Middle layer in api code example

Example: web application layers

There are 3 layers

Front End:
Presentation layer 
-Html, css, javascript codes for human eye

Back End:
Data Layer
Relational Database =
-Tables are structured related to each other
-Each specific type of domain data is strored
 it's own table

Non-Relational Database =
-There is no relation at all between tables
-Mostly key+value pair : like jSon

Business Logic Layer:
This is where backend code is written
and it can be exposed as rest api.
We can access over the internet