Routes and controllers: This layer is responsible for defining the API routes and handling incoming requests. It will typically parse request parameters, call appropriate services or models, and send back responses to the client-side application.

Services: This layer is responsible for implementing the business logic of the application. It will typically contain functions or methods to perform CRUD operations on the database, validate user input, and perform other tasks required by the application.

Models: This layer is responsible for defining the data models used by the application. It will typically define the schema for the database tables or collections, and provide functions or methods to interact with the database.

Middleware: This layer is responsible for adding additional functionality to the server, such as authentication or logging. Middleware functions can intercept incoming requests or outgoing responses, and modify them as necessary.