Enterprise Architecture Patterns

Enterprise Architecture Patterns

A comprehensive skill for mastering enterprise architecture patterns, distributed systems design, and scalable application development. This skill covers strategic and tactical patterns for building robust, maintainable, and scalable enterprise systems.

When to Use This Skill

Use this skill when:

• Designing microservices architectures for distributed systems
• Implementing domain-driven design (DDD) in complex business domains
• Building event-driven architectures with event sourcing and CQRS
• Designing saga patterns for distributed transactions
• Implementing API gateways and service mesh architectures
• Scaling applications horizontally and vertically
• Building resilient systems with fault tolerance
• Migrating monoliths to microservices
• Designing multi-tenant SaaS architectures
• Implementing backend-for-frontend (BFF) patterns
• Building real-time systems with event streaming
• Architecting cloud-native applications

Core Architectural Concepts

System Design Fundamentals

Separation of Concerns Divide systems into distinct sections where each section addresses a separate concern, reducing coupling and increasing cohesion.

Modularity Design systems as collections of independent modules that can be developed, tested, and deployed separately.

Abstraction Hide complex implementation details behind simple interfaces, making systems easier to understand and modify.

Scalability Dimensions

• Horizontal scaling: Add more machines/instances
• Vertical scaling: Add more resources to existing machines
• Data scaling: Partition data across multiple stores
• Functional scaling: Decompose by business capability

Consistency Models

• Strong consistency: All nodes see the same data at the same time
• Eventual consistency: All nodes will eventually see the same data
• Causal consistency: Related operations see consistent state
• Read-your-writes consistency: Users see their own updates immediately

CAP Theorem In distributed systems, you can only guarantee two of three properties:

• Consistency: All nodes see the same data
• Availability: Every request receives a response
• Partition tolerance: System continues despite network failures

Distributed Computing Fallacies 1. The network is reliable 2. Latency is zero 3. Bandwidth is infinite 4. The network is secure 5. Topology doesn't change 6. There is one administrator 7. Transport cost is zero 8. The network is homogeneous

Domain-Driven Design (DDD)

Strategic Design Patterns

Bounded Context

A bounded context is an explicit boundary within which a domain model is consistent and valid. It defines the scope where particular terms, definitions, and rules apply.

Key Principles:

• Each bounded context has its own ubiquitous language
• Models within a context are consistent
• Cross-context integration requires translation
• Contexts align with business capabilities

Implementation:

// Example: E-commerce system with multiple bounded contexts

// Sales Context
namespace Sales {
  class Customer {
    customerId: string;
    email: string;
    orderHistory: Order[];

    placeOrder(order: Order): void {
      // Sales-specific logic
    }
  }
}

// Billing Context
namespace Billing {
  class Customer {
    customerId: string;
    paymentMethods: PaymentMethod[];
    invoices: Invoice[];

    processPayment(invoice: Invoice): void {
      // Billing-specific logic
    }
  }
}

// Different models for Customer in different contexts

Context Mapping Patterns:

1. Shared Kernel: Two contexts share a subset of the domain model

• Use when: Teams are closely coordinated
• Risk: Changes affect multiple contexts

2. Customer-Supplier: Downstream context depends on upstream

• Use when: Clear dependency direction exists
• Pattern: Upstream provides defined API

3. Conformist: Downstream conforms to upstream model

• Use when: Upstream is external/unchangeable
• Pattern: Adapt to external API

4. Anti-Corruption Layer: Translate between contexts

• Use when: Protecting from legacy or external systems
• Pattern: Adapter/facade to translate models

5. Separate Ways: Contexts are completely independent

• Use when: No integration needed
• Pattern: Duplicate functionality if necessary

6. Open Host Service: Well-defined protocol for integration

• Use when: Multiple consumers need access
• Pattern: REST API, GraphQL, gRPC

7. Published Language: Shared, well-documented language

• Use when: Industry standards exist
• Pattern: XML schemas, JSON schemas, OpenAPI

Ubiquitous Language

A shared vocabulary between developers and domain experts used consistently in code, documentation, and conversations.

Building Ubiquitous Language:

// Bad: Generic technical terms
class DataProcessor {
  processData(data: any): void {
    // Unclear what this does in business terms
  }
}

// Good: Business domain terms
class OrderFulfillment {
  fulfillOrder(order: Order): void {
    this.pickItems(order.items);
    this.packForShipment(order);
    this.scheduleDelivery(order);
  }

  private pickItems(items: OrderItem[]): void {
    // Business logic using domain language
  }
}

Tactical Design Patterns

Entities

Objects with unique identity that persist over time, tracking continuity and lifecycle.

Characteristics:

• Unique identifier (ID)
• Mutable state
• Lifecycle (created, modified, deleted)
• Equality based on identity, not attributes

Implementation:

class Order {
  private readonly orderId: string;
  private orderItems: OrderItem[];
  private status: OrderStatus;
  private orderDate: Date;
  private customerId: string;

  constructor(orderId: string, customerId: string) {
    this.orderId = orderId;
    this.customerId = customerId;
    this.orderItems = [];
    this.status = OrderStatus.Draft;
    this.orderDate = new Date();
  }

  // Business behavior
  addItem(product: Product, quantity: number): void {
    if (this.status !== OrderStatus.Draft) {
      throw new Error("Cannot modify confirmed order");
    }
    this.orderItems.push(new OrderItem(product, quantity));
  }

  confirm(): void {
    if (this.orderItems.length === 0) {
      throw new Error("Cannot confirm empty order");
    }
    this.status = OrderStatus.Confirmed;
  }

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

Value Objects

Immutable objects defined by their attributes rather than identity, representing descriptive aspects of the domain.

Characteristics:

• No unique identifier
• Immutable (cannot change state)
• Equality based on all attributes
• Often passed by value
• Can be shared safely

Implementation:

class Money {
  readonly amount: number;
  readonly currency: string;

  constructor(amount: number, currency: string) {
    if (amount < 0) {
      throw new Error("Amount cannot be negative");
    }
    this.amount = amount;
    this.currency = currency;
  }

  // Operations return new instances
  add(other: Money): Money {
    if (this.currency !== other.currency) {
      throw new Error("Cannot add different currencies");
    }
    return new Money(this.amount + other.amount, this.currency);
  }

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

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

class Address {
  readonly street: string;
  readonly city: string;
  readonly state: string;
  readonly zipCode: string;
  readonly country: string;

  constructor(
    street: string,
    city: string,
    state: string,
    zipCode: string,
    country: string
  ) {
    this.street = street;
    this.city = city;
    this.state = state;
    this.zipCode = zipCode;
    this.country = country;
  }

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

Aggregates

Clusters of entities and value objects with clear consistency boundaries, accessed through a single root entity.

Key Principles:

• One aggregate = one transaction boundary
• External references only to aggregate root
• Root enforces all invariants
• Small aggregates perform better
• Eventual consistency between aggregates

Design Rules: 1. Model true invariants in consistency boundaries 2. Design small aggregates 3. Reference other aggregates by identity only 4. Update other aggregates using eventual consistency 5. Use repositories to retrieve aggregates

Implementation:

// Aggregate Root
class Order {
  private readonly orderId: string;
  private readonly customerId: string; // Reference by ID only
  private orderItems: OrderItem[] = [];
  private shippingAddress: Address;
  private status: OrderStatus;
  private totalAmount: Money;

  constructor(orderId: string, customerId: string, shippingAddress: Address) {
    this.orderId = orderId;
    this.customerId = customerId;
    this.shippingAddress = shippingAddress;
    this.status = OrderStatus.Draft;
    this.totalAmount = new Money(0, "USD");
  }

  // Public methods enforce invariants
  addItem(product: Product, quantity: number): void {
    if (this.status !== OrderStatus.Draft) {
      throw new Error("Cannot modify confirmed order");
    }

    if (quantity <= 0) {
      throw new Error("Quantity must be positive");
    }

    const existingItem = this.findItem(product.id);
    if (existingItem) {
      existingItem.increaseQuantity(quantity);
    } else {
      this.orderItems.push(new OrderItem(product, quantity));
    }

    this.recalculateTotal();
  }

  removeItem(productId: string): void {
    if (this.status !== OrderStatus.Draft) {
      throw new Error("Cannot modify confirmed order");
    }

    this.orderItems = this.orderItems.filter(
      item => item.productId !== productId
    );
    this.recalculateTotal();
  }

  confirm(): void {
    if (this.orderItems.length === 0) {
      throw new Error("Cannot confirm empty order");
    }

    if (!this.shippingAddress) {
      throw new Error("Shipping address required");
    }

    this.status = OrderStatus.Confirmed;
  }

  private recalculateTotal(): void {
    this.totalAmount = this.orderItems.reduce(
      (total, item) => total.add(item.subtotal),
      new Money(0, "USD")
    );
  }

  private findItem(productId: string): OrderItem | undefined {
    return this.orderItems.find(item => item.productId === productId);
  }

  // Getters for read-only access
  get id(): string { return this.orderId; }
  get total(): Money { return this.totalAmount; }
  get items(): readonly OrderItem[] { return this.orderItems; }
}

// Entity within aggregate
class OrderItem {
  readonly productId: string;
  readonly productName: string;
  readonly unitPrice: Money;
  private quantity: number;

  constructor(product: Product, quantity: number) {
    this.productId = product.id;
    this.productName = product.name;
    this.unitPrice = product.price;
    this.quantity = quantity;
  }

  increaseQuantity(amount: number): void {
    this.quantity += amount;
  }

  get subtotal(): Money {
    return this.unitPrice.multiply(this.quantity);
  }
}

Domain Events

Events that represent something significant that happened in the domain, enabling loose coupling and eventual consistency.

Characteristics:

• Past tense naming (OrderPlaced, PaymentProcessed)
• Immutable
• Include all necessary information
• Timestamped
• Often include aggregate ID

Implementation:

interface DomainEvent {
  eventId: string;
  occurredAt: Date;
  aggregateId: string;
  eventType: string;
}

class OrderPlacedEvent implements DomainEvent {
  readonly eventId: string;
  readonly occurredAt: Date;
  readonly aggregateId: string;
  readonly eventType = "OrderPlaced";

  readonly orderId: string;
  readonly customerId: string;
  readonly totalAmount: Money;
  readonly items: OrderItemDto[];

  constructor(order: Order) {
    this.eventId = generateId();
    this.occurredAt = new Date();
    this.aggregateId = order.id;
    this.orderId = order.id;
    this.customerId = order.customerId;
    this.totalAmount = order.total;
    this.items = order.items.map(item => ({
      productId: item.productId,
      quantity: item.quantity,
      price: item.unitPrice
    }));
  }
}

// Domain event publisher
class DomainEventPublisher {
  private handlers: Map<string, Function[]> = new Map();

  subscribe(eventType: string, handler: Function): void {
    if (!this.handlers.has(eventType)) {
      this.handlers.set(eventType, []);
    }
    this.handlers.get(eventType)!.push(handler);
  }

  async publish(event: DomainEvent): Promise<void> {
    const handlers = this.handlers.get(event.eventType) || [];
    await Promise.all(handlers.map(handler => handler(event)));
  }
}

// Usage
class OrderService {
  constructor(
    private orderRepository: OrderRepository,
    private eventPublisher: DomainEventPublisher
  ) {}

  async placeOrder(order: Order): Promise<void> {
    order.confirm();
    await this.orderRepository.save(order);

    const event = new OrderPlacedEvent(order);
    await this.eventPublisher.publish(event);
  }
}

Repositories

Abstraction for accessing aggregates, providing collection-like interface while hiding persistence details.

Principles:

• One repository per aggregate root
• Collection-oriented interface
• Hide database implementation
• Return fully-formed aggregates
• Support querying by ID and business criteria

Implementation:

interface OrderRepository {
  save(order: Order): Promise<void>;
  findById(orderId: string): Promise<Order | null>;
  findByCustomer(customerId: string): Promise<Order[]>;
  findByStatus(status: OrderStatus): Promise<Order[]>;
  delete(orderId: string): Promise<void>;
}

class OrderRepositoryImpl implements OrderRepository {
  constructor(private db: Database) {}

  async save(order: Order): Promise<void> {
    const data = this.toDataModel(order);
    await this.db.orders.upsert(data);
  }

  async findById(orderId: string): Promise<Order | null> {
    const data = await this.db.orders.findOne({ id: orderId });
    if (!data) return null;
    return this.toDomainModel(data);
  }

  async findByCustomer(customerId: string): Promise<Order[]> {
    const results = await this.db.orders.find({ customerId });
    return results.map(data => this.toDomainModel(data));
  }

  async findByStatus(status: OrderStatus): Promise<Order[]> {
    const results = await this.db.orders.find({ status });
    return results.map(data => this.toDomainModel(data));
  }

  async delete(orderId: string): Promise<void> {
    await this.db.orders.delete({ id: orderId });
  }

  private toDataModel(order: Order): any {
    // Convert domain model to database model
    return {
      id: order.id,
      customerId: order.customerId,
      items: order.items.map(item => ({
        productId: item.productId,
        quantity: item.quantity,
        price: item.unitPrice.amount
      })),
      total: order.total.amount,
      status: order.status
    };
  }

  private toDomainModel(data: any): Order {
    // Reconstruct domain model from database data
    const order = new Order(data.id, data.customerId, data.shippingAddress);
    // Restore items and state
    return order;
  }
}

Domain Services

Operations that don't naturally belong to any entity or value object, encapsulating domain logic that involves multiple aggregates.

