Persistence layer is a group of files which is used to communicate between the application and DB. 2. Business logic layer is the rules of how to retrieve the data information from the database, and then the sever takes those information to display on the user presentation layer
This two layers look the same
What do they mean in real scenario? Also do they have difference in code?
The idea for layering a system is isolation. One layer is independent of the other.
How many different Databases are out there? Postgresql, MySql, MongoDB, Cassandra... The persistence layer (or Data Access Layer) will provide an interface to your your system. Let's say you system needs to find an User by its ID.
public interface UserRepository {
User findByID (Long id);
}
For each database, the implementation will change, but for the application consuming it, does it really matter? No, as long as the contract provided by the interface is not broken.
Once you have the data, the business logic will dictate what and how you will deal with it. For a MVC point of view, the business logic also defines the transaction scope (more at: Why we shouldn't make a Spring MVC controller @Transactional?). Let's say that you have your User that you retrieved using the previous interface. But you need to return additional attributes, for example, its salary. But there is no Salary Attribute on the User POJO. And also, to calculate the salary, you system needs to call an external system. Your business logic will take care of that and then return the condensed object (known as Data Transfer Obejct) to the caller.
Some resources:
https://blog.cleancoder.com/uncle-bob/2012/08/13/the-clean-architecture.html
