The hexagonal architecture, or ports and adapters architecture, is an architectural pattern used in

software design Software design is the process of conceptualizing how a software system will work before it is implemented or modified. Software design also refers to the direct result of the design process the concepts of how the software will work which co ...

. It aims at creating loosely coupled application components that can be easily connected to their software environment by means of ports and adapters. This makes components exchangeable at any level and facilitates test automation.

Origin

The hexagonal architecture was invented by

Alistair Cockburn Alistair Cockburn ( ) is an American computer scientist, known as one of the initiators of the agile movement in software development. He cosigned (with 16 others) the Manifesto for Agile Software Development. Life and career Cockburn starte ...

in an attempt to avoid known structural pitfalls in object-oriented software design, such as undesired dependencies between layers and contamination of

user interface In the industrial design field of human–computer interaction, a user interface (UI) is the space where interactions between humans and machines occur. The goal of this interaction is to allow effective operation and control of the machine fro ...

code with

business logic In computer software, business logic or domain logic is the part of the program that encodes the real-world business rules that determine how data can be created, stored, and changed. It is contrasted with the remainder of the software that might ...

. It was discussed at first on the

Portland Pattern Repository The Portland Pattern Repository (PPR) is an online repository for computer programming software design patterns. It was accompanied by the website WikiWikiWeb, the world's first wiki. The repository has an emphasis on extreme programming, and i ...

wiki; in 2005 Cockburn renamed it "Ports and adapters". In April 2024, Cockburn published a comprehensive book on the subject, coauthored with Juan Manuel Garrido de Paz. The term "hexagonal" comes from the graphical conventions that shows the application component like a

hexagon In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°. Regular hexagon A regular hexagon is de ...

al cell. The purpose was not to suggest that there would be six borders/ports, but to leave enough space to represent the different interfaces needed between the component and the external world.

Principle

The hexagonal architecture divides a system into several loosely-coupled interchangeable components, such as the application core, the database, the user interface, test scripts and interfaces with other systems. This approach is an alternative to the traditional layered architecture. Each component is connected to the others through a number of exposed "ports". Communication through these ports follow a given protocol depending on their purpose. Ports and protocols define an abstract

API An application programming interface (API) is a connection between computers or between computer programs. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how to build ...

that can be implemented by any suitable technical means (e.g. method invocation in an

object-oriented language Object-oriented programming (OOP) is a programming paradigm based on the concept of '' objects''. Objects can contain data (called fields, attributes or properties) and have actions they can perform (called procedures or methods and impleme ...

remote procedure call In distributed computing, a remote procedure call (RPC) is when a computer program causes a procedure (subroutine) to execute in a different address space (commonly on another computer on a shared computer network), which is written as if it were a ...

s, or

Web service A web service (WS) is either: * a service offered by an electronic device to another electronic device, communicating with each other via the Internet, or * a server running on a computer device, listening for requests at a particular port over a n ...

s). The granularity of the ports and their number is not constrained: * a single port could in some case be sufficient (e.g. in the case of a simple service consumer); * typically, there are ports for event sources (user interface, automatic feeding), notifications (outgoing notifications),

database In computing, a database is an organized collection of data or a type of data store based on the use of a database management system (DBMS), the software that interacts with end users, applications, and the database itself to capture and a ...

(in order to interface the component with any suitable DBMS), and administration (for controlling the component); * in an extreme case, there could be a different port for every

use case In both software and systems engineering, a use case is a structured description of a system’s behavior as it responds to requests from external actors, aiming to achieve a specific goal. It is used to define and validate functional requireme ...

, if needed. Adapters are the glue between components and the outside world. They tailor the exchanges between the external world and the ports that represent the requirements of the inside of the application component. There can be several adapters for one port, for example, data can be provided by a user through a GUI or a command-line interface, by an automated data source, or by test scripts.

Criticism

The term "hexagonal" implies that there are 6 parts to the concept, whereas there are only 4 key areas. The term’s usage comes from the graphical conventions that shows the application component like a

al cell. The purpose was not to suggest that there would be six borders/ports, but to leave enough space to represent the different interfaces needed between the component and the external world. According to Martin Fowler, the hexagonal architecture has the benefit of using similarities between presentation layer and data source layer to create symmetric components made of a core surrounded by interfaces, but with the drawback of hiding the inherent asymmetry between a service provider and a service consumer that would better be represented as layers.

Evolution

According to some authors, the hexagonal architecture is at the origin of the

microservices In software engineering, a microservice architecture is an architectural pattern that organizes an application into a collection of loosely coupled, fine-grained services that communicate through lightweight protocols. This pattern is characterize ...

architecture.

Variants

The onion architecture proposed by Jeffrey Palermo in 2008 is similar to the hexagonal architecture: it also externalizes the infrastructure with interfaces to ensure loose coupling between the application and the database. It decomposes further the application core into several concentric rings using inversion of control. The clean architecture proposed by Robert C. Martin in 2012 combines the principles of the hexagonal architecture, the onion architecture and several other variants. It provides additional levels of detail of the component, which are presented as concentric rings. It isolates adapters and interfaces (user interface, databases, external systems, devices) in the outer rings of the architecture and leaves the inner rings for

s and entities. The clean architecture uses the principle of dependency inversion with the strict rule that dependencies shall only exist between an outer ring to an inner ring and never the contrary.

References