# Tactical DDD Patterns

Tactical DDD patterns are code-level building blocks for implementing a rich domain model. They help express domain concepts in code that mirrors how domain experts think.

## Entity

### Definition

An object defined by its identity rather than its attributes. Two entities with the same attribute values but different identities are different things.

### Characteristics

- Has a unique identifier that persists through state changes
- Identity established at creation, immutable thereafter
- Equality based on identity, not attribute values
- Has a lifecycle (created, modified, potentially deleted)
- Contains behavior relevant to the domain concept it represents

### When to Use

- The object represents something tracked over time
- "Is this the same one?" is a meaningful question
- The object needs to be referenced from other parts of the system
- State changes are important to track

### Implementation

```typescript
// Entity with identity and behavior
class Order {
  private readonly _id: OrderId;
  private _status: OrderStatus;
  private _items: OrderItem[];
  private _shippingAddress: Address;

  constructor(id: OrderId, items: OrderItem[], shippingAddress: Address) {
    this._id = id;
    this._items = items;
    this._shippingAddress = shippingAddress;
    this._status = OrderStatus.Pending;
  }

  get id(): OrderId {
    return this._id;
  }

  // Behavior, not just data access
  confirm(): void {
    if (this._items.length === 0) {
      throw new EmptyOrderError(this._id);
    }
    this._status = OrderStatus.Confirmed;
  }

  ship(trackingNumber: TrackingNumber): void {
    if (this._status !== OrderStatus.Confirmed) {
      throw new InvalidOrderStateError(this._id, this._status, 'ship');
    }
    this._status = OrderStatus.Shipped;
    // Domain event raised
  }

  addItem(item: OrderItem): void {
    if (this._status !== OrderStatus.Pending) {
      throw new OrderModificationError(this._id);
    }
    this._items.push(item);
  }

  // Identity-based equality
  equals(other: Order): boolean {
    return this._id.equals(other._id);
  }
}

// Strongly-typed identity
class OrderId {
  constructor(private readonly value: string) {
    if (!value || value.trim() === '') {
      throw new InvalidOrderIdError();
    }
  }

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

  toString(): string {
    return this.value;
  }
}
```

### Entity vs Data Structure

```typescript
// Bad: Anemic entity (data structure)
class Order {
  id: string;
  status: string;
  items: Item[];

  // Only getters/setters, no behavior
}

// Good: Rich entity with behavior
class Order {
  private _id: OrderId;
  private _status: OrderStatus;
  private _items: OrderItem[];

  confirm(): void { /* enforces rules */ }
  cancel(reason: CancellationReason): void { /* enforces rules */ }
  addItem(item: OrderItem): void { /* enforces rules */ }
}
```

## Value Object

### Definition

An object defined entirely by its attributes. Two value objects with the same attributes are interchangeable. Has no identity.

### Characteristics

- Immutable - once created, never changes
- Equality based on attributes, not identity
- Self-validating - always in a valid state
- Side-effect free - methods return new instances
- Conceptually whole - attributes form a complete concept

### When to Use

- The concept has no lifecycle or identity
- "Are these the same?" means "do they have the same values?"
- Measurement, description, or quantification
- Combinations of attributes that belong together

### Implementation

```typescript
// Value Object: Money
class Money {
  private constructor(
    private readonly amount: number,
    private readonly currency: Currency
  ) {}

  // Factory method with validation
  static of(amount: number, currency: Currency): Money {
    if (amount < 0) {
      throw new NegativeMoneyError(amount);
    }
    return new Money(amount, currency);
  }

  // Immutable operations - return new instances
  add(other: Money): Money {
    this.ensureSameCurrency(other);
    return Money.of(this.amount + other.amount, this.currency);
  }

  subtract(other: Money): Money {
    this.ensureSameCurrency(other);
    return Money.of(this.amount - other.amount, this.currency);
  }

  multiply(factor: number): Money {
    return Money.of(this.amount * factor, this.currency);
  }

  // Value-based equality
  equals(other: Money): boolean {
    return this.amount === other.amount &&
           this.currency.equals(other.currency);
  }

  private ensureSameCurrency(other: Money): void {
    if (!this.currency.equals(other.currency)) {
      throw new CurrencyMismatchError(this.currency, other.currency);
    }
  }
}

// Value Object: Address
class Address {
  private constructor(
    readonly street: string,
    readonly city: string,
    readonly postalCode: string,
    readonly country: Country
  ) {}

  static create(street: string, city: string, postalCode: string, country: Country): Address {
    if (!street || !city || !postalCode) {
      throw new InvalidAddressError();
    }
    if (!country.validatePostalCode(postalCode)) {
      throw new InvalidPostalCodeError(postalCode, country);
    }
    return new Address(street, city, postalCode, country);
  }

  // Returns new instance with modified value
  withStreet(newStreet: string): Address {
    return Address.create(newStreet, this.city, this.postalCode, this.country);
  }

  equals(other: Address): boolean {
    return this.street === other.street &&
           this.city === other.city &&
           this.postalCode === other.postalCode &&
           this.country.equals(other.country);
  }
}

// Value Object: DateRange
class DateRange {
  private constructor(
    readonly start: Date,
    readonly end: Date
  ) {}

  static create(start: Date, end: Date): DateRange {
    if (end < start) {
      throw new InvalidDateRangeError(start, end);
    }
    return new DateRange(start, end);
  }

  contains(date: Date): boolean {
    return date >= this.start && date <= this.end;
  }

  overlaps(other: DateRange): boolean {
    return this.start <= other.end && this.end >= other.start;
  }

  durationInDays(): number {
    return Math.floor((this.end.getTime() - this.start.getTime()) / (1000 * 60 * 60 * 24));
  }
}
```