When to Use:

• Logic spans multiple aggregates
• Operation is a significant domain concept
• Behavior doesn't fit naturally in entity or value object

Implementation:

class PricingService {
  calculateOrderTotal(
    items: OrderItem[],
    customer: Customer,
    promotions: Promotion[]
  ): Money {
    let total = items.reduce(
      (sum, item) => sum.add(item.subtotal),
      new Money(0, "USD")
    );

    // Apply customer discount
    if (customer.isPremium) {
      total = total.multiply(0.9); // 10% discount
    }

    // Apply promotions
    for (const promo of promotions) {
      total = promo.apply(total);
    }

    return total;
  }
}

class TransferService {
  transfer(
    fromAccount: Account,
    toAccount: Account,
    amount: Money
  ): void {
    if (!fromAccount.canWithdraw(amount)) {
      throw new Error("Insufficient funds");
    }

    fromAccount.withdraw(amount);
    toAccount.deposit(amount);
  }
}

Event Sourcing

Event sourcing persists the state of a system as a sequence of events rather than storing current state. The current state is derived by replaying events.

Core Concepts

Event Store Append-only log of all events that have occurred in the system.

Event Stream Sequence of events for a specific aggregate, ordered by time.

Projection Read model built by processing events, optimized for queries.

Snapshot Cached state at a point in time to avoid replaying all events.

Implementation

// Event interface
interface Event {
  eventId: string;
  eventType: string;
  aggregateId: string;
  aggregateType: string;
  version: number;
  timestamp: Date;
  data: any;
  metadata?: any;
}

// Account aggregate with event sourcing
class Account {
  private accountId: string;
  private balance: number = 0;
  private isActive: boolean = true;
  private version: number = 0;
  private uncommittedEvents: Event[] = [];

  constructor(accountId: string) {
    this.accountId = accountId;
  }

  // Command handlers
  open(initialBalance: number): void {
    if (this.version > 0) {
      throw new Error("Account already opened");
    }
    this.applyEvent({
      eventType: "AccountOpened",
      data: { accountId: this.accountId, initialBalance }
    });
  }

  deposit(amount: number): void {
    if (!this.isActive) {
      throw new Error("Account is closed");
    }
    if (amount <= 0) {
      throw new Error("Amount must be positive");
    }
    this.applyEvent({
      eventType: "MoneyDeposited",
      data: { amount }
    });
  }

  withdraw(amount: number): void {
    if (!this.isActive) {
      throw new Error("Account is closed");
    }
    if (amount <= 0) {
      throw new Error("Amount must be positive");
    }
    if (this.balance < amount) {
      throw new Error("Insufficient funds");
    }
    this.applyEvent({
      eventType: "MoneyWithdrawn",
      data: { amount }
    });
  }

  close(): void {
    if (!this.isActive) {
      throw new Error("Account already closed");
    }
    if (this.balance > 0) {
      throw new Error("Cannot close account with positive balance");
    }
    this.applyEvent({
      eventType: "AccountClosed",
      data: {}
    });
  }

  // Event application
  private applyEvent(eventData: Partial<Event>): void {
    const event: Event = {
      eventId: generateId(),
      eventType: eventData.eventType!,
      aggregateId: this.accountId,
      aggregateType: "Account",
      version: this.version + 1,
      timestamp: new Date(),
      data: eventData.data,
      metadata: eventData.metadata
    };

    this.apply(event);
    this.uncommittedEvents.push(event);
  }

  // Event handlers (state mutations)
  private apply(event: Event): void {
    switch (event.eventType) {
      case "AccountOpened":
        this.balance = event.data.initialBalance;
        this.isActive = true;
        break;

      case "MoneyDeposited":
        this.balance += event.data.amount;
        break;

      case "MoneyWithdrawn":
        this.balance -= event.data.amount;
        break;

      case "AccountClosed":
        this.isActive = false;
        break;

      default:
        throw new Error(`Unknown event type: ${event.eventType}`);
    }

    this.version = event.version;
  }

  // Replay events to rebuild state
  static fromEvents(events: Event[]): Account {
    if (events.length === 0) {
      throw new Error("Cannot create account from empty event stream");
    }

    const account = new Account(events[0].aggregateId);
    events.forEach(event => account.apply(event));
    return account;
  }

  getUncommittedEvents(): Event[] {
    return this.uncommittedEvents;
  }

  markEventsAsCommitted(): void {
    this.uncommittedEvents = [];
  }
}

// Event store interface
interface EventStore {
  append(events: Event[]): Promise<void>;
  getEvents(aggregateId: string, fromVersion?: number): Promise<Event[]>;
  getAllEvents(fromTimestamp?: Date): Promise<Event[]>;
}

// Event store implementation
class InMemoryEventStore implements EventStore {
  private events: Map<string, Event[]> = new Map();
  private allEvents: Event[] = [];

  async append(events: Event[]): Promise<void> {
    for (const event of events) {
      // Store in aggregate stream
      if (!this.events.has(event.aggregateId)) {
        this.events.set(event.aggregateId, []);
      }
      this.events.get(event.aggregateId)!.push(event);

      // Store in global stream
      this.allEvents.push(event);
    }
  }

  async getEvents(
    aggregateId: string,
    fromVersion: number = 0
  ): Promise<Event[]> {
    const events = this.events.get(aggregateId) || [];
    return events.filter(e => e.version > fromVersion);
  }

  async getAllEvents(fromTimestamp?: Date): Promise<Event[]> {
    if (!fromTimestamp) {
      return this.allEvents;
    }
    return this.allEvents.filter(e => e.timestamp >= fromTimestamp);
  }
}

// Repository with event sourcing
class EventSourcedAccountRepository {
  constructor(private eventStore: EventStore) {}

  async save(account: Account): Promise<void> {
    const events = account.getUncommittedEvents();
    if (events.length > 0) {
      await this.eventStore.append(events);
      account.markEventsAsCommitted();
    }
  }

  async findById(accountId: string): Promise<Account | null> {
    const events = await this.eventStore.getEvents(accountId);
    if (events.length === 0) {
      return null;
    }
    return Account.fromEvents(events);
  }
}

Snapshots

Optimize performance by periodically saving aggregate state:

interface Snapshot {
  aggregateId: string;
  version: number;
  timestamp: Date;
  state: any;
}

class SnapshotStore {
  private snapshots: Map<string, Snapshot> = new Map();

  async save(snapshot: Snapshot): Promise<void> {
    this.snapshots.set(snapshot.aggregateId, snapshot);
  }

  async getLatest(aggregateId: string): Promise<Snapshot | null> {
    return this.snapshots.get(aggregateId) || null;
  }
}

class EventSourcedAccountRepositoryWithSnapshots {
  constructor(
    private eventStore: EventStore,
    private snapshotStore: SnapshotStore,
    private snapshotInterval: number = 100
  ) {}

  async save(account: Account): Promise<void> {
    const events = account.getUncommittedEvents();
    await this.eventStore.append(events);
    account.markEventsAsCommitted();

    // Create snapshot every N events
    if (account.version % this.snapshotInterval === 0) {
      await this.snapshotStore.save({
        aggregateId: account.id,
        version: account.version,
        timestamp: new Date(),
        state: account.toSnapshot()
      });
    }
  }

  async findById(accountId: string): Promise<Account | null> {
    // Try to load from snapshot
    const snapshot = await this.snapshotStore.getLatest(accountId);
    let account: Account;
    let fromVersion = 0;

    if (snapshot) {
      account = Account.fromSnapshot(snapshot.state);
      fromVersion = snapshot.version;
    } else {
      account = new Account(accountId);
    }

    // Apply events after snapshot
    const events = await this.eventStore.getEvents(accountId, fromVersion);
    events.forEach(event => account.apply(event));

    return account;
  }
}

CQRS (Command Query Responsibility Segregation)

Separate read and write operations into different models, optimizing each for its specific use case.

Architecture

Commands → Command Handlers → Aggregates → Events → Event Store
                                                   ↓
                                              Event Bus
                                                   ↓
                                           Projections → Read Models → Queries

Implementation

// Commands (write operations)
interface Command {
  commandId: string;
  timestamp: Date;
}

class CreateAccountCommand implements Command {
  commandId: string;
  timestamp: Date;
  accountId: string;
  initialBalance: number;

  constructor(accountId: string, initialBalance: number) {
    this.commandId = generateId();
    this.timestamp = new Date();
    this.accountId = accountId;
    this.initialBalance = initialBalance;
  }
}

class DepositMoneyCommand implements Command {
  commandId: string;
  timestamp: Date;
  accountId: string;
  amount: number;

  constructor(accountId: string, amount: number) {
    this.commandId = generateId();
    this.timestamp = new Date();
    this.accountId = accountId;
    this.amount = amount;
  }
}

// Command handlers
class AccountCommandHandler {
  constructor(
    private repository: EventSourcedAccountRepository,
    private eventBus: EventBus
  ) {}

  async handle(command: Command): Promise<void> {
    if (command instanceof CreateAccountCommand) {
      await this.handleCreateAccount(command);
    } else if (command instanceof DepositMoneyCommand) {
      await this.handleDepositMoney(command);
    }
  }

  private async handleCreateAccount(
    command: CreateAccountCommand
  ): Promise<void> {
    const account = new Account(command.accountId);
    account.open(command.initialBalance);
    await this.repository.save(account);

    // Publish events
    const events = account.getUncommittedEvents();
    await this.eventBus.publish(events);
  }

  private async handleDepositMoney(
    command: DepositMoneyCommand
  ): Promise<void> {
    const account = await this.repository.findById(command.accountId);
    if (!account) {
      throw new Error("Account not found");
    }

    account.deposit(command.amount);
    await this.repository.save(account);

    const events = account.getUncommittedEvents();
    await this.eventBus.publish(events);
  }
}

// Read models (optimized for queries)
interface AccountReadModel {
  accountId: string;
  balance: number;
  status: string;
  lastActivity: Date;
  transactionCount: number;
}

interface AccountSummaryReadModel {
  accountId: string;
  balance: number;
  status: string;
}

// Projections (build read models from events)
class AccountProjection {
  constructor(private db: ReadDatabase) {}

  async handleEvent(event: Event): Promise<void> {
    switch (event.eventType) {
      case "AccountOpened":
        await this.handleAccountOpened(event);
        break;
      case "MoneyDeposited":
        await this.handleMoneyDeposited(event);
        break;
      case "MoneyWithdrawn":
        await this.handleMoneyWithdrawn(event);
        break;
      case "AccountClosed":
        await this.handleAccountClosed(event);
        break;
    }
  }

  private async handleAccountOpened(event: Event): Promise<void> {
    await this.db.accounts.insert({
      accountId: event.aggregateId,
      balance: event.data.initialBalance,
      status: "Active",
      lastActivity: event.timestamp,
      transactionCount: 0
    });
  }

  private async handleMoneyDeposited(event: Event): Promise<void> {
    await this.db.accounts.update(
      { accountId: event.aggregateId },
      {
        $inc: { balance: event.data.amount, transactionCount: 1 },
        $set: { lastActivity: event.timestamp }
      }
    );
  }

  private async handleMoneyWithdrawn(event: Event): Promise<void> {
    await this.db.accounts.update(
      { accountId: event.aggregateId },
      {
        $inc: { balance: -event.data.amount, transactionCount: 1 },
        $set: { lastActivity: event.timestamp }
      }
    );
  }

  private async handleAccountClosed(event: Event): Promise<void> {
    await this.db.accounts.update(
      { accountId: event.aggregateId },
      {
        $set: {
          status: "Closed",
          lastActivity: event.timestamp
        }
      }
    );
  }
}

// Query service (read-only)
class AccountQueryService {
  constructor(private db: ReadDatabase) {}

  async getAccount(accountId: string): Promise<AccountReadModel | null> {
    return await this.db.accounts.findOne({ accountId });
  }

  async getAccountsByStatus(status: string): Promise<AccountSummaryReadModel[]> {
    return await this.db.accounts.find(
      { status },
      { projection: { accountId: 1, balance: 1, status: 1 } }
    );
  }

  async getHighBalanceAccounts(
    minBalance: number
  ): Promise<AccountSummaryReadModel[]> {
    return await this.db.accounts.find(
      { balance: { $gte: minBalance } },
      { projection: { accountId: 1, balance: 1, status: 1 } }
    );
  }
}

// Event bus for publishing events
class EventBus {
  private subscribers: Map<string, Function[]> = new Map();

  subscribe(eventType: string, handler: Function): void {
    if (!this.subscribers.has(eventType)) {
      this.subscribers.set(eventType, []);
    }
    this.subscribers.get(eventType)!.push(handler);
  }

  subscribeToAll(handler: Function): void {
    this.subscribe("*", handler);
  }

  async publish(events: Event[]): Promise<void> {
    for (const event of events) {
      // Call specific handlers
      const handlers = this.subscribers.get(event.eventType) || [];
      await Promise.all(handlers.map(h => h(event)));

      // Call wildcard handlers
      const allHandlers = this.subscribers.get("*") || [];
      await Promise.all(allHandlers.map(h => h(event)));
    }
  }
}

Saga Pattern

Manage distributed transactions across multiple services using a sequence of local transactions coordinated by a saga.

Orchestration-Based Saga

A central orchestrator coordinates all saga participants.

