Real-World Patterns

Overview

The team adopted a DDD architecture. Entities must be sealed, Command handlers must implement specific interfaces, and the domain layer must not reference the adapter layer. This was documented, but as the project grows, the rules gradually begin to crumble.

Architecture documents can go unread, but failing tests cannot be ignored. Let’s apply the verification techniques learned in Parts 1—3 to real-world layer architectures.

In this part, you will learn how to enforce architecture patterns used in real projects through tests. This covers DDD tactical patterns, Command/Query patterns, Port & Adapter patterns, and layer dependency rules.

Learning Objectives

Core Learning Goals

1. Comprehensive domain layer rule application
   • Verify structural rules for Entity, Value Object, Domain Event, and Domain Service
   • Unify immutability, inheritance, and naming rules into a single test suite
2. Enforcing application layer patterns
   • Verify Command/Query separation rules and Usecase handler signatures
   • Ensure structural consistency of DTOs and nested class patterns
3. Verifying Port & Adapter rules
   • Enforce the relationship between port interfaces and adapter implementations
   • Automatically verify that adapters implement only the correct ports
4. Automated layer dependency direction verification
   • Combine ArchUnitNET native API with Functorium API
   • Test dependency rules in the Domain -> Application -> Adapter direction

What You Will Verify Through Practice

• Verify that Entity classes are public sealed and inherit the correct base class
• Confirm that Command handlers have exactly one Execute method
• Automatically verify that the domain layer does not reference the adapter layer

Chapter Structure

Chapter	Title	Key Content
Chapter 1	Domain Layer Rules	Entity, Value Object, Domain Event, Domain Service
Chapter 2	Application Layer Rules	Command/Query pattern, nested classes, DTOs
Chapter 3	Adapter Layer Rules	Port interfaces, adapter implementations, dependency verification
Chapter 4	Layer Dependency Rules	Multi-layer dependencies, ArchUnitNET + Functorium combination

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In the first chapter, we implement 21 rules for Entity, Value Object, Domain Event, Specification, and Domain Service across 6 categories.

-> Ch 1: Domain Layer Rules