Domain-Driven Design Skill

DDD manages complexity through alignment between software and business reality. Strategic design (boundaries, language, subdomains) provides more value than tactical patterns (aggregates, repositories).

When to Apply DDD

Apply DDD when:

• Domain has intricate business rules
• System is long-lived and high-value
• Domain experts are available
• Multiple teams/departments involved
• Software represents competitive advantage

DDD is overkill when:

• Simple CRUD applications
• Tight deadlines, limited budgets
• No domain experts available
• Complexity is purely technical, not business

Core Workflow

1. Domain Discovery → 2. Bounded Context Definition → 3. Context Mapping → 4. Architecture Selection → 5. Tactical Implementation

See WORKFLOW.md for detailed step-by-step instructions for each phase.

Quick Reference

Subdomain Types (Problem Space)

Type	Investment	Example
Core	Maximum - competitive advantage	Recommendation engine, trading logic
Supporting	Custom but quality tradeoffs OK	Inventory management
Generic	Buy/outsource	Auth, email, payments

Key Decision: Entity vs Value Object

• Entity: Has identity, tracked through time, mutable → Customer, Order
• Value Object: Defined by attributes, immutable, interchangeable → Money, Address, Email

Default to value objects. Only use entities when identity matters.

Aggregate Design Rules (Vaughn Vernon)

1. Model true invariants in consistency boundaries
2. Design small aggregates (~70% should be root + value objects only)
3. Reference other aggregates by ID only
4. Use eventual consistency outside the boundary

Architecture Decision

Start with modular monolith when:
├── Team < 20 developers
├── Domain boundaries unclear
├── Time-to-market critical
└── Strong consistency required

Consider microservices when:
├── Bounded contexts have distinct languages
├── Teams can own full contexts
├── Independent scaling required
└── DevOps maturity exists

Detailed References

• Strategic Patterns: See references/STRATEGIC-PATTERNS.md for subdomains, bounded contexts, context mapping, event storming
• Tactical Patterns: See references/TACTICAL-PATTERNS.md for entities, value objects, aggregates, services, repositories
• Architecture Alignment: See references/ARCHITECTURE-ALIGNMENT.md for clean/hexagonal architecture, modular monolith, microservices
• Workflow: See WORKFLOW.md for detailed step-by-step DDD implementation process
• Anti-Patterns: See references/ANTI-PATTERNS.md for common pitfalls and how to avoid them
• Examples: See EXAMPLES.md for scenario walkthroughs applying DDD concepts
• Troubleshooting: See TROUBLESHOOTING.md for common issues and solutions

Critical Reminders

1. Ubiquitous language first - Code should read like business language
2. Strategic before tactical - Understand boundaries before implementing patterns
3. Apply tactical patterns selectively - Only in core domains where complexity warrants
4. One aggregate per transaction - Cross-aggregate consistency via domain events
5. Persistence ignorance - Domain layer has no infrastructure dependencies

Related Skills

Need	Skill
Data layer implementation	spring-boot-data-ddd — JPA/JDBC aggregates, repositories, transactions
REST API layer	spring-boot-web-api — Controllers, validation, exception handling
Module boundaries	spring-boot-modulith — Module structure, event-driven communication
Testing patterns	spring-boot-testing — Aggregate tests, module tests, Scenario API
Security for domains	spring-boot-security — Method-level authorization, role-based access