+-------------------------------------------------------------+ | Application Layer | | (Orchestrates use cases, manages transactions) | +-------------------------------------------------------------+ | v +-------------------------------------------------------------+ | Domain Layer | | +------------------+ +-------------------+ | | | Aggregate | | Domain Event | | | | (Root Entity) |------------->| "OrderSubmitted" | | | +------------------+ +-------------------+ | | | | ^ | | v v | | | [Entity] [Value Object] | | | | | | +-----------------------------------------------------+ | | | Domain Service | | | | (Handles multi-aggregate logic) | | | +-----------------------------------------------------+ | +-------------------------------------------------------------+ ^ | (Dependency Inversion) +-------------------------------------------------------------+ | Infrastructure Layer | | (Repositories, Database Access, External APIs) | +-------------------------------------------------------------+ Entities vs. Value Objects
// Order.ts (Aggregate Root) import Price from "./Price"; export class Order { private constructor( public readonly id: string, private items: OrderItem[], private status: "Pending" | "Paid" | "Shipped" ) {} public static create(orderId: string): Order return new Order(orderId, [], "Pending"); // Business Invariant: Cannot add items if already processed public addItem(productId: string, price: Price): void if (this.status !== "Pending") throw new Error("Cannot modify order after payment processing."); this.items.push(new OrderItem(productId, price)); // State mutation protected by business rules public markAsPaid(): void if (this.items.length === 0) throw new Error("Cannot pay for an empty order."); this.status = "Paid"; // Raise a Domain Event here if needed: OrderPaidEvent } Use code with caution. ⚠️ Common Pitfalls to Avoid implementing domain-driven design pdf github
Defining boundaries within which a domain model applies. private items: OrderItem[]
Domain-Driven Design (DDD) is a software development philosophy that aligns complex business needs directly with software architecture. Originally introduced by Eric Evans, DDD addresses the challenges of large-scale enterprise systems by focusing on the "domain"—the core business logic and problem space. this.status = "Paid"
1. Understanding "Implementing Domain-Driven Design" (The Red Book)