### Common Value Objects

| Domain | Value Objects |
|--------|--------------|
| **E-commerce** | Money, Price, Quantity, SKU, Address, PhoneNumber |
| **Healthcare** | BloodPressure, Dosage, DateRange, PatientId |
| **Finance** | AccountNumber, IBAN, TaxId, Percentage |
| **Shipping** | Weight, Dimensions, TrackingNumber, PostalCode |
| **General** | Email, URL, PhoneNumber, Name, Coordinates |

## Aggregate

### Definition

A cluster of entities and value objects with defined boundaries. Has an aggregate root entity that serves as the single entry point. External objects can only reference the root.

### Characteristics

- Defines a transactional consistency boundary
- Aggregate root is the only externally accessible object
- Enforces invariants across the cluster
- Loaded and saved as a unit
- Other aggregates referenced by identity only

### Design Rules

1. **Protect invariants** - All rules that must be consistent are inside the boundary
2. **Small aggregates** - Prefer single-entity aggregates; add children only when invariants require
3. **Reference by identity** - Never hold direct references to other aggregates
4. **Update one per transaction** - Eventual consistency between aggregates
5. **Design around invariants** - Identify what must be immediately consistent

### Implementation

```typescript
// Aggregate: Order (root) with OrderItems (child entities)
class Order {
  private readonly _id: OrderId;
  private _items: Map<ProductId, OrderItem>;
  private _status: OrderStatus;

  // Invariant: Order total cannot exceed credit limit
  private _creditLimit: Money;

  private constructor(
    id: OrderId,
    creditLimit: Money
  ) {
    this._id = id;
    this._items = new Map();
    this._status = OrderStatus.Draft;
    this._creditLimit = creditLimit;
  }

  static create(id: OrderId, creditLimit: Money): Order {
    return new Order(id, creditLimit);
  }

  // All modifications go through aggregate root
  addItem(productId: ProductId, quantity: Quantity, unitPrice: Money): void {
    this.ensureCanModify();

    const newItem = OrderItem.create(productId, quantity, unitPrice);
    const projectedTotal = this.calculateTotalWith(newItem);

    // Invariant enforcement
    if (projectedTotal.isGreaterThan(this._creditLimit)) {
      throw new CreditLimitExceededError(projectedTotal, this._creditLimit);
    }

    this._items.set(productId, newItem);
  }

  removeItem(productId: ProductId): void {
    this.ensureCanModify();
    this._items.delete(productId);
  }

  updateItemQuantity(productId: ProductId, newQuantity: Quantity): void {
    this.ensureCanModify();

    const item = this._items.get(productId);
    if (!item) {
      throw new ItemNotFoundError(productId);
    }

    const updatedItem = item.withQuantity(newQuantity);
    const projectedTotal = this.calculateTotalWithUpdate(productId, updatedItem);

    if (projectedTotal.isGreaterThan(this._creditLimit)) {
      throw new CreditLimitExceededError(projectedTotal, this._creditLimit);
    }

    this._items.set(productId, updatedItem);
  }

  submit(): OrderSubmitted {
    if (this._items.size === 0) {
      throw new EmptyOrderError();
    }
    this._status = OrderStatus.Submitted;

    return new OrderSubmitted(this._id, this.total(), new Date());
  }

  // Read-only access to child entities
  get items(): ReadonlyArray<OrderItem> {
    return Array.from(this._items.values());
  }

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

  private ensureCanModify(): void {
    if (this._status !== OrderStatus.Draft) {
      throw new OrderNotModifiableError(this._id, this._status);
    }
  }

  private calculateTotalWith(newItem: OrderItem): Money {
    return this.total().add(newItem.subtotal());
  }

  private calculateTotalWithUpdate(productId: ProductId, updatedItem: OrderItem): Money {
    const currentItem = this._items.get(productId)!;
    return this.total().subtract(currentItem.subtotal()).add(updatedItem.subtotal());
  }
}

// Child entity - only accessible through aggregate root
class OrderItem {
  private constructor(
    private readonly _productId: ProductId,
    private _quantity: Quantity,
    private readonly _unitPrice: Money
  ) {}

  static create(productId: ProductId, quantity: Quantity, unitPrice: Money): OrderItem {
    return new OrderItem(productId, quantity, unitPrice);
  }

  get productId(): ProductId { return this._productId; }
  get quantity(): Quantity { return this._quantity; }
  get unitPrice(): Money { return this._unitPrice; }

  subtotal(): Money {
    return this._unitPrice.multiply(this._quantity.value);
  }

  withQuantity(newQuantity: Quantity): OrderItem {
    return new OrderItem(this._productId, newQuantity, this._unitPrice);
  }
}
```