// Saga state
enum SagaStatus {
  Started = "Started",
  Completed = "Completed",
  Compensating = "Compensating",
  Compensated = "Compensated",
  Failed = "Failed"
}

interface SagaStep {
  name: string;
  action: () => Promise<void>;
  compensation: () => Promise<void>;
}

class OrderSaga {
  private sagaId: string;
  private status: SagaStatus;
  private completedSteps: string[] = [];
  private currentStep: number = 0;

  private steps: SagaStep[] = [
    {
      name: "CreateOrder",
      action: async () => await this.createOrder(),
      compensation: async () => await this.cancelOrder()
    },
    {
      name: "ReserveInventory",
      action: async () => await this.reserveInventory(),
      compensation: async () => await this.releaseInventory()
    },
    {
      name: "ProcessPayment",
      action: async () => await this.processPayment(),
      compensation: async () => await this.refundPayment()
    },
    {
      name: "ArrangeShipment",
      action: async () => await this.arrangeShipment(),
      compensation: async () => await this.cancelShipment()
    }
  ];

  constructor(
    private orderId: string,
    private customerId: string,
    private items: OrderItem[]
  ) {
    this.sagaId = generateId();
    this.status = SagaStatus.Started;
  }

  async execute(): Promise<void> {
    try {
      // Execute each step
      for (let i = 0; i < this.steps.length; i++) {
        this.currentStep = i;
        const step = this.steps[i];

        console.log(`Executing step: ${step.name}`);
        await step.action();
        this.completedSteps.push(step.name);
      }

      this.status = SagaStatus.Completed;
      console.log("Saga completed successfully");

    } catch (error) {
      console.error(`Saga failed at step ${this.currentStep}:`, error);
      await this.compensate();
    }
  }

  private async compensate(): Promise<void> {
    this.status = SagaStatus.Compensating;
    console.log("Starting compensation");

    // Compensate in reverse order
    for (let i = this.completedSteps.length - 1; i >= 0; i--) {
      const stepName = this.completedSteps[i];
      const step = this.steps.find(s => s.name === stepName);

      if (step) {
        try {
          console.log(`Compensating step: ${step.name}`);
          await step.compensation();
        } catch (error) {
          console.error(`Compensation failed for ${step.name}:`, error);
          // Log for manual intervention
        }
      }
    }

    this.status = SagaStatus.Compensated;
    console.log("Compensation completed");
  }

  // Step implementations
  private async createOrder(): Promise<void> {
    await orderService.create({
      orderId: this.orderId,
      customerId: this.customerId,
      items: this.items
    });
  }

  private async cancelOrder(): Promise<void> {
    await orderService.cancel(this.orderId);
  }

  private async reserveInventory(): Promise<void> {
    await inventoryService.reserve(this.orderId, this.items);
  }

  private async releaseInventory(): Promise<void> {
    await inventoryService.release(this.orderId);
  }

  private async processPayment(): Promise<void> {
    const total = this.calculateTotal();
    await paymentService.charge(this.customerId, total);
  }

  private async refundPayment(): Promise<void> {
    const total = this.calculateTotal();
    await paymentService.refund(this.customerId, total);
  }

  private async arrangeShipment(): Promise<void> {
    await shippingService.createShipment(this.orderId);
  }

  private async cancelShipment(): Promise<void> {
    await shippingService.cancelShipment(this.orderId);
  }

  private calculateTotal(): Money {
    return this.items.reduce(
      (sum, item) => sum.add(item.price.multiply(item.quantity)),
      new Money(0, "USD")
    );
  }
}

// Orchestrator service
class SagaOrchestrator {
  private activeSagas: Map<string, OrderSaga> = new Map();

  async startOrderSaga(
    orderId: string,
    customerId: string,
    items: OrderItem[]
  ): Promise<void> {
    const saga = new OrderSaga(orderId, customerId, items);
    this.activeSagas.set(saga.sagaId, saga);

    try {
      await saga.execute();
    } finally {
      this.activeSagas.delete(saga.sagaId);
    }
  }

  getSagaStatus(sagaId: string): SagaStatus | null {
    const saga = this.activeSagas.get(sagaId);
    return saga ? saga.status : null;
  }
}

Choreography-Based Saga

Services coordinate through events without central orchestrator.

// Event-driven saga with choreography
class OrderCreatedEvent {
  constructor(
    public orderId: string,
    public customerId: string,
    public items: OrderItem[]
  ) {}
}

class InventoryReservedEvent {
  constructor(
    public orderId: string,
    public reservationId: string
  ) {}
}

class PaymentProcessedEvent {
  constructor(
    public orderId: string,
    public paymentId: string
  ) {}
}

class ShipmentArrangedEvent {
  constructor(
    public orderId: string,
    public shipmentId: string
  ) {}
}

// Compensation events
class InventoryReservationFailedEvent {
  constructor(
    public orderId: string,
    public reason: string
  ) {}
}

class PaymentFailedEvent {
  constructor(
    public orderId: string,
    public reason: string
  ) {}
}

// Order service
class OrderService {
  constructor(private eventBus: EventBus) {
    // Subscribe to compensation events
    eventBus.subscribe("InventoryReservationFailed",
      this.handleInventoryReservationFailed.bind(this));
    eventBus.subscribe("PaymentFailed",
      this.handlePaymentFailed.bind(this));
  }

  async createOrder(order: CreateOrderRequest): Promise<void> {
    // Create order in pending state
    await this.repository.save({
      ...order,
      status: "Pending"
    });

    // Publish event to trigger next step
    await this.eventBus.publish(
      new OrderCreatedEvent(order.orderId, order.customerId, order.items)
    );
  }

  private async handleInventoryReservationFailed(
    event: InventoryReservationFailedEvent
  ): Promise<void> {
    await this.repository.updateStatus(event.orderId, "Cancelled");
    console.log(`Order ${event.orderId} cancelled: ${event.reason}`);
  }

  private async handlePaymentFailed(event: PaymentFailedEvent): Promise<void> {
    await this.repository.updateStatus(event.orderId, "PaymentFailed");
    // Trigger inventory release
    await this.eventBus.publish(
      new ReleaseInventoryCommand(event.orderId)
    );
  }
}

// Inventory service
class InventoryService {
  constructor(private eventBus: EventBus) {
    eventBus.subscribe("OrderCreated",
      this.handleOrderCreated.bind(this));
    eventBus.subscribe("ReleaseInventory",
      this.handleReleaseInventory.bind(this));
  }

  private async handleOrderCreated(event: OrderCreatedEvent): Promise<void> {
    try {
      // Reserve inventory
      const reservationId = await this.reserveItems(event.items);

      // Publish success event
      await this.eventBus.publish(
        new InventoryReservedEvent(event.orderId, reservationId)
      );
    } catch (error) {
      // Publish failure event
      await this.eventBus.publish(
        new InventoryReservationFailedEvent(
          event.orderId,
          error.message
        )
      );
    }
  }

  private async handleReleaseInventory(
    command: ReleaseInventoryCommand
  ): Promise<void> {
    await this.releaseReservation(command.orderId);
  }
}

// Payment service
class PaymentService {
  constructor(private eventBus: EventBus) {
    eventBus.subscribe("InventoryReserved",
      this.handleInventoryReserved.bind(this));
    eventBus.subscribe("RefundPayment",
      this.handleRefundPayment.bind(this));
  }

  private async handleInventoryReserved(
    event: InventoryReservedEvent
  ): Promise<void> {
    try {
      // Process payment
      const paymentId = await this.chargeCustomer(event.orderId);

      // Publish success event
      await this.eventBus.publish(
        new PaymentProcessedEvent(event.orderId, paymentId)
      );
    } catch (error) {
      // Publish failure event
      await this.eventBus.publish(
        new PaymentFailedEvent(event.orderId, error.message)
      );
    }
  }

  private async handleRefundPayment(
    command: RefundPaymentCommand
  ): Promise<void> {
    await this.refund(command.orderId);
  }
}

API Gateway Pattern

Single entry point for clients, routing requests to appropriate microservices and handling cross-cutting concerns.

Implementation

class APIGateway {
  constructor(
    private router: Router,
    private authService: AuthService,
    private rateLimiter: RateLimiter,
    private circuitBreaker: CircuitBreaker,
    private loadBalancer: LoadBalancer
  ) {
    this.setupRoutes();
  }

  private setupRoutes(): void {
    // User service routes
    this.router.get("/api/users/:id",
      this.authenticate.bind(this),
      this.rateLimit.bind(this),
      this.getUserHandler.bind(this)
    );

    // Order service routes
    this.router.post("/api/orders",
      this.authenticate.bind(this),
      this.rateLimit.bind(this),
      this.createOrderHandler.bind(this)
    );

    // Product service routes
    this.router.get("/api/products",
      this.rateLimit.bind(this),
      this.getProductsHandler.bind(this)
    );
  }

  // Middleware: Authentication
  private async authenticate(
    req: Request,
    res: Response,
    next: NextFunction
  ): Promise<void> {
    try {
      const token = req.headers.authorization?.replace("Bearer ", "");
      if (!token) {
        res.status(401).json({ error: "Unauthorized" });
        return;
      }

      const user = await this.authService.validateToken(token);
      req.user = user;
      next();
    } catch (error) {
      res.status(401).json({ error: "Invalid token" });
    }
  }

  // Middleware: Rate limiting
  private async rateLimit(
    req: Request,
    res: Response,
    next: NextFunction
  ): Promise<void> {
    const clientId = req.ip || req.user?.id;

    if (await this.rateLimiter.isAllowed(clientId)) {
      next();
    } else {
      res.status(429).json({ error: "Too many requests" });
    }
  }

  // Handler: Get user
  private async getUserHandler(req: Request, res: Response): Promise<void> {
    try {
      const userId = req.params.id;

      // Call user service with circuit breaker
      const user = await this.circuitBreaker.execute(
        "user-service",
        async () => {
          const instance = this.loadBalancer.getInstance("user-service");
          return await instance.getUser(userId);
        }
      );

      res.json(user);
    } catch (error) {
      res.status(500).json({ error: "Internal server error" });
    }
  }

  // Handler: Create order
  private async createOrderHandler(
    req: Request,
    res: Response
  ): Promise<void> {
    try {
      // Aggregate data from multiple services
      const [user, products, inventory] = await Promise.all([
        this.callUserService(req.user.id),
        this.callProductService(req.body.items),
        this.callInventoryService(req.body.items)
      ]);

      // Call order service
      const order = await this.callOrderService({
        user,
        items: req.body.items,
        inventory
      });

      res.status(201).json(order);
    } catch (error) {
      res.status(500).json({ error: "Internal server error" });
    }
  }

  private async callUserService(userId: string): Promise<User> {
    return await this.circuitBreaker.execute("user-service", async () => {
      const instance = this.loadBalancer.getInstance("user-service");
      return await instance.getUser(userId);
    });
  }

  private async callProductService(items: any[]): Promise<Product[]> {
    return await this.circuitBreaker.execute("product-service", async () => {
      const instance = this.loadBalancer.getInstance("product-service");
      return await instance.getProducts(items.map(i => i.productId));
    });
  }
}

// Rate limiter implementation
class RateLimiter {
  private requests: Map<string, number[]> = new Map();

  constructor(
    private maxRequests: number = 100,
    private windowMs: number = 60000 // 1 minute
  ) {}

  async isAllowed(clientId: string): Promise<boolean> {
    const now = Date.now();
    const windowStart = now - this.windowMs;

    // Get existing requests
    const clientRequests = this.requests.get(clientId) || [];

    // Filter out old requests
    const recentRequests = clientRequests.filter(t => t > windowStart);

    // Check if under limit
    if (recentRequests.length < this.maxRequests) {
      recentRequests.push(now);
      this.requests.set(clientId, recentRequests);
      return true;
    }

    return false;
  }
}

// Load balancer
class LoadBalancer {
  private services: Map<string, ServiceInstance[]> = new Map();
  private currentIndex: Map<string, number> = new Map();

  registerService(name: string, instance: ServiceInstance): void {
    if (!this.services.has(name)) {
      this.services.set(name, []);
      this.currentIndex.set(name, 0);
    }
    this.services.get(name)!.push(instance);
  }

  getInstance(serviceName: string): ServiceInstance {
    const instances = this.services.get(serviceName);
    if (!instances || instances.length === 0) {
      throw new Error(`No instances available for ${serviceName}`);
    }

    // Round-robin selection
    const index = this.currentIndex.get(serviceName)!;
    const instance = instances[index];

    // Update index for next call
    this.currentIndex.set(
      serviceName,
      (index + 1) % instances.length
    );

    return instance;
  }
}

Backend for Frontend (BFF) Pattern

// Separate BFFs for different clients
class WebBFF {
  constructor(
    private userService: UserService,
    private productService: ProductService,
    private orderService: OrderService
  ) {}

  // Optimized for web client needs
  async getHomePage(userId: string): Promise<WebHomePageData> {
    const [user, recommendations, recentOrders] = await Promise.all([
      this.userService.getUser(userId),
      this.productService.getRecommendations(userId, 10),
      this.orderService.getRecentOrders(userId, 5)
    ]);

    return {
      user: {
        name: user.name,
        email: user.email,
        avatar: user.avatarUrl
      },
      recommendations: recommendations.map(p => ({
        id: p.id,
        name: p.name,
        price: p.price,
        imageUrl: p.images[0], // Full images for web
        rating: p.averageRating
      })),
      recentOrders: recentOrders.map(o => ({
        orderId: o.id,
        date: o.createdAt,
        total: o.totalAmount,
        status: o.status,
        itemCount: o.items.length
      }))
    };
  }
}

class MobileBFF {
  constructor(
    private userService: UserService,
    private productService: ProductService,
    private orderService: OrderService
  ) {}

  // Optimized for mobile client needs
  async getHomePage(userId: string): Promise<MobileHomePageData> {
    const [user, recommendations, recentOrders] = await Promise.all([
      this.userService.getUser(userId),
      this.productService.getRecommendations(userId, 5), // Fewer items
      this.orderService.getRecentOrders(userId, 3)
    ]);

    return {
      user: {
        name: user.name,
        avatar: user.avatarThumbnailUrl // Smaller images for mobile
      },
      recommendations: recommendations.map(p => ({
        id: p.id,
        name: p.name,
        price: p.price,
        thumbnail: p.thumbnails.small, // Optimized image size
        rating: Math.round(p.averageRating) // Simplified rating
      })),
      recentOrders: recentOrders.map(o => ({
        id: o.id,
        date: o.createdAt.toISOString(),
        total: o.totalAmount,
        status: o.status
      }))
    };
  }
}

Service Mesh Pattern

Infrastructure layer for service-to-service communication providing observability, traffic management, and security.

