Hexagonal architecture, or the "Ports and Adapters" pattern, is designed to make applications change-tolerant. By isolating core business rules, you can swap out databases (e.g., moving from Oracle to MongoDB) or communication protocols (REST to gRPC) without touching your core logic.
The domain model represents the business logic of the application. In this case, we'll define a simple User entity:
: These are the interfaces that define how the core communicates with the outside world.