### Aggregate Reference Patterns

```typescript
// Bad: Direct object reference across aggregates
class Order {
  private customer: Customer;  // Holds the entire aggregate!
}

// Good: Reference by identity
class Order {
  private customerId: CustomerId;

  // If customer data needed, load separately
  getCustomerAddress(customerRepository: CustomerRepository): Address {
    const customer = customerRepository.findById(this.customerId);
    return customer.shippingAddress;
  }
}
```

## Domain Event

### Definition

A record of something significant that happened in the domain. Captures state changes that domain experts care about.

### Characteristics

- Named in past tense (OrderPlaced, PaymentReceived)
- Immutable - records historical fact
- Contains all relevant data about what happened
- Published after state change is committed
- May trigger reactions in same or different bounded contexts

### When to Use

- Domain experts talk about "when X happens, Y should happen"
- Need to communicate changes across aggregate boundaries
- Maintaining an audit trail
- Implementing eventual consistency
- Integration with other bounded contexts

### Implementation

```typescript
// Base domain event
abstract class DomainEvent {
  readonly occurredAt: Date;
  readonly eventId: string;

  constructor() {
    this.occurredAt = new Date();
    this.eventId = generateUUID();
  }

  abstract get eventType(): string;
}

// Specific domain events
class OrderPlaced extends DomainEvent {
  constructor(
    readonly orderId: OrderId,
    readonly customerId: CustomerId,
    readonly totalAmount: Money,
    readonly items: ReadonlyArray<OrderItemSnapshot>
  ) {
    super();
  }

  get eventType(): string {
    return 'order.placed';
  }
}

class OrderShipped extends DomainEvent {
  constructor(
    readonly orderId: OrderId,
    readonly trackingNumber: TrackingNumber,
    readonly carrier: string,
    readonly estimatedDelivery: Date
  ) {
    super();
  }

  get eventType(): string {
    return 'order.shipped';
  }
}

class PaymentReceived extends DomainEvent {
  constructor(
    readonly orderId: OrderId,
    readonly amount: Money,
    readonly paymentMethod: PaymentMethod,
    readonly transactionId: string
  ) {
    super();
  }

  get eventType(): string {
    return 'payment.received';
  }
}

// Entity raising events
class Order {
  private _domainEvents: DomainEvent[] = [];

  submit(): void {
    // State change
    this._status = OrderStatus.Submitted;

    // Raise event
    this._domainEvents.push(
      new OrderPlaced(
        this._id,
        this._customerId,
        this.total(),
        this.itemSnapshots()
      )
    );
  }

  pullDomainEvents(): DomainEvent[] {
    const events = [...this._domainEvents];
    this._domainEvents = [];
    return events;
  }
}

// Event handler
class OrderPlacedHandler {
  constructor(
    private inventoryService: InventoryService,
    private emailService: EmailService
  ) {}

  async handle(event: OrderPlaced): Promise<void> {
    // Reserve inventory (different aggregate)
    await this.inventoryService.reserveItems(event.items);

    // Send confirmation email
    await this.emailService.sendOrderConfirmation(
      event.customerId,
      event.orderId,
      event.totalAmount
    );
  }
}
```