Key Features

// Service mesh configuration example (Istio)
const serviceMeshConfig = {
  // Traffic management
  virtualService: {
    name: "product-service",
    hosts: ["product-service"],
    http: [
      {
        match: [{ uri: { prefix: "/api/v1" } }],
        route: [
          {
            destination: {
              host: "product-service",
              subset: "v1"
            },
            weight: 90
          },
          {
            destination: {
              host: "product-service",
              subset: "v2"
            },
            weight: 10 // Canary deployment
          }
        ],
        retries: {
          attempts: 3,
          perTryTimeout: "2s"
        },
        timeout: "10s"
      }
    ]
  },

  // Destination rules
  destinationRule: {
    name: "product-service",
    host: "product-service",
    trafficPolicy: {
      connectionPool: {
        tcp: {
          maxConnections: 100
        },
        http: {
          http1MaxPendingRequests: 50,
          http2MaxRequests: 100,
          maxRequestsPerConnection: 2
        }
      },
      loadBalancer: {
        simple: "ROUND_ROBIN"
      },
      outlierDetection: {
        consecutive5xxErrors: 5,
        interval: "30s",
        baseEjectionTime: "30s",
        maxEjectionPercent: 50
      }
    },
    subsets: [
      {
        name: "v1",
        labels: { version: "v1" }
      },
      {
        name: "v2",
        labels: { version: "v2" }
      }
    ]
  },

  // Circuit breaker
  circuitBreaker: {
    consecutiveErrors: 5,
    interval: "30s",
    baseEjectionTime: "30s",
    maxEjectionPercent: 50
  }
};

Resilience Patterns

Circuit Breaker

Prevent cascading failures by stopping requests to failing services.

enum CircuitState {
  Closed = "Closed",     // Normal operation
  Open = "Open",         // Blocking requests
  HalfOpen = "HalfOpen"  // Testing if service recovered
}

class CircuitBreaker {
  private state: CircuitState = CircuitState.Closed;
  private failureCount: number = 0;
  private successCount: number = 0;
  private lastFailureTime: number = 0;

  constructor(
    private failureThreshold: number = 5,
    private successThreshold: number = 2,
    private timeout: number = 60000 // 1 minute
  ) {}

  async execute<T>(operation: () => Promise<T>): Promise<T> {
    if (this.state === CircuitState.Open) {
      if (Date.now() - this.lastFailureTime > this.timeout) {
        this.state = CircuitState.HalfOpen;
        this.successCount = 0;
      } else {
        throw new Error("Circuit breaker is OPEN");
      }
    }

    try {
      const result = await operation();
      this.onSuccess();
      return result;
    } catch (error) {
      this.onFailure();
      throw error;
    }
  }

  private onSuccess(): void {
    this.failureCount = 0;

    if (this.state === CircuitState.HalfOpen) {
      this.successCount++;
      if (this.successCount >= this.successThreshold) {
        this.state = CircuitState.Closed;
        this.successCount = 0;
      }
    }
  }

  private onFailure(): void {
    this.failureCount++;
    this.lastFailureTime = Date.now();

    if (this.failureCount >= this.failureThreshold) {
      this.state = CircuitState.Open;
    }
  }

  getState(): CircuitState {
    return this.state;
  }
}

Retry Pattern

Automatically retry failed operations with exponential backoff.

class RetryPolicy {
  constructor(
    private maxRetries: number = 3,
    private initialDelayMs: number = 100,
    private maxDelayMs: number = 5000,
    private backoffMultiplier: number = 2
  ) {}

  async execute<T>(
    operation: () => Promise<T>,
    isRetryable: (error: Error) => boolean = () => true
  ): Promise<T> {
    let lastError: Error;

    for (let attempt = 0; attempt <= this.maxRetries; attempt++) {
      try {
        return await operation();
      } catch (error) {
        lastError = error as Error;

        if (attempt === this.maxRetries || !isRetryable(lastError)) {
          throw lastError;
        }

        const delay = this.calculateDelay(attempt);
        await this.sleep(delay);
      }
    }

    throw lastError!;
  }

  private calculateDelay(attempt: number): number {
    const delay = this.initialDelayMs * Math.pow(this.backoffMultiplier, attempt);
    return Math.min(delay, this.maxDelayMs);
  }

  private sleep(ms: number): Promise<void> {
    return new Promise(resolve => setTimeout(resolve, ms));
  }
}

// Usage
const retryPolicy = new RetryPolicy(3, 100, 5000, 2);

await retryPolicy.execute(
  async () => await apiClient.get("/users/123"),
  (error) => error.statusCode >= 500 // Only retry server errors
);

Bulkhead Pattern

Isolate resources to prevent failures from affecting entire system.

class Bulkhead {
  private activeRequests: number = 0;
  private queue: Array<{
    resolve: (value: any) => void;
    reject: (error: any) => void;
    operation: () => Promise<any>;
  }> = [];

  constructor(
    private maxConcurrent: number = 10,
    private maxQueueSize: number = 100
  ) {}

  async execute<T>(operation: () => Promise<T>): Promise<T> {
    if (this.activeRequests < this.maxConcurrent) {
      return await this.executeOperation(operation);
    }

    if (this.queue.length >= this.maxQueueSize) {
      throw new Error("Bulkhead queue full");
    }

    return new Promise((resolve, reject) => {
      this.queue.push({ resolve, reject, operation });
    });
  }

  private async executeOperation<T>(
    operation: () => Promise<T>
  ): Promise<T> {
    this.activeRequests++;

    try {
      const result = await operation();
      this.processQueue();
      return result;
    } catch (error) {
      this.processQueue();
      throw error;
    } finally {
      this.activeRequests--;
    }
  }

  private processQueue(): void {
    if (this.queue.length > 0 &&
        this.activeRequests < this.maxConcurrent) {
      const { resolve, reject, operation } = this.queue.shift()!;

      this.executeOperation(operation)
        .then(resolve)
        .catch(reject);
    }
  }
}

Timeout Pattern

Prevent indefinite waits by setting time limits.

class TimeoutPolicy {
  constructor(private timeoutMs: number = 30000) {}

  async execute<T>(operation: () => Promise<T>): Promise<T> {
    return Promise.race([
      operation(),
      this.timeout()
    ]);
  }

  private timeout(): Promise<never> {
    return new Promise((_, reject) => {
      setTimeout(() => {
        reject(new Error(`Operation timed out after ${this.timeoutMs}ms`));
      }, this.timeoutMs);
    });
  }
}

Fallback Pattern

Provide alternative response when operation fails.

class FallbackPolicy<T> {
  constructor(private fallbackFn: () => Promise<T>) {}

  async execute(operation: () => Promise<T>): Promise<T> {
    try {
      return await operation();
    } catch (error) {
      console.warn("Operation failed, using fallback:", error);
      return await this.fallbackFn();
    }
  }
}

// Usage
const getUserWithFallback = new FallbackPolicy(async () => ({
  id: "default",
  name: "Guest User",
  email: "guest@example.com"
}));

const user = await getUserWithFallback.execute(
  async () => await userService.getUser(userId)
);

Combined Resilience Strategy

class ResilientClient {
  private circuitBreaker: CircuitBreaker;
  private retryPolicy: RetryPolicy;
  private timeoutPolicy: TimeoutPolicy;
  private fallbackPolicy: FallbackPolicy<any>;
  private bulkhead: Bulkhead;

  constructor() {
    this.circuitBreaker = new CircuitBreaker(5, 2, 60000);
    this.retryPolicy = new RetryPolicy(3, 100, 5000, 2);
    this.timeoutPolicy = new TimeoutPolicy(10000);
    this.fallbackPolicy = new FallbackPolicy(async () => null);
    this.bulkhead = new Bulkhead(10, 100);
  }

  async call<T>(
    operation: () => Promise<T>,
    options?: {
      timeout?: number;
      retries?: number;
      fallback?: () => Promise<T>;
    }
  ): Promise<T> {
    const timeoutPolicy = options?.timeout
      ? new TimeoutPolicy(options.timeout)
      : this.timeoutPolicy;

    const fallbackPolicy = options?.fallback
      ? new FallbackPolicy(options.fallback)
      : this.fallbackPolicy;

    return await fallbackPolicy.execute(async () => {
      return await this.bulkhead.execute(async () => {
        return await this.circuitBreaker.execute(async () => {
          return await this.retryPolicy.execute(async () => {
            return await timeoutPolicy.execute(operation);
          });
        });
      });
    });
  }
}

Scalability Patterns

Horizontal Scaling (Scale Out)

Add more instances to handle increased load.

// Load balancer for horizontal scaling
class RoundRobinLoadBalancer {
  private instances: string[] = [];
  private currentIndex: number = 0;

  addInstance(url: string): void {
    this.instances.push(url);
  }

  removeInstance(url: string): void {
    this.instances = this.instances.filter(i => i !== url);
  }

  getNextInstance(): string {
    if (this.instances.length === 0) {
      throw new Error("No instances available");
    }

    const instance = this.instances[this.currentIndex];
    this.currentIndex = (this.currentIndex + 1) % this.instances.length;
    return instance;
  }
}

// Auto-scaling based on metrics
class AutoScaler {
  constructor(
    private minInstances: number = 2,
    private maxInstances: number = 10,
    private targetCPU: number = 70
  ) {}

  async scale(currentInstances: number, currentCPU: number): Promise<number> {
    if (currentCPU > this.targetCPU) {
      // Scale up
      const desiredInstances = Math.ceil(
        currentInstances * (currentCPU / this.targetCPU)
      );
      return Math.min(desiredInstances, this.maxInstances);
    } else if (currentCPU < this.targetCPU * 0.5) {
      // Scale down
      const desiredInstances = Math.floor(
        currentInstances * (currentCPU / this.targetCPU)
      );
      return Math.max(desiredInstances, this.minInstances);
    }

    return currentInstances;
  }
}

Caching Strategies

// Cache-aside pattern
class CacheAsideRepository {
  constructor(
    private cache: Cache,
    private database: Database,
    private ttl: number = 3600
  ) {}

  async get(id: string): Promise<any> {
    // Try cache first
    const cached = await this.cache.get(id);
    if (cached) {
      return cached;
    }

    // Cache miss - get from database
    const data = await this.database.findById(id);
    if (data) {
      await this.cache.set(id, data, this.ttl);
    }

    return data;
  }

  async update(id: string, data: any): Promise<void> {
    // Update database
    await this.database.update(id, data);

    // Invalidate cache
    await this.cache.delete(id);
  }
}

// Write-through cache
class WriteThroughCache {
  constructor(
    private cache: Cache,
    private database: Database
  ) {}

  async write(id: string, data: any): Promise<void> {
    // Write to both cache and database
    await Promise.all([
      this.cache.set(id, data),
      this.database.save(id, data)
    ]);
  }
}

// Write-behind cache
class WriteBehindCache {
  private writeQueue: Map<string, any> = new Map();

  constructor(
    private cache: Cache,
    private database: Database,
    private flushInterval: number = 5000
  ) {
    this.startFlushInterval();
  }

  async write(id: string, data: any): Promise<void> {
    // Write to cache immediately
    await this.cache.set(id, data);

    // Queue for database write
    this.writeQueue.set(id, data);
  }

  private startFlushInterval(): void {
    setInterval(async () => {
      await this.flush();
    }, this.flushInterval);
  }

  private async flush(): Promise<void> {
    const entries = Array.from(this.writeQueue.entries());
    this.writeQueue.clear();

    await Promise.all(
      entries.map(([id, data]) => this.database.save(id, data))
    );
  }
}

Database Sharding

// Shard key-based routing
class ShardRouter {
  private shards: Map<number, Database> = new Map();
  private totalShards: number;

  constructor(shards: Database[]) {
    this.totalShards = shards.length;
    shards.forEach((shard, index) => {
      this.shards.set(index, shard);
    });
  }

  private getShardIndex(key: string): number {
    // Hash-based sharding
    const hash = this.hashCode(key);
    return Math.abs(hash) % this.totalShards;
  }

  private hashCode(str: string): number {
    let hash = 0;
    for (let i = 0; i < str.length; i++) {
      const char = str.charCodeAt(i);
      hash = ((hash << 5) - hash) + char;
      hash = hash & hash; // Convert to 32-bit integer
    }
    return hash;
  }

  getShard(key: string): Database {
    const index = this.getShardIndex(key);
    return this.shards.get(index)!;
  }

  async save(key: string, data: any): Promise<void> {
    const shard = this.getShard(key);
    await shard.save(key, data);
  }

  async find(key: string): Promise<any> {
    const shard = this.getShard(key);
    return await shard.findById(key);
  }
}

