DDD Architecture Explained: Layers, Hexagonal Architecture, and Examples

A Deep Analysis of Essential DDD Concepts to Create Clear and Robust Architectures

Overview of Layered Architecture in DDD

What is Layered Architecture?

Layered Architecture is a structured approach to software design that enforces a separation of responsibilities by organizing code into distinct layers. This approach allows for better maintainability, scalability, and testability by ensuring that each layer has a specific role within the system.

In Domain-Driven Design (DDD), Layered Architecture provides a way to structure applications so that the integrity of the domain model is preserved while managing interactions with external systems.

The Four Main Layers in Traditional DDD Architecture

A DDD-based Layered Architecture typically consists of the following layers:

Domain Layer (The Core) - Contains business rules and domain logic.
Application Layer (Orchestration) - Handles use cases, coordinating domain operations.
Infrastructure Layer (Technical Details) - Manages databases, APIs, messaging, and persistence.
Presentation Layer (User Interaction) - Handles UI, API endpoints, or CLI interactions.

Each layer is designed to communicate in a structured way, typically with dependencies flowing toward the domain core.

Detailed Analysis of Each Layer in DDD Architecture

Domain Layer (The Heart of the System)

The Domain Layer is the most important layer in DDD-based applications. It contains:

Entities & Value Objects: Represent domain concepts and ensure consistency of business logic.
Aggregates & Repositories: Manage domain state and encapsulate persistence concerns.
Domain Services: Handle domain logic that does not fit within an entity.
Domain Events: Enable decoupled business logic by triggering actions based on domain changes.

This layer must be independent of external frameworks and infrastructure details to remain pure and reusable.

Application Layer (Orchestration Layer)

The Application Layer acts as an intermediary between the domain and the outside world, orchestrating workflows and applying business processes. Its key responsibilities include:

Coordinating Use Cases: Managing business flows without implementing domain logic.
Handling Transactions: Ensuring consistency across multiple domain operations.
Interacting with External Systems: Communicating with APIs, databases, or other services.

This layer should be thin, delegating business logic to the domain layer and focusing only on orchestration.

Infrastructure Layer (Technical Implementation)

The Infrastructure Layer provides the technical implementation needed to support the application. This includes:

Persistence Mechanisms: Databases, file storage, or caching systems.
External Integrations: APIs, messaging queues, or third-party services.
Dependency Injection and Configuration: Managing dependencies and framework-level concerns.

Although this layer contains necessary technical details, it should remain separate from domain logic.

Presentation Layer (User Interaction)

The Presentation Layer handles user interactions and communication with the outside. It can include:

A Web or Mobile UI: Frontends in React, Angular, or Swift.
An API Layer: REST or GraphQL endpoints.
A Command Line Interface (CLI): For automation and scripts.

The key principle here is separation of concerns—presentation logic should not contain domain logic but delegate it to the application layer.

Alternative Architectural Approaches in DDD

Hexagonal Architecture (Ports and Adapters)

Source:https://medium.com/ssense-tech/hexagonal-architecture-there-are-always-two-sides-to-every-story-bc0780ed7d9c

Also known as Ports and Adapters, Hexagonal Architecture isolates domain logic from external systems using ports (interfaces) and adapters (implementations).

Why Use It?
- Improves testability and flexibility.
- Allows easy replacement of external dependencies (e.g., switching databases).
- Encourages a clear separation between domain logic and technical concerns.

CQRS (Command Query Responsibility Segregation)

Source:https://medium.com/design-microservices-architecture-with-patterns/cqrs-design-pattern-in-microservices-architectures-5d41e359768c

CQRS separates commands (write operations) from queries (read operations), optimizing performance and scalability.

Why Use It?
- Improves scalability by handling reads and writes differently.
- Enhances security by allowing separate permissions for read and write actions.
- Facilitates event sourcing, where changes are stored as a sequence of domain events.

Event-Driven Architecture & Microservices

Source:https://medium.com/@seetharamugn/the-complete-guide-to-event-driven-architecture-b25226594227

Event-Driven Architecture (EDA) enables decoupled systems that react asynchronously to domain events.

Why Use It?
- Improves system resilience by decoupling services.
- Increases scalability by processing events in a distributed manner.
- Supports microservices architectures where each service manages its own domain.

Best Practices for Applying DDD Architecture

Keep the Domain Layer Independent

The domain layer should not depend on frameworks, databases, or external services.
Use inversion of control to inject dependencies instead of hardcoding them.

Use Dependency Injection to Decouple Components

Avoid tight coupling by injecting repositories, services, and adapters when necessary.
This makes the system easier to test and adaptable to changes.

Respect Clear Boundaries Between Layers

Keep domain logic within the domain layer.
The application layer should only coordinate, not implement business rules.
The infrastructure layer should focus on technical concerns without business logic.

Use Domain Events for Communication Between Layers

Implement domain events to decouple components and improve extensibility.
For example, when a user registers, trigger a UserRegistered event instead of notifying other services directly.

Leverage Application Services for Use Cases, Not Business Logic

Application Services should only orchestrate workflows, delegating business rules to the domain layer.
Avoid embedding domain logic in controllers or infrastructure components.

Conclusion

Domain-Driven Design offers a structured way to build scalable and maintainable applications by focusing on domain logic, while ensuring separation of concerns.

Layered Architecture provides a clear structure but can be rigid.
Hexagonal Architecture offers better decoupling and flexibility.
CQRS optimizes read and write operations to improve scalability.
Event-Driven Architecture enhances decoupling and microservices integration.

By following best practices—such as keeping the domain layer independent, using dependency injection, and leveraging domain events—developers can build resilient, scalable, and maintainable systems.

These are the next topics we will discuss in this From Good to Excellent in DDD series. I hope we navigate together through this important architecture:

Ready to structure your applications with an effective layered architecture in DDD?

At Kranio, we have software architecture experts who will help you implement layered architecture in your projects, ensuring a solid and scalable structure. Contact us and discover how we can improve the architecture of your systems.

‍

DDD Architecture Explained Simply (Practical Guide with Examples) 7/10