### Event Publishing Patterns

```typescript
// Pattern 1: Collect and dispatch after save
class OrderApplicationService {
  async placeOrder(command: PlaceOrderCommand): Promise<OrderId> {
    const order = Order.create(command);

    await this.orderRepository.save(order);

    // Dispatch events after successful save
    const events = order.pullDomainEvents();
    await this.eventDispatcher.dispatchAll(events);

    return order.id;
  }
}

// Pattern 2: Outbox pattern (reliable publishing)
class OrderApplicationService {
  async placeOrder(command: PlaceOrderCommand): Promise<OrderId> {
    await this.unitOfWork.transaction(async () => {
      const order = Order.create(command);
      await this.orderRepository.save(order);

      // Save events to outbox in same transaction
      const events = order.pullDomainEvents();
      await this.outbox.saveEvents(events);
    });

    // Separate process publishes from outbox
    return order.id;
  }
}
```

## Repository

### Definition

Mediates between the domain and data mapping layers. Provides collection-like interface for accessing aggregates.

### Characteristics

- One repository per aggregate root
- Interface defined in domain layer, implementation in infrastructure
- Returns fully reconstituted aggregates
- Abstracts persistence concerns from domain

### Interface Design

```typescript
// Domain layer interface
interface OrderRepository {
  findById(id: OrderId): Promise<Order | null>;
  save(order: Order): Promise<void>;
  delete(order: Order): Promise<void>;

  // Domain-specific queries
  findPendingOrdersFor(customerId: CustomerId): Promise<Order[]>;
  findOrdersToShipBefore(deadline: Date): Promise<Order[]>;
}

// Infrastructure implementation
class PostgresOrderRepository implements OrderRepository {
  constructor(private db: Database) {}

  async findById(id: OrderId): Promise<Order | null> {
    const row = await this.db.query(
      'SELECT * FROM orders WHERE id = $1',
      [id.toString()]
    );

    if (!row) return null;

    const items = await this.db.query(
      'SELECT * FROM order_items WHERE order_id = $1',
      [id.toString()]
    );

    return this.reconstitute(row, items);
  }

  async save(order: Order): Promise<void> {
    await this.db.transaction(async (tx) => {
      await tx.query(
        'INSERT INTO orders (id, status, customer_id) VALUES ($1, $2, $3) ON CONFLICT (id) DO UPDATE SET status = $2',
        [order.id.toString(), order.status, order.customerId.toString()]
      );

      // Save items
      for (const item of order.items) {
        await tx.query(
          'INSERT INTO order_items (order_id, product_id, quantity, unit_price) VALUES ($1, $2, $3, $4) ON CONFLICT DO UPDATE...',
          [order.id.toString(), item.productId.toString(), item.quantity.value, item.unitPrice.amount]
        );
      }
    });
  }

  private reconstitute(orderRow: any, itemRows: any[]): Order {
    // Rebuild aggregate from persistence data
    return Order.reconstitute({
      id: OrderId.from(orderRow.id),
      status: OrderStatus[orderRow.status],
      customerId: CustomerId.from(orderRow.customer_id),
      items: itemRows.map(row => OrderItem.reconstitute({
        productId: ProductId.from(row.product_id),
        quantity: Quantity.of(row.quantity),
        unitPrice: Money.of(row.unit_price, Currency.USD)
      }))
    });
  }
}
```

### Repository vs DAO

```typescript
// DAO: Data-centric, returns raw data
interface OrderDao {
  findById(id: string): Promise<OrderRow>;
  findItems(orderId: string): Promise<OrderItemRow[]>;
  insert(row: OrderRow): Promise<void>;
}

// Repository: Domain-centric, returns aggregates
interface OrderRepository {
  findById(id: OrderId): Promise<Order | null>;
  save(order: Order): Promise<void>;
}
```

## Domain Service

### Definition

Stateless operations that represent domain concepts but don't naturally belong to any entity or value object.

### When to Use

- The operation involves multiple aggregates
- The operation represents a domain concept
- Putting the operation on an entity would create awkward dependencies
- The operation is stateless

