Architecture: The 'Hexagonal' (Ports and Adapters) Pattern

Hexagonal architecture decouples the core business logic from external concerns by using 'Ports' (interfaces) and 'Adapters' (implementations).

In this architecture, the 'Domain' sits in the center. It doesn't know about Express or MongoDB. It defines a Port (interface). The Adapter implements that port, making the code highly testable and adaptable.