Need for a domain model in a service in 3 tier architecture?

Question

I am building an API Rest with Spring Boot and I would like to clarify a concept about my architecture, to see if the community can help me.

Imagine that in my database I have a table called Person. I am mounting an architecture based on three layer architecture. The general scheme would be as follows:

• On the one hand I have the PersonDao.java, which will be in charge of accessing the database to retrieve the tuples from the Person table. To do this, it uses a class called PersonEntity.java, which contains (as attributes) all the columns of the table. PersonDao.java is going to return an object from the PersonModel.java class. PersonModel.java is the class that represents the Person business model.

• On the other hand, I have the PersonService.java, which is responsible for carrying out the business logic of my application. This service calls PersonaDao.java to access the information stored in the database. PersonService.java works with objects of the PersonModel.java class, since this class represents the business objects of my application. PersonService.java will always return a PersonDto,java.

• Finally, PersonController.java. This controller will be the one that exposes the connection interface of the Rest API. He always works with DTO and communicates with PersonService.java through DTO as well.

PersonController <-> (PersonDto) <-> PersonService <-> (PersonModel) <-> PersonDao <-> (PersonEntity) <-> DB

The question is: Is it necessary to use the PersonModel.java class so that PersonService.java only works with objects of this class? Or would it be better for PersonService.java to work directly with objects from class PersonEntity.java? If I do it this way it is to maintain the principle of single responsibility, so that each layer only works with objects of its scope.

If the answer is that PersonModel.java is necessary to maintain the principle of single responsibility of each layer. Would something change if JpaRepository were used? If I ask this question it is because in many tutorials and examples I see that when JpaRepository is used, the services work directly with the entities. In this case, shouldn't we create a class that represents the business object for the services?

EDIT: In the answer to this question (https://stackoverflow.com/questions/34084203), the architecture that makes sense in my head would be reflected, but surely it is not the most correct thing. The conclusion I come to is that each layer would use its own kind of objects. Copy / paste of the answer:

Typically you have different layers:

• A persistence layer to store data
• Business layer to operate on data
• A presentation layer to expose data

Typically, each layer would use its own kind of objects:

• Persistence Layer: Repositories, Entities
• Business Layer: Services, Domain Objects
• Presentation Layer: Controllers, DTOs

This means each layer would only work with its own objects and never ever pass them to another layer.

Thanks in advance.

Answer 1

In domain driven design, the "entity" is the core business object which contains both data and related business logic. In your example, the combination of PersonModel and PersonService is the entity (I would just call it Person).

What you call PersonEntity is an implementation detail of the DAO/repository. It is not part of your domain, and you may not have this intermediate mapping object depending on your mapping technology (for example, you mentioned JPA, you might just directly annotate your real Person entity in that case).

DTOs are also not part of the domain layer; as the name suggests, they are for data transfer to and from the outside world. You want have a separate application service which uses DTOs for input/output and internally loads entities from the repository and invokes methods on them.