Building Better Software with Domain-Driven Design and Hexagonal Architecture

In today’s fast-paced software landscape, creating scalable, maintainable, and flexible systems is crucial. Two proven approaches—Domain-Driven Design (DDD) and Hexagonal Architecture (Ports and Adapters)—help teams build robust, adaptable software that mirrors real-world complexity.

What is Domain-Driven Design (DDD)?

Domain-Driven Design, introduced by Eric Evans, is a methodology that centers on understanding the core business domain your software serves. Rather than starting with technical details, DDD begins with the business problem.

Core Concepts

• Domain: The subject area your application addresses (e.g., eCommerce, banking).
• Entities: Objects with unique identity (e.g., Customer, Order).
• Value Objects: Objects defined only by their attributes (e.g., Money, Address).
• Aggregates: Groups of domain objects treated as a single unit.
• Repositories: Mechanisms to retrieve and store aggregates.
• Services: Operations that don’t naturally belong to a single object.
• Domain Events: Events representing meaningful domain occurrences.

Example: In an eCommerce software system, an OrderPlaced event might trigger inventory checks and email notifications.

What is Hexagonal Architecture?

Hexagonal Architecture is a form of Clean Architecture which structures your backend application to keep business logic at the center, decoupling it from external systems like databases, UIs, or third-party services.

Key Elements

• Core Domain: Pure business logic, independent of infrastructure.
• Ports (Interfaces): Define operations your domain requires or offers.
• Adapters: Implementations of ports (e.g., REST controllers, database gateways).

Think of the core app as a fortress, with bridges (ports) connecting to the outside world via plug-in adapters.

How DDD and Hexagonal Architecture Work Together

Combining DDD and Hexagonal Architecture provides:

• A rich, expressive domain model immune to infrastructure changes.
• An easily testable core, isolated from external systems.
• Flexible boundaries to plug in new technologies with minimal friction.

Example: Online Bookstore

Domain

• Entities: Book, Customer, Order
• Value Objects: Money, Address
• Aggregates: Order (with line items)
• Services: OrderService handles order placement
• Domain Events: OrderConfirmed

Hexagonal Layers

• Core Domain: Pure Java/TypeScript classes for models and services.
• Inbound Adapters: REST API controllers, CLI interfaces.
• Outbound Adapters: Repositories (JPA, MongoDB), email services.

Benefits

• Decoupling: Change your database, UI, or frameworks without rewriting business logic.
• Testability: Test the domain in isolation using mock ports.
• Maintainability: Evolve complex logic as domain understanding grows.
• Clarity: Code reflects business language and rules.

Getting Started with DDD & Hexagonal Design

Ready to stop building brittle software systems? Our course will guide you through a proven method for building adaptable systems. We’ll show you how to start by deeply understanding the business domain, then use that knowledge to craft a powerful domain model with aggregates, entities, and value objects. From there, you’ll learn to design the ports and adapters that create a clean separation between your core business logic and everything else. The result? Software that’s not only easier to change but also a joy to test.

When to Use DDD

DDD is most effective for complex domains where business logic is central. For simple CRUD applications, the overhead may not be justified.

Final Thoughts

DDD and Hexagonal Architecture are more than buzzwords—they’re mindsets. By focusing on what your software system should do, not just how, you’ll create systems that are understandable and resilient to change.

References

1. Implementing Domain Driven Design with Spring by Maciej Walkowiak at springIO24
2. Implementing DDD with Spring Ecosystem by Michael Plod at SpringIO18
3. DDD By Examples
4. Getting Modules Right with Domain-driven Design by Michael Plod
5. DDD-crew
6. DDD-By-Examples

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