DDD Refactor Skill

Comprehensive Domain-Driven Design refactoring framework based on Eric Evans' DDD principles.

When to Use This Skill

• Refactoring domain models and business logic
• Identifying anemic domain models
• Defining bounded contexts
• Applying tactical DDD patterns (Entity, Value Object, Aggregate)
• Improving domain clarity and ubiquitous language
• Analyzing code for DDD anti-patterns
• Architecting domain-centric systems

Prerequisites

CRITICAL: Always load the DDD principles first:

Read: plugins/ddd-refactor/resources/ddd-principles.json

This JSON contains 19+ principles with definitions, code smells, and refactoring guidance.

Refactoring Approach

Four-Phase Strategy

Phase 1: Discovery & Analysis (15-20 min)

Understand the Domain:

1. Identify core domain vs supporting/generic subdomains
2. Map existing bounded contexts (explicit or implicit)
3. Document current ubiquitous language usage
4. Analyze domain model structure

Scan for Code Smells:

• Anemic domain models (all logic in services)
• Missing ubiquitous language (technical jargon)
• Blurred bounded context boundaries
• Mutable value objects
• Large aggregates without clear boundaries
• Domain logic in application/infrastructure layers
• Missing domain events
• Repositories exposing internal entities
• Bidirectional dependencies across contexts

Technical Exploration:

# Find potential entities
grep -r "class.*Entity" --include="*.{js,ts,java,cs,py}"

# Find service classes (potential logic that belongs in domain)
grep -r "class.*Service" --include="*.{js,ts,java,cs,py}"

# Find repositories
grep -r "Repository" --include="*.{js,ts,java,cs,py}"

# Find domain events (or lack thereof)
grep -r "Event\|event" --include="*.{js,ts,java,cs,py}"

Phase 2: Strategic Refactoring Plan (10-15 min)

Based on loaded DDD principles:

1. Define Bounded Contexts

   • Map natural domain boundaries
   • Identify context relationships (Shared Kernel, Customer-Supplier, etc.)
   • Plan integration strategies

2. Establish Ubiquitous Language

   • List terms needing renaming
   • Extract implicit concepts into explicit models
   • Create domain glossary

3. Prioritize Refactoring

   • Critical: Core domain models, aggregate boundaries, data consistency
   • High: Entity/Value Object patterns, domain events, repositories
   • Medium: Services, factories, layering
   • Low: Supporting subdomains, infrastructure

Phase 3: Tactical Pattern Application (30-45 min)

Apply patterns systematically:

1. Entities - Identity-based objects

• Extract objects with lifecycle and identity
• Add identity equality (not value equality)
• Encapsulate business rules within entities
• Make state changes explicit through methods

2. Value Objects - Immutable descriptive objects

• Convert primitive obsession to value objects
• Make all value objects immutable
• Implement value-based equality
• Create self-validating value objects

3. Aggregates - Consistency boundaries

• Group related entities under aggregate roots
• Enforce invariants at aggregate boundaries
• Make internal entities inaccessible from outside
• Use aggregate roots for all external access

4. Domain Events - Significant occurrences

• Identify state changes that matter to the domain
• Publish events from aggregates after state changes
• Use events to decouple bounded contexts
• Record event history for audit/debugging

5. Repositories - Aggregate persistence

• Create repositories only for aggregate roots
• Return reconstituted aggregates (not raw data)
• Abstract all storage concerns
• Use specification pattern for complex queries

6. Domain Services - Stateless operations

• Extract operations that don't belong to any entity
• Keep services thin (orchestration, not logic)
• Separate Domain/Application/Infrastructure services
• Make service operations explicit domain concepts

7. Factories - Complex creation

• Encapsulate complex aggregate construction
• Hide creation details from clients
• Ensure invariants are met at creation
• Provide convenient creation methods

8. Layered Architecture

• Move domain logic from controllers to domain layer
• Isolate external systems with anti-corruption layers
• Ensure dependencies point inward (domain is innermost)
• Keep domain layer free of infrastructure concerns

Phase 4: Validation & Testing (10-15 min)

