Domain Model & Boundaries Mapper

Domain Model & Boundaries Mapper

Map domain boundaries and ownership using Domain-Driven Design.

Domain Map Template

# Domain Map: E-Commerce Platform

## Bounded Contexts

### 1. Customer Management

**Core Domain:** User accounts, profiles, preferences
**Owner:** Customer Team
**Ubiquitous Language:**

- Customer: Registered user with account
- Profile: Customer personal information
- Preferences: User settings and choices

**Entities:**

- Customer (id, email, name)
- Address (id, customer_id, street, city)
- PaymentMethod (id, customer_id, type, token)

**Bounded Context Diagram:**

┌─────────────────────────────┐ │ Customer Management │ │ ┌─────────┐ ┌──────────┐ │ │ │Customer │ │ Address │ │ │ └─────────┘ └──────────┘ │ │ ┌─────────────────────┐ │ │ │ Payment Method │ │ │ └─────────────────────┘ │ └─────────────────────────────┘

### 2. Order Management
**Core Domain:** Order processing, fulfillment
**Owner:** Orders Team
**Ubiquitous Language:**
- Order: Purchase request with line items
- LineItem: Product quantity in order
- Fulfillment: Physical delivery of order

**Entities:**
- Order (id, customer_id, status, total)
- LineItem (id, order_id, product_id, quantity)
- Shipment (id, order_id, tracking_number)

### 3. Product Catalog
**Core Domain:** Product information, inventory
**Owner:** Catalog Team
**Ubiquitous Language:**
- Product: Sellable item
- SKU: Stock keeping unit
- Inventory: Available stock

**Entities:**
- Product (id, name, price, description)
- Inventory (sku, quantity, warehouse_id)

## Context Relationships

Customer Management ──────▶ Order Management (customer_id)

Product Catalog ──────▶ Order Management (product_id)

Order Management ──────▶ Fulfillment (order events)

## Anti-Corruption Layers

### Order Management → Customer Management
**Problem:** Orders need customer data but shouldn't depend on Customer domain model

**Solution:** Customer Adapter
```typescript
// Order domain's view of customer
interface CustomerForOrder {
  id: string;
  shippingAddress: Address;
  billingAddress: Address;
}

// Adapter translates Customer domain to Order domain
class CustomerAdapter {
  async getCustomerForOrder(customerId: string): Promise<CustomerForOrder> {
    const customer = await customerService.getCustomer(customerId);
    return {
      id: customer.id,
      shippingAddress: this.toOrderAddress(customer.defaultShippingAddress),
      billingAddress: this.toOrderAddress(customer.defaultBillingAddress),
    };
  }
}

Dependency Map

┌──────────────┐
│   Customer   │
└──────┬───────┘
       │
       ▼
┌──────────────┐      ┌────────────┐
│    Orders    │─────▶│  Products  │
└──────┬───────┘      └────────────┘
       │
       ▼
┌──────────────┐
│ Fulfillment  │
└──────────────┘

Dependency Rules:

• Customer has no dependencies
• Orders depends on Customer (read) and Products (read)
• Fulfillment depends on Orders (events)

Interface Contracts

Customer Management → Orders

// Public interface exposed by Customer domain
interface CustomerService {
  getCustomer(id: string): Promise<Customer>;
  getCustomerAddresses(id: string): Promise<Address[]>;
}

// Events published
interface CustomerUpdated {
  customerId: string;
  email: string;
  name: string;
}

Product Catalog → Orders

interface ProductService {
  getProduct(id: string): Promise<Product>;
  checkAvailability(sku: string, quantity: number): Promise<boolean>;
  reserveInventory(items: ReservationRequest[]): Promise<Reservation>;
}

Refactor Recommendations

Problem 1: Tight Coupling

Current: Orders directly queries Customer database Issue: Breaks bounded context, creates coupling Recommendation: Use Customer API instead

// ❌ Before: Direct database access
const customer = await db.customers.findById(customerId);

// ✅ After: API call through adapter
const customer = await customerAdapter.getCustomerForOrder(customerId);

Problem 2: Shared Models

Current: Same User model used across contexts Issue: Changes in one context affect others Recommendation: Separate models per context

// Customer context
interface Customer {
  id: string;
  email: string;
  profile: CustomerProfile;
  preferences: CustomerPreferences;
}

// Order context (different model!)
interface OrderCustomer {
  id: string;
  shippingAddress: Address;
  billingAddress: Address;
}

Problem 3: God Service

Current: OrderService handles orders, inventory, payments, shipping Issue: Single service owns too much Recommendation: Extract bounded contexts

• OrderService: Order lifecycle
• InventoryService: Stock management
• PaymentService: Payment processing
• FulfillmentService: Shipping

Strategic Design Patterns

Pattern 1: Shared Kernel

When: Two contexts must share some code Example: Common value objects (Money, Address)

// Shared kernel (minimal!)
class Money {
  constructor(public amount: number, public currency: string) {}
}

Pattern 2: Customer/​Supplier

When: One context depends on another Example: Orders (customer) depends on Products (supplier)

• Supplier defines interface
• Customer adapts to their needs

Pattern 3: Published Language

When: Many contexts need same data Example: Product events

interface ProductCreated {
  productId: string;
  name: string;
  price: Money;
  publishedAt: Date;
}

Migration Strategy

Phase 1: Identify Boundaries

• Map existing code to domains
• Identify coupling points
• Document dependencies

Phase 2: Define Interfaces

• Design APIs between contexts
• Create adapter layers
• Define event contracts

Phase 3: Decouple

• Replace direct DB access with APIs
• Introduce anti-corruption layers
• Separate models per context

Phase 4: Extract Services (optional)

• Move contexts to separate services
• Implement API gateways
• Set up event bus

Best Practices

1. Ubiquitous language: Same terms in code and domain
2. Bounded contexts: Clear boundaries, separate models
3. Context maps: Document relationships
4. Anti-corruption layers: Protect domain integrity
5. Event-driven: Loose coupling via events
6. Separate databases: Context owns its data

Output Checklist

• Bounded contexts identified (3-7)
• Core domain vs supporting domains
• Ubiquitous language defined per context
• Entity/​aggregate definitions
• Context relationship diagram
• Dependency map
• Interface contracts defined
• Anti-corruption layers designed
• Refactor recommendations
• Migration strategy