According to many programers, DAO layer can be bypassed when using JPA.
While using DDD approach, domain layer composes of infrastructure region (containing external resources like repositories implementations) and domain region (with entities, needed value objects and repositories interfaces and services etc...).
Thus, if DOA layer is skipped, should the infrastructure region be part of domain layer within a package called 'infrastructure' for instance ?
If infrastructure part should be moved in a separated layer (separated project to get things cleaner), are circular dependencies acceptable between domain layer and infrastrure layer? Indeed, entities and interface repositories must be shared.
Otherwise, should I separate entities and repositories interfaces from Domain layer in order to be considered as an independent thing shared by domain and infrastructure ?
What is the good practice ?
In DDD, the data access objects (DAOs) are Repositories. There's no "DAO layer", persistence is part of the Infrastructure layer.
As you mentioned, the contracts (interfaces) of Repositories are defined in the Domain layer while their concrete implementation resides in the Infrastructure layer.
There's no need for the Domain layer to reference the Infrastructure since entities are supposed to be pure domain objects, unaware of how they are persisted, transferred to other systems, and so on.
"In other words, each of the layers uses an abstract interface that
represents it's infrastructure needs. It does not know what
infrastructure it will be using. It simply states it's needs through
an abstract interface and expects the infrastructure to implement that
interface and supply the required functionality."
http://www.artima.com/weblogs/viewpost.jsp?thread=35139
A DAO layer and a infrastructure region/domain region are not the same things. DAO layers are used when implementing the infrastructure/domain regions.
Your programmers are correct, JPA is the DAO layer. You still need the infrastructure region and domain regions. They'll just be smaller/differnet then if you had to implement a DAO layer within these two regions.
Related
I am trying to follow clean architecture in my software. I have implemented 3 layers: data layer, business layer and presentation layer.
As far as I understand dependency will follow outside to inside like P->B->D.
But my question is should I do a singleton data layer executor injection into presentation? Doesn't that break this logic?
Or without DI, only create an abstraction between layers I think creates tight coupling.
So referring to some data layer dependency from inside the business layer - doesn't that make the layers tightly coupled?
public class ViewModel<T> extends GenericRouter {
IPresentation ip = new BusinessUseCaseImpl();
public abstract class BusinessUseCase<T extends HashMap> implements IPresentation<T> {
UserRepository urepo = new UserRepository();
In the clean architecture, the inner layers are logic, not infrastructure.
When the business layer instantiates the data layer, it injects any infrastructure dependencies that it requires.
Similarly, when the presentation layer instantiates the business layer, it passes in any infrastructure dependencies that it requires. This is how the business layer gets the infrastructure it needs to pass to the data layer.
And initially, when the application/system instantiates the presentation layer, it passes in the infrastructure that it will need. The humble object pattern should be used to implement these dependencies, because they are the only part of the system that can't be tested independently from infrastructure.
It depends on what you mean by "tight coupling".
Following the 3 layered architecture is ok for the data layer to be a dependency on the business layer and the business layer to be a dependency on the presentation layer.
Instead of injecting a concrete class, you can always define the dependency injection on an Interface. With this, you can always switch the implementation as long as you follow the agreed interface.
However, my experience tells me that you rarely or never do this for a basic and regular 3 layered architecture. You have always a single Service and a single Repository/DAO implementation so having an Interface for each of them is kind of redundant and useless.
I think that the most important thing when you work with 3 layered architecture is to never bypass layers. Examples:
You should never make the presentation layer depend directly on the data layer. Always go through the business layer.
The relation between the layers should be always 1:1. This means you should never make a Controller call a Service that is associated with another Controller.
Not following these rules will for sure make your code incredibly hard to read and understand. Of course, for every rule, there is an exception, but try to stick to them.
Using Spring MVC to develop a web application. Earlier I was using Controller layer, Service layer(business logic), Model layer(entity) and DAO(DB) layer.
But someone pointed out that i should introduce two more layer ie. dto layer for collecting data from front end and transforming layer which will convert than dto into model(entity) layer objects.
Now I am using:
Controller layer (which will send the data to DTO layer)
DTO layer (which will send the its data to transforming layer)
Transforming layer(for converting dto layer objects into entity layer objects)
Service layer(Business logic)
Entity layer(POJO which will map with database)
DAO(which will use entity objects to store the database)
In this way we can keep front end and backend data different. Please help me out, is this a proper structure for Spring MVC ?
There is an interesting question answer thread on MVC here:https://softwareengineering.stackexchange.com/questions/127624/what-is-mvc-really
Some of the key points to remember when designing an application should be; layered and loosely coupled.
In your scenario, having additional transform layer does not necessarily make our break MVC pattern. It is just an additional layer you have introduced in MVC; a design strategy followed by many.
DTO is just a pattern to encapsulate data. Normally is used to be returned in your controllers, but you don't need always to create a DTO, you can use your entities for this. I just use a Dto when i need a different data structure, that any entity support, like a report for example.