Verify Improvements:

• Ubiquitous language is reflected in code
• Business rules are explicit and testable
• Bounded contexts have clear boundaries
• Aggregates enforce their invariants
• Domain events capture significant changes
• Repositories work with aggregate roots
• Domain layer is free of infrastructure
• Integration points are explicit and controlled

Testing Strategy:

• Unit test entity/value object logic
• Test aggregate invariant enforcement
• Test domain event publication
• Integration test repositories
• Test anti-corruption layers

Core DDD Principles Reference

Strategic Design

1. Ubiquitous Language - Domain vocabulary in code
2. Bounded Context - Explicit model boundaries
3. Context Map - Relationships between contexts
4. Continuous Integration - Prevent fragmentation

Tactical Patterns

5. Entity - Identity-based objects with lifecycle
6. Value Object - Immutable, attribute-based objects
7. Aggregate - Consistency boundary and transaction scope
8. Repository - Collection-like access to aggregates
9. Factory - Complex object creation
10. Domain Service - Stateless domain operations
11. Domain Event - Record of domain occurrence

Architecture

12. Layered Architecture - Separation of concerns
13. Anti-Corruption Layer - Isolation from external systems

Context Mapping

14. Shared Kernel - Explicit sharing between contexts
15. Customer-Supplier - Upstream-downstream relationship
16. Conformist - Adopt upstream model
17. Open Host Service - Standardized API
18. Published Language - Documented interchange format

Code Smell Detection Checklist

Strategic Anti-Patterns

• No explicit bounded contexts
• Technical names instead of domain language
• Mixed business logic across contexts
• No context integration strategy
• Same entity meaning different things in different places

Tactical Anti-Patterns

• Anemic domain model (getters/setters only)
• Entities without clear identity
• Mutable value objects
• Large aggregates (>5-7 entities)
• Repositories for non-root entities
• Domain logic in services instead of entities
• No domain events for state changes
• Primitive obsession (no value objects)

Architecture Anti-Patterns

• Domain logic in controllers/UI
• Direct database access from domain
• No anti-corruption layer for external systems
• Infrastructure concerns in domain layer
• Bidirectional dependencies between contexts

Output Format

1. Anti-Pattern Identified

File: src/orders/OrderService.java:45-78
Smell: Anemic domain model - Order entity has only getters/setters
Principle Violated: Entity Pattern
Impact: Business logic scattered in services, hard to test and maintain

2. DDD Principle to Apply

Principle: Entity (from ddd-principles.json)
Category: Tactical Pattern
Key Point: Entities should encapsulate business rules and behavior
When to Apply: Objects with identity that change over time

3. Refactoring Steps

Step 1: Move validation logic from OrderService to Order entity
Step 2: Add domain methods: order.cancel(), order.ship(), order.addItem()
Step 3: Enforce invariants within entity methods
Step 4: Raise domain events for significant state changes
Step 5: Update OrderService to orchestrate, not contain logic

4. Code Example

// BEFORE: Anemic Domain Model (Anti-pattern)
class Order {
  private id: string;
  private items: OrderItem[];
  private status: string;

  // Only getters and setters
  getId(): string { return this.id; }
  getStatus(): string { return this.status; }
  setStatus(status: string): void { this.status = status; }
}

class OrderService {
  cancelOrder(order: Order): void {
    // Business logic in service layer
    if (order.getStatus() === 'SHIPPED') {
      throw new Error('Cannot cancel shipped order');
    }
    order.setStatus('CANCELLED');
    this.emailService.sendCancellationEmail(order);
  }
}

// AFTER: Rich Domain Model (DDD Pattern)
class Order {
  private id: OrderId;
  private items: OrderItem[];
  private status: OrderStatus;
  private events: DomainEvent[] = [];

  constructor(id: OrderId, items: OrderItem[]) {
    this.id = id;
    this.items = items;
    this.status = OrderStatus.PENDING;
  }