Best Practices

Architecture Design

1. Start with the domain: Understand business requirements before choosing patterns 2. Keep it simple: Don't over-engineer; add complexity only when needed 3. Design for failure: Assume services will fail; build resilience 4. Loose coupling: Services should be independent and deployable separately 5. High cohesion: Group related functionality together 6. API-first design: Define contracts before implementation 7. Versioning strategy: Plan for API evolution from the start 8. Observability: Build in logging, metrics, and tracing from day one

Domain-Driven Design

1. Collaborate with domain experts: Build shared understanding 2. Use ubiquitous language: Consistent terminology everywhere 3. Model bounded contexts: Clear boundaries prevent model confusion 4. Small aggregates: Better performance and clearer boundaries 5. Reference by ID: Aggregates reference others by identity 6. Protect invariants: Aggregate roots enforce all business rules 7. Domain events: Capture important business occurrences 8. Repository per aggregate: One repository per aggregate root

Event Sourcing & CQRS

1. Event naming: Use past tense, business-meaningful names 2. Event immutability: Never modify published events 3. Event versioning: Plan for event schema evolution 4. Snapshots: Use for long event streams 5. Idempotent handlers: Events may be processed multiple times 6. Separate concerns: Different models for read and write 7. Eventual consistency: Accept and communicate delay 8. Monitoring: Track projection lag and event processing

Microservices

1. Service size: Small enough to understand, large enough to provide value 2. Data ownership: Each service owns its data 3. Asynchronous communication: Prefer events over synchronous calls 4. Service discovery: Dynamic service location 5. Configuration management: Centralized, environment-specific config 6. Deployment independence: Services deploy without coordinating 7. Failure isolation: Circuit breakers and bulkheads 8. Distributed tracing: Correlation IDs across service calls

Performance & Scalability

1. Measure first: Profile before optimizing 2. Cache strategically: Right layer, right data, right TTL 3. Async processing: Move slow operations to background 4. Connection pooling: Reuse database/HTTP connections 5. Pagination: Never return unbounded result sets 6. Compression: Reduce network transfer size 7. CDN usage: Serve static assets from edge locations 8. Database indexes: Index query patterns, not all columns

Security

1. Defense in depth: Multiple security layers 2. Least privilege: Minimal permissions necessary 3. Encrypt in transit: TLS for all network communication 4. Encrypt at rest: Sensitive data encrypted in storage 5. Input validation: Validate and sanitize all inputs 6. Authentication: Verify identity (JWT, OAuth) 7. Authorization: Verify permissions (RBAC, ABAC) 8. Audit logging: Track security-relevant events

Testing

1. Test pyramid: Many unit tests, fewer integration, few E2E 2. Test behavior: Focus on business logic, not implementation 3. Contract testing: Verify API contracts between services 4. Chaos engineering: Test failure scenarios 5. Performance testing: Load test before production 6. Security testing: Automated vulnerability scanning 7. Smoke tests: Quick validation after deployment 8. Canary deployments: Gradual rollout to detect issues

Monitoring & Observability

1. Structured logging: JSON logs with context 2. Metrics collection: RED metrics (Rate, Errors, Duration) 3. Distributed tracing: Request flow across services 4. Health checks: Liveness and readiness endpoints 5. Alerting: Alert on symptoms, not causes 6. Dashboards: Key metrics visible at a glance 7. SLO/SLA: Define and track service levels 8. Incident response: Runbooks for common issues

Resources

Books

• "Domain-Driven Design" by Eric Evans
• "Implementing Domain-Driven Design" by Vaughn Vernon
• "Microservices Patterns" by Chris Richardson
• "Building Microservices" by Sam Newman
• "Designing Data-Intensive Applications" by Martin Kleppmann

Online Resources

• https://microservices.io/patterns - Microservices pattern catalog
• https://martinfowler.com - Architecture articles and patterns
• https://learn.microsoft.com/en-us/azure/architecture - Azure Architecture Center
• https://aws.amazon.com/architecture - AWS Architecture resources
• https://cloud.google.com/architecture - Google Cloud Architecture

---

Skill Version: 1.0.0 Last Updated: October 2025 Skill Category: Enterprise Architecture, System Design, Distributed Systems

Enterprise Architecture Patterns - Examples

Comprehensive examples demonstrating enterprise architecture patterns in real-world scenarios.

Table of Contents

1. E-Commerce Order Management System 2. Banking Transaction System with Event Sourcing 3. Multi-Tenant SaaS Application 4. Microservices Communication Patterns 5. Distributed Payment Processing 6. Real-Time Analytics Platform 7. Content Management System with CQRS 8. Hotel Booking System with Saga 9. Social Media Feed Architecture 10. IoT Device Management Platform 11. Healthcare Patient Records System 12. Supply Chain Management 13. Video Streaming Platform 14. Financial Trading System 15. Customer Support Ticketing System 16. Inventory Management with Eventual Consistency 17. Multi-Region Deployment Architecture 18. API Rate Limiting and Throttling 19. Event-Driven Notification System 20. Serverless Microservices Architecture 21. GraphQL API Gateway Pattern 22. Zero-Downtime Deployment Strategy

---

Example 1: E-Commerce Order Management System

Complete implementation of an order management system using DDD, CQRS, and Event Sourcing.

Architecture Diagram

┌─────────────┐
│   Client    │
└──────┬──────┘
       │
       ▼
┌─────────────────────────────────────────┐
│          API Gateway                     │
│  - Authentication                        │
│  - Rate Limiting                         │
│  - Request Routing                       │
└──────┬──────────────────────┬───────────┘
       │                      │
       ▼                      ▼
┌──────────────┐      ┌──────────────┐
│   Command    │      │    Query     │
│   Service    │      │   Service    │
└──────┬───────┘      └──────┬───────┘
       │                      │
       ▼                      ▼
┌──────────────┐      ┌──────────────┐
│ Event Store  │──────▶│ Read Model   │
└──────────────┘      └──────────────┘
       │
       ▼
┌──────────────┐
│  Event Bus   │
└──────┬───────┘
       │
       ▼
┌─────────────────────────────────────────┐
│       Domain Services                   │
│  - Inventory Service                    │
│  - Payment Service                      │
│  - Shipping Service                     │
└─────────────────────────────────────────┘

Domain Model

// Value Objects
class Money {
  constructor(
    readonly amount: number,
    readonly currency: string
  ) {
    if (amount < 0) throw new Error("Amount cannot be negative");
  }

  add(other: Money): Money {
    if (this.currency !== other.currency) {
      throw new Error("Cannot add different currencies");
    }
    return new Money(this.amount + other.amount, this.currency);
  }

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

  equals(other: Money): boolean {
    return this.amount === other.amount &&
           this.currency === other.currency;
  }
}

class Address {
  constructor(
    readonly street: string,
    readonly city: string,
    readonly state: string,
    readonly zipCode: string,
    readonly country: string
  ) {}
}

class ProductDetails {
  constructor(
    readonly productId: string,
    readonly name: string,
    readonly sku: string
  ) {}
}

// Entities
class OrderItem {
  constructor(
    readonly product: ProductDetails,
    readonly unitPrice: Money,
    private quantity: number
  ) {
    if (quantity <= 0) {
      throw new Error("Quantity must be positive");
    }
  }

  increaseQuantity(amount: number): void {
    if (amount <= 0) {
      throw new Error("Amount must be positive");
    }
    this.quantity += amount;
  }

  decreaseQuantity(amount: number): void {
    if (amount <= 0 || amount > this.quantity) {
      throw new Error("Invalid quantity decrease");
    }
    this.quantity -= amount;
  }

  get subtotal(): Money {
    return this.unitPrice.multiply(this.quantity);
  }

  getQuantity(): number {
    return this.quantity;
  }
}

// Domain Events
class OrderCreatedEvent {
  constructor(
    readonly orderId: string,
    readonly customerId: string,
    readonly createdAt: Date
  ) {}
}

class OrderItemAddedEvent {
  constructor(
    readonly orderId: string,
    readonly productId: string,
    readonly quantity: number,
    readonly price: Money
  ) {}
}

class OrderConfirmedEvent {
  constructor(
    readonly orderId: string,
    readonly total: Money,
    readonly confirmedAt: Date
  ) {}
}

class OrderCancelledEvent {
  constructor(
    readonly orderId: string,
    readonly reason: string,
    readonly cancelledAt: Date
  ) {}
}

class OrderShippedEvent {
  constructor(
    readonly orderId: string,
    readonly trackingNumber: string,
    readonly shippedAt: Date
  ) {}
}

// Aggregate Root
class Order {
  private items: Map<string, OrderItem> = new Map();
  private status: OrderStatus = OrderStatus.Draft;
  private version: number = 0;
  private uncommittedEvents: any[] = [];

  constructor(
    readonly orderId: string,
    readonly customerId: string,
    private shippingAddress: Address
  ) {}

  // Commands
  addItem(product: ProductDetails, price: Money, quantity: number): void {
    if (this.status !== OrderStatus.Draft) {
      throw new Error("Cannot modify confirmed order");
    }

    const existingItem = this.items.get(product.productId);
    if (existingItem) {
      existingItem.increaseQuantity(quantity);
    } else {
      this.items.set(
        product.productId,
        new OrderItem(product, price, quantity)
      );
    }

    this.addEvent(new OrderItemAddedEvent(
      this.orderId,
      product.productId,
      quantity,
      price
    ));
  }

  removeItem(productId: string): void {
    if (this.status !== OrderStatus.Draft) {
      throw new Error("Cannot modify confirmed order");
    }

    if (!this.items.has(productId)) {
      throw new Error("Item not in order");
    }

    this.items.delete(productId);
  }

  updateShippingAddress(address: Address): void {
    if (this.status !== OrderStatus.Draft) {
      throw new Error("Cannot modify confirmed order");
    }

    this.shippingAddress = address;
  }

  confirm(): void {
    if (this.status !== OrderStatus.Draft) {
      throw new Error("Order already confirmed");
    }

    if (this.items.size === 0) {
      throw new Error("Cannot confirm empty order");
    }

    this.status = OrderStatus.Confirmed;

    this.addEvent(new OrderConfirmedEvent(
      this.orderId,
      this.calculateTotal(),
      new Date()
    ));
  }

  cancel(reason: string): void {
    if (this.status === OrderStatus.Cancelled ||
        this.status === OrderStatus.Shipped) {
      throw new Error(`Cannot cancel order in ${this.status} status`);
    }

    this.status = OrderStatus.Cancelled;

    this.addEvent(new OrderCancelledEvent(
      this.orderId,
      reason,
      new Date()
    ));
  }

  ship(trackingNumber: string): void {
    if (this.status !== OrderStatus.Confirmed) {
      throw new Error("Can only ship confirmed orders");
    }

    this.status = OrderStatus.Shipped;

    this.addEvent(new OrderShippedEvent(
      this.orderId,
      trackingNumber,
      new Date()
    ));
  }

  // Queries
  calculateTotal(): Money {
    return Array.from(this.items.values()).reduce(
      (total, item) => total.add(item.subtotal),
      new Money(0, "USD")
    );
  }

  getItems(): OrderItem[] {
    return Array.from(this.items.values());
  }

  getStatus(): OrderStatus {
    return this.status;
  }

  // Event management
  private addEvent(event: any): void {
    this.uncommittedEvents.push(event);
  }

  getUncommittedEvents(): any[] {
    return [...this.uncommittedEvents];
  }

  markEventsAsCommitted(): void {
    this.uncommittedEvents = [];
  }
}

enum OrderStatus {
  Draft = "Draft",
  Confirmed = "Confirmed",
  Shipped = "Shipped",
  Delivered = "Delivered",
  Cancelled = "Cancelled"
}

Command Handler

class CreateOrderCommand {
  constructor(
    readonly orderId: string,
    readonly customerId: string,
    readonly shippingAddress: Address
  ) {}
}

class AddItemToOrderCommand {
  constructor(
    readonly orderId: string,
    readonly product: ProductDetails,
    readonly price: Money,
    readonly quantity: number
  ) {}
}

class ConfirmOrderCommand {
  constructor(readonly orderId: string) {}
}

class OrderCommandHandler {
  constructor(
    private orderRepository: OrderRepository,
    private eventBus: EventBus
  ) {}

  async handleCreateOrder(command: CreateOrderCommand): Promise<void> {
    const order = new Order(
      command.orderId,
      command.customerId,
      command.shippingAddress
    );

    await this.orderRepository.save(order);
    await this.publishEvents(order);
  }

  async handleAddItem(command: AddItemToOrderCommand): Promise<void> {
    const order = await this.orderRepository.findById(command.orderId);
    if (!order) {
      throw new Error("Order not found");
    }

    order.addItem(command.product, command.price, command.quantity);

    await this.orderRepository.save(order);
    await this.publishEvents(order);
  }

  async handleConfirmOrder(command: ConfirmOrderCommand): Promise<void> {
    const order = await this.orderRepository.findById(command.orderId);
    if (!order) {
      throw new Error("Order not found");
    }

    order.confirm();

    await this.orderRepository.save(order);
    await this.publishEvents(order);
  }

  private async publishEvents(order: Order): Promise<void> {
    const events = order.getUncommittedEvents();
    await this.eventBus.publishAll(events);
    order.markEventsAsCommitted();
  }
}

Read Model and Projection

// Read model for order list view
interface OrderSummaryReadModel {
  orderId: string;
  customerId: string;
  customerName: string;
  total: number;
  currency: string;
  itemCount: number;
  status: string;
  createdAt: Date;
  confirmedAt?: Date;
}

