.increaseQuantity(quantity);
    } else {
      this.items.push(new OrderItem(product.id, quantity, price));
    }
  }

  submit(): DomainEvent[] {
    if (this.items.length === 0) throw new DomainError('Cannot submit empty order');
    this.status = 'submitted';
    return [new OrderSubmitted(this.id, this.total(), this.items.length)];
  }

  get total(): Money {
    return this.items.reduce((sum, item) => sum.add(item.subtotal()), Money.zero());
  }
}