The hexagonal architecture, also known as Ports and Adapters architecture, is a software architectural pattern that aims to create software systems that are independent of frameworks, databases, and other external dependencies. It promotes the separation of concerns by dividing the application into three main layers: domain, application, and infrastructure.

The domain layer contains the core business logic and entities of the application. The application layer contains the use cases or services that orchestrate the domain logic. The infrastructure layer provides the adapters that connect the application to external systems, such as databases, user interfaces, and other services.

The hexagonal architecture emphasizes the use of interfaces, or ports, to decouple the application from external dependencies. Adapters are used to implement these interfaces, allowing the application to communicate with external systems while maintaining its independence. This makes the application more modular and easier to maintain, as changes to external systems can be accommodated without affecting the core business logic.

The hexagonal architecture is often used in combination with domain-driven design (DDD) and test-driven development (TDD), as it helps to keep the domain objects separate from the infrastructure, making it easier to test them in isolation.

Example

Let’s consider a simple example of a blog application that allows users to create and manage blog posts. Here’s a basic overview of the hexagonal architecture for the blog application:

• The domain layer contains the core business logic of the application, including entities, value objects, and domain services. In our blog example, the domain layer would define the BlogPost entity, which represents a single blog post, as well as other domain objects like Author and Comment.
• The application layer contains the use cases and business rules that are specific to the application. This layer is responsible for coordinating the interactions between the external interfaces and the domain layer. In our blog example, the application layer would define use cases such as CreatePost, UpdatePost, and DeletePost, which would invoke the appropriate methods on the BlogPost entity.
• The external interfaces layer contains the adapters that allow the application to communicate with external systems and services. This layer includes both inbound and outbound adapters. In our blog example, the external interfaces layer might include a REST API for creating and updating blog posts, as well as a database adapter for persisting blog post data.
• The infrastructure layer contains the implementation details that support the application, including libraries, frameworks, and other third-party tools. This layer is responsible for handling cross-cutting concerns such as security, logging, and caching. In our blog example, the infrastructure layer might include a database ORM library and a logging framework.

The hexagonal architecture allows the application to be decoupled from the external systems and services it interacts with, which makes it easier to test and maintain. This is because the external interfaces and infrastructure are isolated from the core business logic of the application, which allows them to be swapped out or replaced without affecting the rest of the system.

Comparison to the layered architecture

The hexagonal architecture and the layered architecture are both architectural styles that are often used in software development. The hexagonal architecture, is a more modern approach that emphasizes loose coupling and separation of concerns, while the layered architecture is a more traditional approach that has been in use for many years.

The hexagonal architecture is designed to be technology-agnostic, meaning that the domain logic is decoupled from the infrastructure and can be easily swapped out or replaced without affecting the rest of the system. This makes it easier to change or upgrade technology in the future, and also allows for more testable and maintainable code.

The layered architecture, on the other hand, is typically organized into three layers: presentation, business logic, and data access. The presentation layer is responsible for displaying information to the user, the business logic layer contains the application’s business rules, and the data access layer is responsible for interacting with the database.

While both architectural styles have their advantages and disadvantages, the hexagonal architecture is generally considered to be more flexible and easier to maintain in the long term, particularly for larger and more complex applications.