At what level of the system architecture is the repository layer interface usually placed?

The repository layer interface is usually placed at the domain layer or business logic layer in the application architecture.

The repository serves as an abstraction for data access and hides the details of data sources (databases, networks, cache) from the upper layers. It provides an interface used by services or use case layers to retrieve and save data.

Thus, the repository interface defines a contract between the business logic layer and the data layer, ensuring loose coupling and ease of testing.