Database Gateways

Between the use case interactors and the database are the database gateways. These gateways are polymorphic interfaces that contain methods for every create, read, update, or delete operation that can be performed by the application on the database.

For example, if the application needs to know the last names of all the users who logged in yesterday, then the UserGateway interface will have a method named getLastNamesOfUsersWhoLoggedInAfter that takes a Date as its argument and returns a list of last names.

Recall that we do not allow SQL in the use cases layer; instead, we use gateway interfaces that have appropriate methods. Those gateways are implemented by classes in the database layer. The gateway implementation is the humble object. It simply uses SQL, or whatever the interface to the database is, to access the data required by each of the methods. The interactors, in contrast, are not humble because they encapsulate application-specific business rules. Although they are not humble, those interactors are testable, because the gateways can be replaced with appropriate stubs and test-doubles.