// Read model for order details view
interface OrderDetailsReadModel {
  orderId: string;
  customerId: string;
  customerName: string;
  items: {
    productId: string;
    productName: string;
    quantity: number;
    unitPrice: number;
    subtotal: number;
  }[];
  shippingAddress: Address;
  total: number;
  currency: string;
  status: string;
  createdAt: Date;
  confirmedAt?: Date;
  shippedAt?: Date;
  trackingNumber?: string;
}

// Projection builder
class OrderProjection {
  constructor(private db: ReadDatabase) {}

  async handleEvent(event: any): Promise<void> {
    if (event instanceof OrderCreatedEvent) {
      await this.handleOrderCreated(event);
    } else if (event instanceof OrderItemAddedEvent) {
      await this.handleOrderItemAdded(event);
    } else if (event instanceof OrderConfirmedEvent) {
      await this.handleOrderConfirmed(event);
    } else if (event instanceof OrderShippedEvent) {
      await this.handleOrderShipped(event);
    }
  }

  private async handleOrderCreated(event: OrderCreatedEvent): Promise<void> {
    await this.db.orderSummaries.insert({
      orderId: event.orderId,
      customerId: event.customerId,
      total: 0,
      currency: "USD",
      itemCount: 0,
      status: "Draft",
      createdAt: event.createdAt
    });

    await this.db.orderDetails.insert({
      orderId: event.orderId,
      customerId: event.customerId,
      items: [],
      total: 0,
      currency: "USD",
      status: "Draft",
      createdAt: event.createdAt
    });
  }

  private async handleOrderItemAdded(event: OrderItemAddedEvent): Promise<void> {
    // Update summary
    await this.db.orderSummaries.update(
      { orderId: event.orderId },
      {
        $inc: {
          total: event.price.amount * event.quantity,
          itemCount: 1
        }
      }
    );

    // Update details
    await this.db.orderDetails.update(
      { orderId: event.orderId },
      {
        $push: {
          items: {
            productId: event.productId,
            quantity: event.quantity,
            unitPrice: event.price.amount,
            subtotal: event.price.amount * event.quantity
          }
        },
        $inc: {
          total: event.price.amount * event.quantity
        }
      }
    );
  }

  private async handleOrderConfirmed(event: OrderConfirmedEvent): Promise<void> {
    await this.db.orderSummaries.update(
      { orderId: event.orderId },
      { $set: { status: "Confirmed", confirmedAt: event.confirmedAt } }
    );

    await this.db.orderDetails.update(
      { orderId: event.orderId },
      { $set: { status: "Confirmed", confirmedAt: event.confirmedAt } }
    );
  }

  private async handleOrderShipped(event: OrderShippedEvent): Promise<void> {
    await this.db.orderSummaries.update(
      { orderId: event.orderId },
      { $set: { status: "Shipped" } }
    );

    await this.db.orderDetails.update(
      { orderId: event.orderId },
      {
        $set: {
          status: "Shipped",
          shippedAt: event.shippedAt,
          trackingNumber: event.trackingNumber
        }
      }
    );
  }
}

// Query service
class OrderQueryService {
  constructor(private db: ReadDatabase) {}

  async getOrderSummary(orderId: string): Promise<OrderSummaryReadModel | null> {
    return await this.db.orderSummaries.findOne({ orderId });
  }

  async getOrderDetails(orderId: string): Promise<OrderDetailsReadModel | null> {
    return await this.db.orderDetails.findOne({ orderId });
  }

  async getCustomerOrders(
    customerId: string,
    status?: string
  ): Promise<OrderSummaryReadModel[]> {
    const filter: any = { customerId };
    if (status) {
      filter.status = status;
    }
    return await this.db.orderSummaries.find(filter).sort({ createdAt: -1 });
  }

  async getRecentOrders(limit: number = 10): Promise<OrderSummaryReadModel[]> {
    return await this.db.orderSummaries
      .find({})
      .sort({ createdAt: -1 })
      .limit(limit);
  }
}

---

Example 2: Banking Transaction System with Event Sourcing

Implementing a banking system where all transactions are stored as events.

Domain Model

// Account aggregate with event sourcing
class BankAccount {
  private accountNumber: string;
  private accountHolder: string;
  private balance: Money = new Money(0, "USD");
  private status: AccountStatus = AccountStatus.Active;
  private version: number = 0;
  private uncommittedEvents: DomainEvent[] = [];

  constructor(accountNumber: string, accountHolder: string) {
    this.accountNumber = accountNumber;
    this.accountHolder = accountHolder;
  }

  // Commands
  open(initialDeposit: Money): void {
    if (this.version > 0) {
      throw new Error("Account already opened");
    }

    if (initialDeposit.amount < 0) {
      throw new Error("Initial deposit must be non-negative");
    }

    this.applyEvent(new AccountOpenedEvent(
      this.accountNumber,
      this.accountHolder,
      initialDeposit,
      new Date()
    ));
  }

  deposit(amount: Money, description: string): void {
    this.ensureAccountActive();

    if (amount.amount <= 0) {
      throw new Error("Deposit amount must be positive");
    }

    this.applyEvent(new MoneyDepositedEvent(
      this.accountNumber,
      amount,
      description,
      new Date()
    ));
  }

  withdraw(amount: Money, description: string): void {
    this.ensureAccountActive();

    if (amount.amount <= 0) {
      throw new Error("Withdrawal amount must be positive");
    }

    if (this.balance.amount < amount.amount) {
      throw new Error("Insufficient funds");
    }

    this.applyEvent(new MoneyWithdrawnEvent(
      this.accountNumber,
      amount,
      description,
      new Date()
    ));
  }

  close(): void {
    this.ensureAccountActive();

    if (this.balance.amount > 0) {
      throw new Error("Cannot close account with positive balance");
    }

    this.applyEvent(new AccountClosedEvent(
      this.accountNumber,
      new Date()
    ));
  }

  freeze(reason: string): void {
    this.ensureAccountActive();

    this.applyEvent(new AccountFrozenEvent(
      this.accountNumber,
      reason,
      new Date()
    ));
  }

  unfreeze(): void {
    if (this.status !== AccountStatus.Frozen) {
      throw new Error("Account is not frozen");
    }

    this.applyEvent(new AccountUnfrozenEvent(
      this.accountNumber,
      new Date()
    ));
  }

  // Event handlers
  private applyEvent(event: DomainEvent): void {
    this.apply(event);
    this.uncommittedEvents.push(event);
  }

  private apply(event: DomainEvent): void {
    if (event instanceof AccountOpenedEvent) {
      this.balance = event.initialDeposit;
      this.status = AccountStatus.Active;
    } else if (event instanceof MoneyDepositedEvent) {
      this.balance = this.balance.add(event.amount);
    } else if (event instanceof MoneyWithdrawnEvent) {
      this.balance = new Money(
        this.balance.amount - event.amount.amount,
        this.balance.currency
      );
    } else if (event instanceof AccountClosedEvent) {
      this.status = AccountStatus.Closed;
    } else if (event instanceof AccountFrozenEvent) {
      this.status = AccountStatus.Frozen;
    } else if (event instanceof AccountUnfrozenEvent) {
      this.status = AccountStatus.Active;
    }

    this.version++;
  }

  // Rebuild from events
  static fromEvents(events: DomainEvent[]): BankAccount {
    if (events.length === 0) {
      throw new Error("Cannot create account from empty event stream");
    }

    const firstEvent = events[0] as AccountOpenedEvent;
    const account = new BankAccount(
      firstEvent.accountNumber,
      firstEvent.accountHolder
    );

    events.forEach(event => account.apply(event));
    return account;
  }

  // Helpers
  private ensureAccountActive(): void {
    if (this.status !== AccountStatus.Active) {
      throw new Error(`Account is ${this.status}`);
    }
  }

  getUncommittedEvents(): DomainEvent[] {
    return [...this.uncommittedEvents];
  }

  markEventsAsCommitted(): void {
    this.uncommittedEvents = [];
  }

  getBalance(): Money {
    return this.balance;
  }

  getStatus(): AccountStatus {
    return this.status;
  }
}

enum AccountStatus {
  Active = "Active",
  Frozen = "Frozen",
  Closed = "Closed"
}

// Domain events
class AccountOpenedEvent {
  readonly eventType = "AccountOpened";

  constructor(
    readonly accountNumber: string,
    readonly accountHolder: string,
    readonly initialDeposit: Money,
    readonly timestamp: Date
  ) {}
}

class MoneyDepositedEvent {
  readonly eventType = "MoneyDeposited";

  constructor(
    readonly accountNumber: string,
    readonly amount: Money,
    readonly description: string,
    readonly timestamp: Date
  ) {}
}

class MoneyWithdrawnEvent {
  readonly eventType = "MoneyWithdrawn";

  constructor(
    readonly accountNumber: string,
    readonly amount: Money,
    readonly description: string,
    readonly timestamp: Date
  ) {}
}

class AccountClosedEvent {
  readonly eventType = "AccountClosed";

  constructor(
    readonly accountNumber: string,
    readonly timestamp: Date
  ) {}
}

class AccountFrozenEvent {
  readonly eventType = "AccountFrozen";

  constructor(
    readonly accountNumber: string,
    readonly reason: string,
    readonly timestamp: Date
  ) {}
}

class AccountUnfrozenEvent {
  readonly eventType = "AccountUnfrozen";

  constructor(
    readonly accountNumber: string,
    readonly timestamp: Date
  ) {}
}

type DomainEvent = AccountOpenedEvent | MoneyDepositedEvent |
                   MoneyWithdrawnEvent | AccountClosedEvent |
                   AccountFrozenEvent | AccountUnfrozenEvent;

Transaction Projection

// Read model for transaction history
interface TransactionReadModel {
  transactionId: string;
  accountNumber: string;
  type: "Deposit" | "Withdrawal";
  amount: number;
  currency: string;
  description: string;
  balanceAfter: number;
  timestamp: Date;
}

class TransactionProjection {
  constructor(private db: ReadDatabase) {}

  async handleEvent(event: DomainEvent): Promise<void> {
    if (event instanceof MoneyDepositedEvent) {
      await this.handleMoneyDeposited(event);
    } else if (event instanceof MoneyWithdrawnEvent) {
      await this.handleMoneyWithdrawn(event);
    }
  }

  private async handleMoneyDeposited(event: MoneyDepositedEvent): Promise<void> {
    // Get current balance
    const account = await this.db.accounts.findOne({
      accountNumber: event.accountNumber
    });

    const balanceAfter = (account?.balance || 0) + event.amount.amount;

    await this.db.transactions.insert({
      transactionId: generateId(),
      accountNumber: event.accountNumber,
      type: "Deposit",
      amount: event.amount.amount,
      currency: event.amount.currency,
      description: event.description,
      balanceAfter,
      timestamp: event.timestamp
    });

    // Update account balance
    await this.db.accounts.update(
      { accountNumber: event.accountNumber },
      { $set: { balance: balanceAfter, lastActivity: event.timestamp } }
    );
  }

  private async handleMoneyWithdrawn(event: MoneyWithdrawnEvent): Promise<void> {
    const account = await this.db.accounts.findOne({
      accountNumber: event.accountNumber
    });

    const balanceAfter = (account?.balance || 0) - event.amount.amount;

    await this.db.transactions.insert({
      transactionId: generateId(),
      accountNumber: event.accountNumber,
      type: "Withdrawal",
      amount: event.amount.amount,
      currency: event.amount.currency,
      description: event.description,
      balanceAfter,
      timestamp: event.timestamp
    });

    await this.db.accounts.update(
      { accountNumber: event.accountNumber },
      { $set: { balance: balanceAfter, lastActivity: event.timestamp } }
    );
  }
}

// Query service
class TransactionQueryService {
  constructor(private db: ReadDatabase) {}

  async getTransactionHistory(
    accountNumber: string,
    fromDate?: Date,
    toDate?: Date
  ): Promise<TransactionReadModel[]> {
    const filter: any = { accountNumber };

    if (fromDate || toDate) {
      filter.timestamp = {};
      if (fromDate) filter.timestamp.$gte = fromDate;
      if (toDate) filter.timestamp.$lte = toDate;
    }

    return await this.db.transactions
      .find(filter)
      .sort({ timestamp: -1 });
  }

  async getAccountStatement(
    accountNumber: string,
    month: number,
    year: number
  ): Promise<{
    openingBalance: number;
    closingBalance: number;
    transactions: TransactionReadModel[];
  }> {
    const startDate = new Date(year, month - 1, 1);
    const endDate = new Date(year, month, 0, 23, 59, 59);

    const transactions = await this.getTransactionHistory(
      accountNumber,
      startDate,
      endDate
    );

    const openingBalance = transactions.length > 0
      ? transactions[transactions.length - 1].balanceAfter -
        transactions.reduce((sum, t) =>
          sum + (t.type === "Deposit" ? t.amount : -t.amount), 0)
      : 0;

    const closingBalance = transactions.length > 0
      ? transactions[0].balanceAfter
      : openingBalance;

    return {
      openingBalance,
      closingBalance,
      transactions
    };
  }
}

---

Example 3: Multi-Tenant SaaS Application

Implementing tenant isolation and resource management in a SaaS platform.

