Understanding Hexagonal Architecture: Ports and Adapters

Hexagonal Architecture decouples business logic from external systems, improving testability, flexibility, and maintainability in software.

Hexagonal Architecture, also known as “Ports and Adapters,” is an architectural style that aims to separate the business logic of an application from its external dependencies, such as databases, frameworks, and APIs. Created by Alistair Cockburn, this architecture addresses the issue of coupling, which occurs when business logic becomes intertwined with infrastructure details, making maintenance, testing, and scalability difficult.

Principles of Hexagonal Architecture

Hexagonal Architecture is guided by several core principles that help keep the application decoupled and easy to evolve:

1. Independence from Frameworks: Business logic does not depend on specific frameworks or libraries. This allows the application to evolve or swap out its dependencies independently of the core logic.
2. Separation of Concerns: By isolating different parts of the system, the code becomes more organized, facilitating readability and maintainability.
3. Ease of Testing: Since the business logic is separated from external dependencies, the domain can be tested in isolation without worrying about infrastructure details.
4. Extensibility and Ease of Replacement: Any external dependency can be easily added or replaced because it is encapsulated within specific adapters.

Components of Hexagonal Architecture

Hexagonal Architecture comprises three main components:

1. Domain (or Core): The domain is where all of the application’s business logic resides. It is the core of the architecture, containing classes and methods that represent the business rules. This core is entirely independent of any infrastructure details.
2. Ports: Ports are interfaces that define how the domain interacts with the external world. They represent the input and output channels of the system, functioning as contracts for communication between the domain and its external dependencies.
3. Adapters: Adapters implement the interfaces defined by the ports. They act as intermediaries that translate requests from one side to another, connecting the domain with external technologies such as databases, APIs, or user interfaces.

How It Works in Practice

To understand how these components interact, let’s consider the flow of an application using Hexagonal Architecture:

• Input: When a request enters the system (for example, a user registration request), it goes through an input adapter, which could be a REST API. This adapter receives the request and forwards it to the domain via a port.
• Processing in the Domain: The domain processes the request using its internal logic. Since the domain is unaware of external details, it uses an interface (port) to interact with its dependencies.
• Output: If the domain needs to store data, it invokes an output port, which in turn connects to a persistence adapter, such as a database. This adapter translates the domain call into an operation that the database understands.

Code Example (in JavaScript)

Let’s go through a simple user registration example to illustrate this interaction. Suppose we have a user service and a database repository.

// Port - Defines the contract
class UserRepository {
    save(user) {
        throw new Error('Method not implemented');
    }
}

// Domain - Business Logic
class UserService {
    constructor(userRepository) {
        this.userRepository = userRepository;
    }

    registerUser(user) {
        // registration logic
        if (!user.name) {
            throw new Error('Name is required');
        }
        this.userRepository.save(user);
    }
}

// Adapter - Implements the Port contract
class UserRepositoryAdapter extends UserRepository {
    save(user) {
        console.log(`Saving user ${user.name} to the database.`);
        // database persistence logic
    }
}

// Usage
const userRepository = new UserRepositoryAdapter();
const userService = new UserService(userRepository);

userService.registerUser({ name: 'John Doe' });        

Benefits of Hexagonal Architecture

Hexagonal Architecture offers several advantages that make software development more sustainable:

• Decoupling: The separation between domain and infrastructure allows the application to change its dependencies without affecting business logic.
• Maintenance and Testing: Since the domain is isolated, the application becomes easier to test and maintain. Any changes to dependencies do not affect core logic.
• Ease of Adding New Features: New adapters (for example, for a different type of database) can be added with minimal impact on the domain.

Challenges and Disadvantages

While Hexagonal Architecture provides numerous benefits, it also presents some challenges:

• Initial Complexity: Setting up Hexagonal Architecture can be complex, especially for smaller projects, where the overhead of organizing the system into layers may not be justified.
• Overhead: The separation into multiple layers adds some complexity to the code and can increase development time.

When to Use Hexagonal Architecture?

Hexagonal Architecture is best suited for projects that require significant flexibility and decoupling. It is ideal for:

• Systems with Complex Integration: Projects that need to connect to different data sources or external services.
• Long-Term Corporate Applications: Projects that will grow over time and need to be maintained sustainably.
• Scalable and Adaptable Systems: Applications that need to be easily scalable or where dependencies may change frequently.

However, for small or short-term projects, a simpler architectural approach may be more efficient.

Conclusion

Hexagonal Architecture is a powerful approach for those seeking a modular, testable, and maintainable application. By separating business logic from external dependencies, it enables more flexible and robust development. Although it may seem complex initially, its long-term benefits far outweigh the initial challenges.

Try implementing Hexagonal Architecture in your next project and see how it can improve the quality and sustainability of your code. Happy coding!