Domain-Driven Design

Core Concepts

Ubiquitous Language

Use the same terminology as domain experts. Code should read like business documentation.

Bounded Context

A boundary within which a particular domain model is defined and applicable.

Context Map

Shows how bounded contexts relate to each other.

Building Blocks

Entity

Has identity that persists over time. Equality based on ID.

typescript

class User {
  constructor(
    public readonly id: UserId,
    public email: Email,
    public name: string
  ) {}
}

Value Object

Immutable, equality based on attributes.

typescript

class Email {
  private constructor(public readonly value: string) {}

  static create(value: string): Email {
    if (!value.includes('@')) {
      throw new Error('Invalid email');
    }
    return new Email(value);
  }

  equals(other: Email): boolean {
    return this.value === other.value;
  }
}

Aggregate

Cluster of entities and value objects with a root entity.

typescript

class Order { // Aggregate Root
  private items: OrderItem[] = [];

  addItem(product: ProductId, quantity: number): void {
    // Business rules enforced here
    this.items.push(new OrderItem(product, quantity));
  }

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

Repository

Abstracts data access for aggregates.

typescript

interface OrderRepository {
  findById(id: OrderId): Promise<Order | null>;
  save(order: Order): Promise<void>;
}

Domain Event

Something that happened in the domain.

typescript

class OrderPlaced {
  constructor(
    public readonly orderId: OrderId,
    public readonly userId: UserId,
    public readonly occurredAt: Date
  ) {}
}

Strategic Patterns

Anti-Corruption Layer

Translate between your model and external systems.

Shared Kernel

Shared subset of domain model between contexts.

Customer-Supplier

Upstream provides what downstream needs.