Tenant Management

// Tenant aggregate
class Tenant {
  constructor(
    readonly tenantId: string,
    readonly name: string,
    private plan: SubscriptionPlan,
    private status: TenantStatus = TenantStatus.Active
  ) {}

  // Resource limits based on plan
  getResourceLimits(): ResourceLimits {
    return {
      maxUsers: this.plan.maxUsers,
      maxStorageGB: this.plan.maxStorageGB,
      maxApiCallsPerDay: this.plan.maxApiCallsPerDay,
      features: this.plan.features
    };
  }

  upgradePlan(newPlan: SubscriptionPlan): void {
    if (newPlan.tier <= this.plan.tier) {
      throw new Error("Cannot downgrade using upgrade method");
    }
    this.plan = newPlan;
  }

  suspend(reason: string): void {
    if (this.status === TenantStatus.Suspended) {
      throw new Error("Tenant already suspended");
    }
    this.status = TenantStatus.Suspended;
  }

  activate(): void {
    if (this.status === TenantStatus.Active) {
      throw new Error("Tenant already active");
    }
    this.status = TenantStatus.Active;
  }

  isActive(): boolean {
    return this.status === TenantStatus.Active;
  }
}

enum TenantStatus {
  Active = "Active",
  Suspended = "Suspended",
  Cancelled = "Cancelled"
}

interface SubscriptionPlan {
  name: string;
  tier: number;
  maxUsers: number;
  maxStorageGB: number;
  maxApiCallsPerDay: number;
  features: string[];
}

interface ResourceLimits {
  maxUsers: number;
  maxStorageGB: number;
  maxApiCallsPerDay: number;
  features: string[];
}

// Tenant context middleware
class TenantContextMiddleware {
  async extractTenant(
    req: Request,
    res: Response,
    next: NextFunction
  ): Promise<void> {
    // Extract tenant from subdomain or header
    const tenantId = this.getTenantId(req);

    if (!tenantId) {
      res.status(400).json({ error: "Tenant ID required" });
      return;
    }

    // Load tenant
    const tenant = await tenantRepository.findById(tenantId);

    if (!tenant) {
      res.status(404).json({ error: "Tenant not found" });
      return;
    }

    if (!tenant.isActive()) {
      res.status(403).json({ error: "Tenant suspended" });
      return;
    }

    // Attach to request
    req.tenant = tenant;
    next();
  }

  private getTenantId(req: Request): string | null {
    // From subdomain: tenant1.app.com
    const subdomain = req.hostname.split('.')[0];
    if (subdomain && subdomain !== 'www') {
      return subdomain;
    }

    // From header: X-Tenant-ID
    return req.headers['x-tenant-id'] as string || null;
  }
}

// Database isolation strategies
class DatabaseIsolationStrategy {
  // Strategy 1: Separate database per tenant
  async getDatabaseConnection(tenantId: string): Promise<Database> {
    const connectionString = `mongodb://localhost/${tenantId}`;
    return await createConnection(connectionString);
  }

  // Strategy 2: Shared database with tenant column
  async query(tenantId: string, collection: string, filter: any): Promise<any[]> {
    return await db.collection(collection).find({
      ...filter,
      tenantId
    });
  }

  // Strategy 3: Separate schema per tenant (PostgreSQL)
  async executeQuery(tenantId: string, sql: string): Promise<any> {
    return await db.query(`
      SET search_path TO tenant_${tenantId};
      ${sql}
    `);
  }
}

Resource Quota Enforcement

class ResourceQuotaService {
  constructor(
    private tenantRepository: TenantRepository,
    private usageRepository: UsageRepository
  ) {}

  async checkQuota(
    tenantId: string,
    resourceType: ResourceType
  ): Promise<boolean> {
    const tenant = await this.tenantRepository.findById(tenantId);
    const limits = tenant.getResourceLimits();
    const usage = await this.usageRepository.getUsage(tenantId);

    switch (resourceType) {
      case ResourceType.Users:
        return usage.userCount < limits.maxUsers;

      case ResourceType.Storage:
        return usage.storageGB < limits.maxStorageGB;

      case ResourceType.ApiCalls:
        return usage.apiCallsToday < limits.maxApiCallsPerDay;

      default:
        return false;
    }
  }

  async recordUsage(
    tenantId: string,
    resourceType: ResourceType,
    amount: number
  ): Promise<void> {
    await this.usageRepository.increment(tenantId, resourceType, amount);
  }

  async getUsageReport(tenantId: string): Promise<UsageReport> {
    const tenant = await this.tenantRepository.findById(tenantId);
    const usage = await this.usageRepository.getUsage(tenantId);
    const limits = tenant.getResourceLimits();

    return {
      tenantId,
      plan: tenant.plan.name,
      usage: {
        users: {
          current: usage.userCount,
          limit: limits.maxUsers,
          percentage: (usage.userCount / limits.maxUsers) * 100
        },
        storage: {
          current: usage.storageGB,
          limit: limits.maxStorageGB,
          percentage: (usage.storageGB / limits.maxStorageGB) * 100
        },
        apiCalls: {
          today: usage.apiCallsToday,
          limit: limits.maxApiCallsPerDay,
          percentage: (usage.apiCallsToday / limits.maxApiCallsPerDay) * 100
        }
      }
    };
  }
}

enum ResourceType {
  Users = "Users",
  Storage = "Storage",
  ApiCalls = "ApiCalls"
}

interface UsageReport {
  tenantId: string;
  plan: string;
  usage: {
    users: { current: number; limit: number; percentage: number };
    storage: { current: number; limit: number; percentage: number };
    apiCalls: { today: number; limit: number; percentage: number };
  };
}

---

Example 4: Microservices Communication Patterns

Various patterns for inter-service communication.

Synchronous Communication with Circuit Breaker

class ProductService {
  private circuitBreaker: CircuitBreaker;

  constructor() {
    this.circuitBreaker = new CircuitBreaker({
      failureThreshold: 5,
      successThreshold: 2,
      timeout: 60000
    });
  }

  async getProductDetails(productId: string): Promise<Product> {
    return await this.circuitBreaker.execute(async () => {
      const response = await fetch(
        `http://product-service/api/products/${productId}`,
        {
          timeout: 5000,
          headers: {
            'X-Request-ID': generateRequestId(),
            'Authorization': `Bearer ${this.getAuthToken()}`
          }
        }
      );

      if (!response.ok) {
        throw new Error(`HTTP ${response.status}`);
      }

      return await response.json();
    });
  }
}

Asynchronous Communication with Message Queue

class OrderEventPublisher {
  constructor(private messageQueue: MessageQueue) {}

  async publishOrderPlaced(order: Order): Promise<void> {
    const event = {
      eventId: generateId(),
      eventType: "OrderPlaced",
      timestamp: new Date(),
      data: {
        orderId: order.id,
        customerId: order.customerId,
        items: order.items.map(item => ({
          productId: item.productId,
          quantity: item.quantity,
          price: item.unitPrice
        })),
        total: order.total
      }
    };

    await this.messageQueue.publish('order-events', event, {
      persistent: true,
      priority: 1
    });
  }
}

class InventoryEventHandler {
  constructor(
    private messageQueue: MessageQueue,
    private inventoryService: InventoryService
  ) {
    this.subscribe();
  }

  private async subscribe(): Promise<void> {
    await this.messageQueue.subscribe('order-events', async (message) => {
      if (message.eventType === "OrderPlaced") {
        await this.handleOrderPlaced(message.data);
      }
    }, {
      prefetchCount: 10,
      autoAck: false
    });
  }

  private async handleOrderPlaced(data: any): Promise<void> {
    try {
      for (const item of data.items) {
        await this.inventoryService.reserveStock(
          item.productId,
          item.quantity,
          data.orderId
        );
      }
      // Acknowledge message
      await this.messageQueue.ack(message);
    } catch (error) {
      // Reject and requeue
      await this.messageQueue.nack(message, { requeue: true });
    }
  }
}

Service-to-Service Communication with Retry

class ResilientHttpClient {
  private retryPolicy: RetryPolicy;
  private timeoutPolicy: TimeoutPolicy;

  constructor() {
    this.retryPolicy = new RetryPolicy({
      maxRetries: 3,
      initialDelayMs: 100,
      maxDelayMs: 5000,
      backoffMultiplier: 2
    });

    this.timeoutPolicy = new TimeoutPolicy(10000);
  }

  async get<T>(url: string, options?: RequestOptions): Promise<T> {
    return await this.retryPolicy.execute(
      async () => {
        return await this.timeoutPolicy.execute(async () => {
          const response = await fetch(url, {
            method: 'GET',
            headers: options?.headers,
            signal: AbortSignal.timeout(10000)
          });

          if (!response.ok) {
            throw new HttpError(response.status, response.statusText);
          }

          return await response.json();
        });
      },
      (error) => {
        // Retry on network errors and 5xx status codes
        return error instanceof NetworkError ||
               (error instanceof HttpError && error.status >= 500);
      }
    );
  }
}

---

Example 5: Distributed Payment Processing with Saga

Coordinating payment across multiple services using saga pattern.

Orchestration-Based Saga

class PaymentSaga {
  private sagaId: string;
  private status: SagaStatus = SagaStatus.Started;
  private completedSteps: string[] = [];

  constructor(
    private paymentId: string,
    private orderId: string,
    private amount: Money,
    private customerId: string
  ) {
    this.sagaId = generateId();
  }

  async execute(): Promise<void> {
    const steps: SagaStep[] = [
      {
        name: "ValidatePaymentMethod",
        action: () => this.validatePaymentMethod(),
        compensation: () => Promise.resolve()
      },
      {
        name: "ReserveAmount",
        action: () => this.reserveAmount(),
        compensation: () => this.releaseReservation()
      },
      {
        name: "ProcessPayment",
        action: () => this.processPayment(),
        compensation: () => this.refundPayment()
      },
      {
        name: "UpdateOrderStatus",
        action: () => this.updateOrderStatus(),
        compensation: () => this.revertOrderStatus()
      },
      {
        name: "SendConfirmation",
        action: () => this.sendConfirmation(),
        compensation: () => Promise.resolve()
      }
    ];

    try {
      for (const step of steps) {
        console.log(`Executing: ${step.name}`);
        await step.action();
        this.completedSteps.push(step.name);
      }

      this.status = SagaStatus.Completed;
      await this.publishSuccess();

    } catch (error) {
      console.error(`Saga failed:`, error);
      await this.compensate(steps);
      this.status = SagaStatus.Failed;
      await this.publishFailure(error);
    }
  }

  private async compensate(steps: SagaStep[]): Promise<void> {
    console.log("Starting compensation");

    for (let i = this.completedSteps.length - 1; i >= 0; i--) {
      const stepName = this.completedSteps[i];
      const step = steps.find(s => s.name === stepName);

      if (step) {
        try {
          console.log(`Compensating: ${step.name}`);
          await step.compensation();
        } catch (error) {
          console.error(`Compensation failed for ${step.name}:`, error);
          // Log for manual intervention
          await this.logCompensationFailure(step.name, error);
        }
      }
    }
  }

  private async validatePaymentMethod(): Promise<void> {
    const paymentMethod = await paymentMethodService.get(this.customerId);

    if (!paymentMethod) {
      throw new Error("No payment method found");
    }

    if (paymentMethod.isExpired()) {
      throw new Error("Payment method expired");
    }
  }

  private async reserveAmount(): Promise<void> {
    await paymentGateway.reserve({
      customerId: this.customerId,
      amount: this.amount,
      reference: this.paymentId
    });
  }

  private async releaseReservation(): Promise<void> {
    await paymentGateway.releaseReservation(this.paymentId);
  }

  private async processPayment(): Promise<void> {
    await paymentGateway.capture({
      paymentId: this.paymentId,
      amount: this.amount
    });
  }

  private async refundPayment(): Promise<void> {
    await paymentGateway.refund({
      paymentId: this.paymentId,
      amount: this.amount
    });
  }

  private async updateOrderStatus(): Promise<void> {
    await orderService.markAsPaid(this.orderId, this.paymentId);
  }

  private async revertOrderStatus(): Promise<void> {
    await orderService.markAsPaymentFailed(this.orderId);
  }

  private async sendConfirmation(): Promise<void> {
    await notificationService.sendPaymentConfirmation(
      this.customerId,
      this.orderId,
      this.amount
    );
  }

  private async publishSuccess(): Promise<void> {
    await eventBus.publish(new PaymentCompletedEvent(
      this.paymentId,
      this.orderId,
      this.amount,
      new Date()
    ));
  }

  private async publishFailure(error: any): Promise<void> {
    await eventBus.publish(new PaymentFailedEvent(
      this.paymentId,
      this.orderId,
      error.message,
      new Date()
    ));
  }

  private async logCompensationFailure(
    stepName: string,
    error: any
  ): Promise<void> {
    await compensationFailureLog.create({
      sagaId: this.sagaId,
      paymentId: this.paymentId,
      stepName,
      error: error.message,
      timestamp: new Date()
    });
  }
}

interface SagaStep {
  name: string;
  action: () => Promise<void>;
  compensation: () => Promise<void>;
}

enum SagaStatus {
  Started = "Started",
  Completed = "Completed",
  Failed = "Failed"
}

---

Example 6: Real-Time Analytics Platform

Event streaming and real-time aggregation architecture.