### Examples

```typescript
// Domain Service: Transfer money between accounts
class MoneyTransferService {
  transfer(
    from: Account,
    to: Account,
    amount: Money
  ): TransferResult {
    // Involves two aggregates
    // Neither account should "own" this operation

    if (!from.canWithdraw(amount)) {
      return TransferResult.insufficientFunds();
    }

    from.withdraw(amount);
    to.deposit(amount);

    return TransferResult.success(
      new MoneyTransferred(from.id, to.id, amount)
    );
  }
}

// Domain Service: Calculate shipping cost
class ShippingCostCalculator {
  constructor(
    private rateProvider: ShippingRateProvider
  ) {}

  calculate(
    items: OrderItem[],
    destination: Address,
    shippingMethod: ShippingMethod
  ): Money {
    const totalWeight = items.reduce(
      (sum, item) => sum.add(item.weight),
      Weight.zero()
    );

    const rate = this.rateProvider.getRate(
      destination.country,
      shippingMethod
    );

    return rate.calculateFor(totalWeight);
  }
}

// Domain Service: Check inventory availability
class InventoryAvailabilityService {
  constructor(
    private inventoryRepository: InventoryRepository
  ) {}

  checkAvailability(
    items: Array<{ productId: ProductId; quantity: Quantity }>
  ): AvailabilityResult {
    const unavailable: ProductId[] = [];

    for (const { productId, quantity } of items) {
      const inventory = this.inventoryRepository.findByProductId(productId);
      if (!inventory || !inventory.hasAvailable(quantity)) {
        unavailable.push(productId);
      }
    }

    return unavailable.length === 0
      ? AvailabilityResult.allAvailable()
      : AvailabilityResult.someUnavailable(unavailable);
  }
}
```

### Domain Service vs Application Service

```typescript
// Domain Service: Domain logic, domain types, stateless
class PricingService {
  calculateDiscountedPrice(product: Product, customer: Customer): Money {
    const basePrice = product.price;
    const discount = customer.membershipLevel.discountPercentage;
    return basePrice.applyDiscount(discount);
  }
}

// Application Service: Orchestration, use cases, transaction boundary
class OrderApplicationService {
  constructor(
    private orderRepository: OrderRepository,
    private pricingService: PricingService,
    private eventPublisher: EventPublisher
  ) {}

  async createOrder(command: CreateOrderCommand): Promise<OrderId> {
    const customer = await this.customerRepository.findById(command.customerId);
    const order = Order.create(command.orderId, customer.id);

    for (const item of command.items) {
      const product = await this.productRepository.findById(item.productId);
      const price = this.pricingService.calculateDiscountedPrice(product, customer);
      order.addItem(item.productId, item.quantity, price);
    }

    await this.orderRepository.save(order);
    await this.eventPublisher.publish(order.pullDomainEvents());

    return order.id;
  }
}
```

## Factory

### Definition

Encapsulates complex object or aggregate creation logic. Creates objects in a valid state.

### When to Use

- Construction logic is complex
- Multiple ways to create the same type of object
- Creation involves other objects or services
- Need to enforce invariants at creation time

### Implementation

```typescript
// Factory as static method
class Order {
  static create(customerId: CustomerId, creditLimit: Money): Order {
    return new Order(
      OrderId.generate(),
      customerId,
      creditLimit,
      OrderStatus.Draft,
      []
    );
  }

  static reconstitute(data: OrderData): Order {
    // For rebuilding from persistence
    return new Order(
      data.id,
      data.customerId,
      data.creditLimit,
      data.status,
      data.items
    );
  }
}

// Factory as separate class
class OrderFactory {
  constructor(
    private creditLimitService: CreditLimitService,
    private idGenerator: IdGenerator
  ) {}

  async createForCustomer(customerId: CustomerId): Promise<Order> {
    const creditLimit = await this.creditLimitService.getLimit(customerId);
    const orderId = this.idGenerator.generate();

    return Order.create(orderId, customerId, creditLimit);
  }

  createFromQuote(quote: Quote): Order {
    const order = Order.create(
      this.idGenerator.generate(),
      quote.customerId,
      quote.creditLimit
    );

    for (const item of quote.items) {
      order.addItem(item.productId, item.quantity, item.agreedPrice);
    }

    return order;
  }
}

// Builder pattern for complex construction
class OrderBuilder {
  private customerId?: CustomerId;
  private items: OrderItemData[] = [];
  private shippingAddress?: Address;
  private billingAddress?: Address;

  forCustomer(customerId: CustomerId): this {
    this.customerId = customerId;
    return this;
  }

  withItem(productId: ProductId, quantity: Quantity, price: Money): this {
    this.items.push({ productId, quantity, price });
    return this;
  }

  shippingTo(address: Address): this {
    this.shippingAddress = address;
    return this;
  }

  billingTo(address: Address): this {
    this.billingAddress = address;
    return this;
  }

  build(): Order {
    if (!this.customerId) throw new Error('Customer required');
    if (!this.shippingAddress) throw new Error('Shipping address required');
    if (this.items.length === 0) throw new Error('At least one item required');

    const order = Order.create(this.customerId);
    order.setShippingAddress(this.shippingAddress);
    order.setBillingAddress(this.billingAddress ?? this.shippingAddress);

    for (const item of this.items) {
      order.addItem(item.productId, item.quantity, item.price);
    }

    return order;
  }
}
```