  // Business logic in entity
  cancel(): void {
    if (!this.canBeCancelled()) {
      throw new OrderCannotBeCancelledException(
        'Cannot cancel order in status: ' + this.status
      );
    }
    this.status = OrderStatus.CANCELLED;
    this.recordEvent(new OrderCancelled(this.id, new Date()));
  }

  private canBeCancelled(): boolean {
    return this.status !== OrderStatus.SHIPPED
        && this.status !== OrderStatus.DELIVERED;
  }

  getDomainEvents(): DomainEvent[] {
    return [...this.events];
  }

  private recordEvent(event: DomainEvent): void {
    this.events.push(event);
  }
}

// Service becomes thin orchestrator
class OrderService {
  constructor(
    private orderRepository: OrderRepository,
    private eventBus: EventBus
  ) {}

  async cancelOrder(orderId: string): Promise<void> {
    const order = await this.orderRepository.findById(orderId);

    order.cancel(); // Domain logic stays in domain

    await this.orderRepository.save(order);

    // Publish events for other bounded contexts
    order.getDomainEvents().forEach(event => {
      this.eventBus.publish(event);
    });
  }
}

5. Impact Assessment

Benefits:

• Business rules are explicit and self-documenting
• Order enforces its own invariants
• Easier to test (unit test the entity)
• Domain events enable loose coupling
• Service layer is thin and focused

Metrics:

• Reduced cyclomatic complexity in services
• Increased testability (pure domain logic)
• Better domain language alignment
• Clearer responsibility boundaries

Language-Specific Patterns

TypeScript/JavaScript

// Value Object with immutability
class Money {
  constructor(
    private readonly amount: number,
    private 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('Currency mismatch');
    }
    return new Money(this.amount + other.amount, this.currency);
  }

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

Java

// Aggregate with encapsulation
public class Order {
  private final OrderId id;
  private final List<OrderLine> lines;
  private OrderStatus status;

  // Package-private constructor (use factory)
  Order(OrderId id) {
    this.id = Objects.requireNonNull(id);
    this.lines = new ArrayList<>();
    this.status = OrderStatus.DRAFT;
  }

  public void addLine(Product product, Quantity quantity) {
    if (this.status != OrderStatus.DRAFT) {
      throw new OrderAlreadySubmittedException();
    }
    this.lines.add(new OrderLine(product, quantity));
  }

  // Factory method
  public static Order create(OrderId id) {
    return new Order(id);
  }
}

Python

from dataclasses import dataclass
from typing import List

# Value Object with frozen dataclass
@dataclass(frozen=True)
class EmailAddress:
    value: str

    def __post_init__(self):
        if '@' not in self.value:
            raise ValueError('Invalid email address')

# Entity with behavior
class Customer:
    def __init__(self, customer_id: str, email: EmailAddress):
        self._id = customer_id
        self._email = email
        self._events: List[DomainEvent] = []

    def change_email(self, new_email: EmailAddress) -> None:
        if self._email != new_email:
            old_email = self._email
            self._email = new_email
            self._events.append(
                EmailChanged(self._id, old_email, new_email)
            )

Best Practices

Do

• Start with the domain model, not the database
• Use domain language everywhere (code, tests, docs)
• Make implicit concepts explicit
• Favor immutability where possible
• Test domain logic in isolation
• Keep aggregates small (2-5 entities max)
• Use domain events for inter-context communication
• Apply anti-corruption layers for external systems

Don't

• Don't let infrastructure drive the domain model
• Don't skip domain events for significant changes
• Don't make value objects mutable
• Don't expose aggregate internals
• Don't create repositories for non-aggregate roots
• Don't put domain logic in services
• Don't share entities across bounded contexts
• Don't over-engineer - apply DDD where complexity justifies it

Resources

• DDD Principles: See resources/ddd-principles.json for complete definitions
• Checklist: See CHECKLIST.md for full anti-pattern list
• Book: "Domain-Driven Design" by Eric Evans

Workflow Integration

This skill can be:

• Invoked from /refactor command
• Used by ddd-analyzer agent for autonomous analysis
• Triggered by pre-commit hooks to check for anti-patterns
• Called from other skills for domain model improvements