Using Hexagonal Architecture and DDD together for robust software design

Hexagonal architecture, also known as “ports and adapters” architecture, is a design pattern that emphasizes separation of concerns, flexibility, and maintainability. By separating the core business logic of a software system from its external interfaces, the hexagonal architecture allows for a more robust and testable system that can be easily adapted to new requirements.

The hexagonal architecture pattern is based on the idea of a “hexagon,” with the core business logic of the system at the center and the various external interfaces (such as web interfaces, databases, and external APIs) surrounding it. The interfaces are connected to the core logic through “ports” which act as a bridge between the internal implementation and the external interface. “Adapters” are used to translate between the internal implementation and the specific external interface.

One of the main benefits of using hexagonal architecture is that it allows for a clear separation of concerns. The core business logic can be developed and tested independently of the external interfaces, which makes it easier to make changes to the system without affecting the interfaces. This also makes it simpler to test the system, as the core logic can be tested without having to set up and configure the external interfaces.

Another advantage of hexagonal architecture is that it allows for greater flexibility in terms of the external interfaces used. The core business logic is not tied to any specific interface, so it can be easily adapted to work with different types of interfaces, such as web interfaces, command line interfaces, or mobile interfaces. This allows the system to be easily adapted to new requirements or technologies as they arise.

Hexagonal architecture is particularly well suited for systems that will be deployed in multiple environments or will have to be integrated with other systems. It’s also a good fit when different stakeholders have different requirements on how the system should be exposed to them. It’s a good way to ease the testing process, as it allows for easy testing of core business logic independently of external interfaces.

One of the key principles of hexagonal architecture is to keep the core business logic independent of external interfaces. This allows for the core logic to be tested in isolation and also makes it easier to make changes to the system without affecting the external interfaces.

Another key principle is the use of interfaces to define the external interfaces of the system. This allows for the core business logic to be implemented in a way that is independent of the specific external interfaces that will be used. Adapters are then used to translate between the internal implementation and the specific external interface.

When implementing a hexagonal architecture, it’s important to keep in mind that the architecture should be driven by the business requirements of the system, rather than by the technical details of the specific interfaces that will be used.

Ports

Port in the context of hexagonal architecture, a “port” refers to an interface that defines the contract for how external components interact with the core of the system. An example of a port in a Java application might be a service interface that defines the methods for creating, reading, updating, and deleting data in a database.

/*"PaymentPort" that defines the methods for interacting with a payment system.*/
public interface PaymentPort {
    boolean processPayment(double amount);
    boolean refundPayment(double amount);
}

Adapter

An “adapter” in hexagonal architecture refers to a class that implements a port and provides the necessary logic to interact with external components. An example of an adapter in a Java application might be a class that implements the service interface described above and uses JDBC to interact with a MySQL database. This adapter would be responsible for translating the method calls from the service interface into SQL statements and executing them on the database.

/* Implement the PaymentPort interface 
in a class called "PaymentAdapter" that communicates 
with a specific payment system (e.g. PayPal, Stripe, etc.).*/

public class PaymentAdapter implements PaymentPort {
    private PaymentSystem paymentSystem;
    public PaymentAdapter(PaymentSystem paymentSystem) {
        this.paymentSystem = paymentSystem;
    }
    public boolean processPayment(double amount) {
        return paymentSystem.processPayment(amount);
    }
    public boolean refundPayment(double amount) {
        return paymentSystem.refundPayment(amount);
    }
}

Domain-Driven Design

Hexagonal architecture and Domain-Driven Design (DDD) can be used together to create robust and maintainable software systems. DDD is a design approach that focuses on the domain (or business) logic of a system, while hexagonal architecture is a design pattern that emphasizes the separation of concerns, flexibility, and maintainability.

When using DDD in conjunction with hexagonal architecture, the core business logic of the system is placed at the center of the hexagon. The ports and adapters that surround the hexagon represent the external interfaces of the system, such as web interfaces, databases, and external APIs.

The ports act as a bridge between the internal implementation of the system and its external interfaces, providing a clean separation between the two. Adapters are used to translate between the internal implementation and the specific external interface.

By applying DDD and hexagonal architecture together, the domain logic of the system is kept separate from the external interfaces and can be developed and tested independently. This allows for greater flexibility and maintainability of the system, as well as easier testing and deployment.

Additionally, DDD provides a way to structure the domain and identify the important concepts and their relationships, which can be used to guide the design of the system. With DDD, the hexagon would be defined by the core Domain Model, which represents the business concepts, rules, and invariants of the system and the ports and adapters would be defined by the use cases and the external interfaces.

Conclusion

In summary, Hexagonal Architecture is a powerful tool for software architects to consider when building systems that will be deployed in multiple environments or will have to be integrated with other systems. By separating the core business logic of a software system from its external interfaces, the hexagonal architecture allows for a more robust and testable system that can be easily adapted to new requirements. It emphasizes the separation of concerns, flexibility, and maintainability, which are critical attributes of any software system.