Stream Processing

class EventStreamProcessor {
  constructor(
    private kafkaConsumer: KafkaConsumer,
    private aggregationStore: AggregationStore
  ) {}

  async start(): Promise<void> {
    await this.kafkaConsumer.subscribe(['user-events'], async (message) => {
      await this.processEvent(message);
    });
  }

  private async processEvent(message: any): Promise<void> {
    const event = JSON.parse(message.value);

    switch (event.eventType) {
      case 'PageView':
        await this.handlePageView(event);
        break;
      case 'ButtonClick':
        await this.handleButtonClick(event);
        break;
      case 'Purchase':
        await this.handlePurchase(event);
        break;
    }
  }

  private async handlePageView(event: any): Promise<void> {
    const minuteKey = this.getMinuteKey(event.timestamp);

    await this.aggregationStore.increment(
      `pageviews:${minuteKey}`,
      1
    );

    await this.aggregationStore.increment(
      `pageviews:${event.page}:${minuteKey}`,
      1
    );

    // Update real-time dashboard
    await this.updateDashboard('pageviews', {
      total: await this.aggregationStore.get(`pageviews:${minuteKey}`),
      byPage: await this.getPageBreakdown(minuteKey)
    });
  }

  private async handlePurchase(event: any): Promise<void> {
    const minuteKey = this.getMinuteKey(event.timestamp);

    await this.aggregationStore.increment(
      `revenue:${minuteKey}`,
      event.amount
    );

    await this.aggregationStore.increment(
      `orders:${minuteKey}`,
      1
    );

    // Calculate running average
    const totalRevenue = await this.aggregationStore.get(`revenue:${minuteKey}`);
    const totalOrders = await this.aggregationStore.get(`orders:${minuteKey}`);
    const averageOrderValue = totalRevenue / totalOrders;

    await this.updateDashboard('sales', {
      revenue: totalRevenue,
      orders: totalOrders,
      averageOrderValue
    });
  }

  private getMinuteKey(timestamp: Date): string {
    const date = new Date(timestamp);
    date.setSeconds(0, 0);
    return date.toISOString();
  }

  private async updateDashboard(metric: string, data: any): Promise<void> {
    await websocketServer.broadcast({
      metric,
      data,
      timestamp: new Date()
    });
  }
}

Time-Series Aggregation

class TimeSeriesAggregator {
  async aggregateHourly(
    metric: string,
    date: Date
  ): Promise<HourlyAggregation[]> {
    const results: HourlyAggregation[] = [];

    for (let hour = 0; hour < 24; hour++) {
      const hourStart = new Date(date);
      hourStart.setHours(hour, 0, 0, 0);

      const hourEnd = new Date(hourStart);
      hourEnd.setHours(hour + 1);

      const value = await this.aggregationStore.sum(
        metric,
        hourStart,
        hourEnd
      );

      results.push({
        hour,
        value,
        timestamp: hourStart
      });
    }

    return results;
  }

  async aggregateDaily(
    metric: string,
    startDate: Date,
    endDate: Date
  ): Promise<DailyAggregation[]> {
    const results: DailyAggregation[] = [];
    const currentDate = new Date(startDate);

    while (currentDate <= endDate) {
      const dayStart = new Date(currentDate);
      dayStart.setHours(0, 0, 0, 0);

      const dayEnd = new Date(dayStart);
      dayEnd.setDate(dayEnd.getDate() + 1);

      const value = await this.aggregationStore.sum(
        metric,
        dayStart,
        dayEnd
      );

      results.push({
        date: new Date(dayStart),
        value
      });

      currentDate.setDate(currentDate.getDate() + 1);
    }

    return results;
  }
}

interface HourlyAggregation {
  hour: number;
  value: number;
  timestamp: Date;
}

interface DailyAggregation {
  date: Date;
  value: number;
}

---

[Continue with 16 more examples covering the remaining topics...]

Due to length constraints, I've provided 6 comprehensive examples. The complete EXAMPLES.md would continue with:

7. Content Management System with CQRS 8. Hotel Booking System with Saga 9. Social Media Feed Architecture 10. IoT Device Management Platform 11. Healthcare Patient Records System 12. Supply Chain Management 13. Video Streaming Platform 14. Financial Trading System 15. Customer Support Ticketing System 16. Inventory Management with Eventual Consistency 17. Multi-Region Deployment Architecture 18. API Rate Limiting and Throttling 19. Event-Driven Notification System 20. Serverless Microservices Architecture 21. GraphQL API Gateway Pattern 22. Zero-Downtime Deployment Strategy

Each example would follow the same comprehensive pattern with architecture diagrams, code implementations, and real-world scenarios.

---

Summary

These examples demonstrate:

• Domain-Driven Design: Rich domain models with business logic
• Event Sourcing: Complete audit trail and temporal queries
• CQRS: Optimized read and write models
• Saga Pattern: Distributed transaction coordination
• Resilience: Circuit breakers, retries, timeouts
• Scalability: Horizontal scaling, caching, sharding
• Multi-tenancy: Resource isolation and quota management
• Real-time Processing: Stream processing and aggregation

Each pattern solves specific architectural challenges while maintaining code quality, testability, and maintainability.

Enterprise Architecture Patterns

Quick reference guide for enterprise architecture patterns, distributed systems design, and scalable application development.

Overview

This skill provides comprehensive coverage of modern enterprise architecture patterns including:

• Domain-Driven Design (DDD) - Strategic and tactical patterns
• Event Sourcing & CQRS - Event-driven architecture patterns
• Saga Patterns - Distributed transaction management
• API Gateway & Service Mesh - Service communication patterns
• Resilience Patterns - Building fault-tolerant systems
• Scalability Patterns - Horizontal and vertical scaling strategies

Quick Start

When to Use This Skill

Use enterprise architecture patterns when:

• Building microservices architectures
• Designing complex business domains
• Implementing event-driven systems
• Managing distributed transactions
• Scaling applications for high traffic
• Building resilient, fault-tolerant systems
• Migrating monoliths to microservices

Core Pattern Categories

1. Strategic DDD: Bounded contexts, context mapping, ubiquitous language 2. Tactical DDD: Entities, value objects, aggregates, repositories, domain events 3. Event Sourcing: Event stores, event streams, projections, snapshots 4. CQRS: Command/query separation, read/write models, eventual consistency 5. Sagas: Orchestration, choreography, compensation 6. API Patterns: API Gateway, BFF, service mesh 7. Resilience: Circuit breaker, retry, bulkhead, timeout, fallback 8. Scalability: Horizontal scaling, caching, sharding, load balancing

Pattern Catalog

Domain-Driven Design Patterns

Strategic Patterns

Tactical Patterns

Event Sourcing & CQRS

Saga Patterns

Service Communication

Resilience Patterns

Scalability Patterns

Common Scenarios

Scenario 1: Building a New Microservices Application

Patterns to Apply: 1. Start with Bounded Contexts (DDD) 2. Define Aggregates and Entities (DDD) 3. Implement API Gateway for client access 4. Add Circuit Breakers for resilience 5. Use Event-Driven communication between services 6. Implement CQRS for read-heavy operations 7. Add Caching for frequently accessed data

Scenario 2: Migrating Monolith to Microservices

Patterns to Apply: 1. Strangler Fig Pattern for gradual migration 2. Anti-Corruption Layer to protect from legacy 3. BFF Pattern for client compatibility 4. Event Sourcing for audit trail 5. Saga Pattern for distributed transactions 6. Service Mesh for observability

Scenario 3: Handling Distributed Transactions

Patterns to Apply: 1. Saga Pattern (Orchestration or Choreography) 2. Event Sourcing for state management 3. Eventual Consistency acceptance 4. Compensation for rollback 5. Idempotent operations 6. Circuit Breaker for failure handling

Scenario 4: Scaling High-Traffic Application

Patterns to Apply: 1. Horizontal Scaling with auto-scaling 2. Load Balancing (Round Robin, Least Connections) 3. Caching (Cache-Aside, Write-Through) 4. Database Sharding for data distribution 5. Read Replicas for read scalability 6. CDN for static assets 7. Async Processing for slow operations

Scenario 5: Building Event-Driven Architecture

Patterns to Apply: 1. Domain Events for business occurrences 2. Event Sourcing for state persistence 3. CQRS for read/write separation 4. Event Bus for event distribution 5. Projections for read models 6. Saga Pattern for long-running processes

Pattern Selection Guide

Choose DDD When:

• Complex business domain with rich behavior
• Need to align software with business language
• Multiple teams working on different areas
• Domain experts available for collaboration
• Long-lived application expected to evolve

Choose Event Sourcing When:

• Need complete audit trail
• Temporal queries required (state at any point in time)
• Event-driven architecture
• High write throughput
• Complex event processing needed

Choose CQRS When:

• Very different read and write patterns
• Read scalability requirements
• Different consistency models for read/write
• Multiple read models needed
• Event sourcing already in use

Choose Saga Pattern When:

• Distributed transactions across services
• Long-running business processes
• Need compensation capability
• Microservices architecture
• Eventual consistency acceptable

Choose API Gateway When:

• Multiple client types (web, mobile, IoT)
• Need centralized authentication/authorization
• Rate limiting required
• Request aggregation needed
• Protocol translation necessary

Choose Service Mesh When:

• Many microservices (10+)
• Need observability across services
• Complex traffic management
• Service-to-service security required
• Kubernetes environment

Architecture Decision Framework

Questions to Ask

Domain Complexity:

• How complex is the business domain?
• Do we need rich domain models?
• Is there a ubiquitous language?

Scalability:

• What are the scalability requirements?
• Read-heavy or write-heavy?
• Global or regional distribution?

Consistency:

• Can we accept eventual consistency?
• What are the consistency boundaries?
• Are there strong consistency requirements?

Resilience:

• What is the acceptable downtime?
• What are the failure scenarios?
• Do we need automatic recovery?

Team Structure:

• How many teams?
• Team size and expertise?
• Geographic distribution?

Operational Maturity:

• DevOps capabilities?
• Monitoring and observability?
• Deployment automation?

Anti-Patterns to Avoid

Architecture Anti-Patterns

1. Distributed Monolith: Microservices that are tightly coupled

• Solution: Define clear boundaries, use async communication

2. Anemic Domain Model: Entities with no behavior, only data

• Solution: Put business logic in domain objects

3. Shared Database: Multiple services sharing one database

• Solution: Database per service, event-driven integration

4. Chatty Services: Too many synchronous calls between services

• Solution: Aggregate data, use async events, implement BFF

5. God Service: One service doing too much

• Solution: Split by business capability, follow SRP

6. No API Versioning: Breaking changes without versioning

• Solution: Version APIs from start, support multiple versions

Implementation Anti-Patterns

1. Large Aggregates: Aggregates with too many entities

• Solution: Keep aggregates small, use references

2. Event Coupling: Events containing too much information

• Solution: Minimal event data, separate queries for details

3. Sync Over Async: Using sync calls when async is better

• Solution: Default to async, use sync only when necessary

4. Premature Optimization: Optimizing before measuring

• Solution: Profile first, optimize bottlenecks

5. Over-Engineering: Adding patterns without need

• Solution: Start simple, add complexity when justified

Implementation Checklist

Starting a New Service

• [ ] Define bounded context and ubiquitous language
• [ ] Identify aggregates and their boundaries
• [ ] Design entities and value objects
• [ ] Define domain events
• [ ] Create repository interfaces
• [ ] Implement API contract (OpenAPI)
• [ ] Add authentication and authorization
• [ ] Implement circuit breaker for external calls
• [ ] Add health check endpoints
• [ ] Configure logging and metrics
• [ ] Set up distributed tracing
• [ ] Write unit and integration tests
• [ ] Document API and architecture decisions
• [ ] Set up CI/CD pipeline
• [ ] Configure monitoring and alerting

Adding Event Sourcing

• [ ] Design event schema
• [ ] Implement event store
• [ ] Create event handlers
• [ ] Build projections for read models
• [ ] Add snapshot mechanism
• [ ] Implement event versioning
• [ ] Add event replay capability
• [ ] Monitor event processing lag
• [ ] Test event ordering
• [ ] Plan for event migration

Implementing CQRS

• [ ] Separate command and query models
• [ ] Design command handlers
• [ ] Create read models
• [ ] Implement projections
• [ ] Set up event bus
• [ ] Add eventual consistency handling
• [ ] Monitor projection lag
• [ ] Implement query optimization
• [ ] Add cache for read models
• [ ] Test consistency scenarios

Adding Saga Pattern

• [ ] Identify saga participants
• [ ] Define saga steps
• [ ] Design compensation logic
• [ ] Choose orchestration or choreography
• [ ] Implement saga state persistence
• [ ] Add timeout handling
• [ ] Implement retry logic
• [ ] Monitor saga execution
• [ ] Test compensation scenarios
• [ ] Handle partial failures

Metrics to Track

Performance Metrics

• Request latency (p50, p95, p99)
• Throughput (requests per second)
• Error rate
• Success rate
• Time to first byte

Availability Metrics

• Uptime percentage
• Mean time between failures (MTBF)
• Mean time to recovery (MTTR)
• Service level indicators (SLIs)
• Service level objectives (SLOs)

Scalability Metrics

• CPU utilization
• Memory usage
• Database connections
• Queue depth
• Cache hit rate

Business Metrics

• Event processing lag
• Saga completion rate
• Projection freshness
• Command processing time
• Query response time

Resources

Official Documentation

• Domain-Driven Design: https://www.domainlanguage.com
• Microservices.io: https://microservices.io
• Martin Fowler: https://martinfowler.com
• Microsoft Architecture: https://learn.microsoft.com/azure/architecture
• AWS Architecture: https://aws.amazon.com/architecture

Tools & Frameworks

• Event Store: EventStoreDB, Axon Framework
• CQRS: MediatR, Axon Framework
• API Gateway: Kong, AWS API Gateway, Azure APIM
• Service Mesh: Istio, Linkerd, Consul
• Circuit Breaker: Resilience4j, Polly, Hystrix

Community

• DDD Community: https://www.dddhub.com
• CQRS/Event Sourcing: https://cqrs.nu
• Microservices Practitioners: Various meetups and conferences

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For detailed examples and implementation code, see EXAMPLES.md