Critical rules

• •ALWAYS keep domain and use-case logic framework-agnostic
• •ALWAYS define ports (interfaces/contracts) in the application or domain layer
• •ALWAYS implement adapters outside the domain and use-case layers
• •NEVER allow infrastructure details to leak into the domain or use-case layers
• •ALWAYS direct dependencies inwards (domain <= application <= adapters)
• •ALWAYS validate architectural boundaries when modifying code

Core Concepts

Layers and Responsibilities

• •Domain: Entities, Value Objects, domain services, domain rules
• •Application (Use Cases): Orchestrates domain logic, defines ports
• •Adapters: Implement ports for external systems (DB, HTTP, messaging, files)
• •Infrastructure: Technical details used by adapters (frameworks, libraries, drivers)

Ports and Adapters

• •Ports are interfaces that describe what the application needs (outbound) or exposes (inbound)
• •Adapters implement ports for concrete technologies
• •Inbound adapters call use cases (HTTP, CLI, jobs)
• •Outbound adapters fulfil ports (repositories, gateways, external services)

Workflow

1. •Identify use cases and name them by business intent
2. •Define domain model with rules enforced by entities and Value Objects
3. •Create ports for external interactions required by use cases
4. •Implement use cases that depend only on ports and domain model
5. •Implement adapters to satisfy ports for specific technologies
6. •Wire dependencies in the composition root only
7. •Verify boundaries with dependency checks and reviews

Decision Trees

Where does this class belong?

Is it a domain rule or invariant?
├─ YES → Domain
└─ NO → Is it a use-case orchestration?
          ├─ YES → Application
          └─ NO → Does it connect to an external system?
                    ├─ YES → Adapter
                    └─ NO → Infrastructure helper

When to create a port?

Does a use case need to reach outside the application boundary?
├─ YES → Define a port in the application or domain layer
└─ NO → Keep logic inside the use case or domain model

Examples

Good: Use Case depends on a port

UseCase depends on UserRepositoryPort (interface)
Adapter implements UserRepositoryPort using a database client
Domain entity is used only in Domain and Use Case

Bad: Use Case depends on infrastructure

UseCase imports a database client or framework types directly
Domain entity annotated with framework annotations

Common Pitfalls

• •Framework leakage — Domain or use case imports infrastructure types
• •Missing ports — Use cases call adapters directly
• •Wrong dependency direction — Adapters depend on domain, not the other way around
• •Anemic domain — Domain holds data only, rules live in use cases or adapters
• •Overly generic ports — Ports that mirror framework APIs rather than domain intent

Checklist

Before submitting code, verify:

• • Domain model contains business rules and invariants
• • Use cases depend only on ports and domain model
• • Ports are interfaces/contracts defined inside the application or domain layer
• • Adapters implement ports and live outside the core layers
• • Infrastructure details are confined to adapters and composition root
• • Dependency direction is strictly inwards
• • No framework annotations or types appear in domain or use-case code

Related Skills

• •code-quality — For Clean Code, SOLID and Object Calisthenics
• •java-language — For Java-specific language guidance