Hexagonal Architecture organizes an application around a business core, separating it from details such as user interfaces, databases and external services. The core connects to those parts through ports and adapters, which gives the architecture its other name: Ports & Adapters.
The aim is to keep business rules stable when external technology changes. The hexagon illustrates the application's boundary; it does not require six components.
The core knows what needs to happen, while adapters handle the technology used to communicate. Application startup code connects the appropriate adapters to the core.
When a customer places an order, the HTTP adapter turns the incoming data into a call to the order-creation operation. The core validates the order, calculates its total and requests storage through the persistence port. The configured database adapter runs the query and returns the result.
The calculation rules live in the core and contain neither HTTP-handling code nor SQL. To test these rules, developers can provide an adapter that stores data in memory, allowing the test to run without a real database connection.
If a command-line ordering tool is added, it can call the same order-creation operation through another input adapter. Developers do not need to rewrite the calculation rules for the new tool.
Hexagonal Architecture suits applications with business rules that need thorough testing, several entry points or frequently changing external integrations. It can organize code within a monolith or within individual microservices; it does not require splitting the application into services.
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