🔲 Microservices Service Boundaries

Technical Documentation for Principal Engineers

1. Overview and Problem Statement 🎯

Definition

Service boundaries in microservices architecture define the scope, responsibilities, and interactions of individual services within a distributed system. They establish clear delineations between different business capabilities and determine how services communicate and share data.

Problems Solved

• Unclear service responsibilities
• Tight coupling between services
• Data redundancy and inconsistency
• Team ownership conflicts
• Scaling bottlenecks
• Complex dependencies

Business Value

• Clear ownership and accountability
• Improved maintainability
• Better scalability
• Faster feature delivery
• Reduced technical debt
• Enhanced team autonomy

2. Detailed Solution/Architecture 🏗️

Core Concepts

2.1 Domain-Driven Design (DDD) Concepts

• Bounded Contexts
• Aggregates
• Entities
• Value Objects
• Domain Events

2.2 Service Boundary Patterns

Key Components

1. Business Capability Boundaries

   • Focused functionality
   • Clear domain scope
   • Independent data ownership
   • Team alignment

2. Communication Boundaries

   • API contracts
   • Event boundaries
   • Data sharing policies
   • Integration patterns

3. Technical Implementation 💻

3.1 Domain Model Implementation

// Order Bounded Context
@Aggregate
public class Order {
    @AggregateIdentifier
    private OrderId id;
    private CustomerId customerId;
    private Money totalAmount;
    private OrderStatus status;
    private List<OrderLine> orderLines;

    public Order(CreateOrderCommand cmd) {
        validateOrder(cmd);
        apply(new OrderCreatedEvent(cmd.getOrderId(), cmd.getCustomerId()));
    }

    public void addOrderLine(AddOrderLineCommand cmd) {
        validateOrderLine(cmd);
        apply(new OrderLineAddedEvent(id, cmd.getProductId(), cmd.getQuantity()));
    }

    @EventSourcingHandler
    public void on(OrderCreatedEvent event) {
        this.id = event.getOrderId();
        this.customerId = event.getCustomerId();
        this.status = OrderStatus.CREATED;
    }
}

3.2 Service API Design

// Order Service API
interface OrderService {
  // Commands
  createOrder(order: CreateOrderDto): Promise<OrderId>;
  updateOrder(orderId: OrderId, update: UpdateOrderDto): Promise<void>;
  cancelOrder(orderId: OrderId): Promise<void>;

  // Queries
  getOrder(orderId: OrderId): Promise<OrderDto>;
  getOrdersByCustomer(customerId: CustomerId): Promise<OrderDto[]>;
  
  // Events
  subscribeToOrderEvents(): Observable<OrderEvent>;
}

// Clear boundary with other services
interface OrderDto {
  orderId: string;
  customerId: string;
  status: OrderStatus;
  totalAmount: Money;
  lines: OrderLineDto[];
}

3.3 Event-Driven Integration

@Service
public class OrderProcessingService {
    private final EventPublisher eventPublisher;
    private final OrderRepository orderRepository;

    @Transactional
    public void processOrder(ProcessOrderCommand cmd) {
        Order order = orderRepository.findById(cmd.getOrderId())
            .orElseThrow(() -> new OrderNotFoundException(cmd.getOrderId()));
            
        // Process within boundary
        order.process();
        orderRepository.save(order);
        
        // Communicate across boundary via events
        eventPublisher.publish(new OrderProcessedEvent(
            order.getId(),
            order.getStatus(),
            order.getTotalAmount()
        ));
    }
}

4. Decision Criteria & Evaluation 📊

Service Boundary Decision Matrix

Criterion	Single Service	Multiple Services
Business Capability	Single, cohesive	Multiple, distinct
Data Ownership	Single model	Separate models
Team Structure	One team	Multiple teams
Change Frequency	Similar pace	Different paces
Performance	Unified	Independent

Boundary Identification Checklist

1. Business capability alignment
2. Data ownership clarity
3. Team organization
4. Change patterns
5. Scalability requirements

5. Anti-Patterns ⚠️

5.1 Shared Domain Models

❌ Wrong Implementation:

// Shared domain model across services
public class Customer {
    private CustomerId id;
    private String name;
    private Address address;
    private PaymentInfo paymentInfo;
    private List<Order> orders;
}

✅ Correct Implementation:

// Order service's view of Customer
public class CustomerReference {
    private CustomerId id;
    private String name;
    // Only necessary customer information for Orders
}

// Order aggregate with proper boundaries
public class Order {
    private OrderId id;
    private CustomerReference customer;
    private List<OrderLine> lines;
}

5.2 Improper Service Communication

❌ Wrong:

@Service
public class OrderService {
    private final CustomerService customerService;
    private final PaymentService paymentService;
    
    public Order createOrder(CreateOrderRequest request) {
        // Direct synchronous calls crossing boundaries
        Customer customer = customerService.getCustomer(request.getCustomerId());
        Payment payment = paymentService.processPayment(request.getPaymentDetails());
        return createOrderWithDetails(customer, payment);
    }
}

✅ Correct:

@Service
public class OrderService {
    private final EventPublisher eventPublisher;
    
    public Order createOrder(CreateOrderRequest request) {
        // Create order within boundary
        Order order = new Order(request);
        
        // Publish event for other bounded contexts
        eventPublisher.publish(new OrderCreatedEvent(order.getId(), request));
        
        return order;
    }
}

6. Best Practices & Guidelines 📚

6.1 Service Boundary Design Principles

1. Single Responsibility

// Good: Focused service boundary
@Service
public class OrderManagementService {
    public Order createOrder(CreateOrderCommand cmd) { /* ... */ }
    public void updateOrder(UpdateOrderCommand cmd) { /* ... */ }
    public void cancelOrder(CancelOrderCommand cmd) { /* ... */ }
}

// Bad: Mixed responsibilities
@Service
public class OrderService {
    public Order createOrder(CreateOrderCommand cmd) { /* ... */ }
    public void processPayment(PaymentDetails details) { /* ... */ }
    public void updateInventory(InventoryUpdate update) { /* ... */ }
}

2. API Design

// Clear API contract at boundary
interface OrderApi {
    // Commands
    createOrder(order: CreateOrderRequest): Promise<OrderId>;
    
    // Queries
    getOrder(orderId: OrderId): Promise<OrderDetails>;
    
    // Events
    orderEvents(): Observable<OrderEvent>;
}

6.2 Data Consistency Patterns

@Service
public class OrderBoundaryService {
    @Transactional
    public void processOrder(OrderId orderId) {
        // Handle consistency within boundary
        Order order = repository.findById(orderId)
            .orElseThrow(() -> new OrderNotFoundException(orderId));
            
        order.process();
        repository.save(order);
        
        // Use eventual consistency across boundaries
        eventPublisher.publish(new OrderProcessedEvent(order));
    }
}

7. Testing Strategies 🧪

7.1 Boundary Testing

@SpringBootTest
class OrderBoundaryTests {
    @Autowired
    private OrderService orderService;
    
    @Test
    void whenCreateOrder_thenStaysWithinBoundary() {
        // Given
        CreateOrderCommand cmd = new CreateOrderCommand(/*...*/);
        
        // When
        OrderId orderId = orderService.createOrder(cmd);
        
        // Then
        Order order = orderService.getOrder(orderId);
        assertThat(order).isNotNull();
        verify(eventPublisher).publish(any(OrderCreatedEvent.class));
        // Verify no direct calls to other services
        verifyNoInteractions(customerService, paymentService);
    }
}

8. Real-world Use Cases 🌍

E-commerce Domain Boundaries

1. Order Management Bounded Context

public class Order {
    private OrderId id;
    private CustomerId customerId;
    private Money totalAmount;
    private OrderStatus status;
    private List<OrderLine> lines;
}

2. Inventory Bounded Context

public class InventoryItem {
    private ProductId productId;
    private int quantityAvailable;
    private WarehouseId warehouseId;
}

3. Customer Bounded Context

public class Customer {
    private CustomerId id;
    private String name;
    private ContactInfo contactInfo;
    private List<Address> addresses;
}

9. References and Additional Resources 📚

Books

• "Domain-Driven Design" by Eric Evans
• "Implementing Domain-Driven Design" by Vaughn Vernon
• "Building Microservices" by Sam Newman

Articles

• Strategic Domain-Driven Design
• Bounded Context Canvas
• Team Topologies

Documentation

• Microsoft Domain-Driven Design fundamentals
• AWS Microservices Boundaries
• DDD Community Resources

For additional information and updates, refer to:

• Microsoft DDD Guide
• Martin Fowler's Blog