About your project layers Model/Dao/Service/Controller it's correct, i strongly recommend for you the book Domain-driven design, it's will help you to build your software architectures!
But someone pointed out that i should introduce two more layer ie. dto layer for collecting data from front end and transforming layer which will convert than dto into model(entity) layer objects.
No, it's incorrect. You should use only layers that are necessary.
Earlier I was using Controller layer, Service layer(business logic), Model layer(entity) and DAO(DB) layer.
This is also incorrect. There's no strict definition of MVC is a pattern, but it's used only on one layer called a view layer, or in other terms presentation layer. Where the model, view, and controller reside. There's another layer called a service layer, which is optional. Then persistence layer, in this layer you model your objects and save them in the database or somewhere else. No more layers are necessary to the simple web application.
A few words about model, it's a set of classes that you want to persist or use in the view layer to show/store the data. Here you can play with it, because someone prefer to use it's own model to show/store the data, and others prefer their own model to persist it. These transformations are usually done in persistence or service, or in both layers. Keep in mind that all layers a loosely coupled to each other, but you can transfer data beans from one layer to another and back without problems. Entities in the detached state or other beans like DTOs are examples of such objects that you can pass and bake between layers. And no additional layers are necessary.
I'm starting with DDD and you can image my brain is boiling.
My question is related to my domain objects (entities, VO, ...) which represents my domain concepts/logic and how to persist/retrieve them.
The blue book says the repository is a way to represent collections on domain objects and is responsible to communicate with the infrastructure layer. I read also at some post the infrastructura layer is where you must use hibernate, JPA or whatever.
Then I see this Spring-data-jpa example http://spring.io/guides/gs/accessing-data-jpa/ and I become crazy.
The slogan say Spring-data-jpa is to create repositories easily and the previous samples seems to merge JPA annotations into a domain object (the customer).
Is the sample right? or Am I right?
If I'm right and the domain and infrastructure must be separated, that means to store a customer I must have:
a Customer class in my domain layer (that represents a customer and has all the logic operations)
a CustomerRepository un my domain layer (that retrieves or stores customers from infrastructure layer)
a Customer class in infrastructure layer, probably annotated with #Entity
Some CustomerReposityJPA that know how to store/retrieve customers from database.
Thanks for any clarification.
In DDD, a repository is an object that participates in the domain but really abstracts away some backing store.
If you annotate your domain objects with JPA annotations, your persistence mechanism has bled into your domain. You have tied your domain structure to your persistence structure which is not ideal.
Your JpaCustomerRepository (implements ICustomerRepository) could map the unannotated domain classes (Customer) into an annotated JPA representation - JPA customer. This keeps the annotations out of your domain classes, so is cleaner. It allows you to vary the JPA persistence structure independently of your domain structure. The cost for this benefit is the complexity of the mapping code.
interface ICustomerRepository {}
class JpaCustomerRepository implements ICustomerRepository {
void save(Customer customer) {
JpaCustomer jpaCustomer = map(customer);
save(jpaCustomer);
}
}
class Customer {}
class JpaCustomer {}
I can't see the link you've posted and I have never apply Domain driven design to the Java world.
Theoretically what you need is Customer aggregate in the domain layer. In your domain layer there's space for the repository (intended as interface), so you'll have ICustomerRepository.
Probably you will have the four prototype for the common persistence issues:
GetById(CustomerId id);
Add(Customer c);
Delete(Customer c);
Update(Customer c);
In the infrastructure layer you will provide the body (for instance CustomerRepository), in the infrastracture layer you can couple yourself with something technological (for example JPA).
Domain layer MUST be completly unaware of technology used in the infrastructure. Doing so you could change completly the implementation details having (almost) no troubles.
We are building an enterprise app that uses Spring MVC in presentation layer, EJB in business layer and JPA for data access. The app needs to support search function for various entities based on several search criteria, as entered by user from GUI. This seems to be fairly common function in most of the enterprise app, and I am wondering if there is any design pattern/principle that provides best approach.
For now, we are having SearchCriteria classes for each business entity (e.g. AccountSearchFilter for Account). Presentation layer builds the object of SearchCriteria class and pass it over to business layer. The business layer builds a JPA query based on the available criteria and creates a collection of entity objects. The collection is passed back to presentation layer, which is then transformed to JSON by presentation layer.
I have two problems with this design:
SearchCriteria classes are part of business layer, but they are used by presentation layer as well.
Business layer returns collections of business entities to presentation layer.
I don't like to share classes among presentation and business layers, but I don't know what we can do to decouple it. I have already searched the existing posts regarding using domain objects in presentation layer, and I don't want to start a debate whether it is good or bad. I have already made up my mind that I don't want presentation layer using domain objects. Also, creating DTOs seems to duplicate code, so if that is the only feasible solution, then I may pass on this one.
What is DAO and Service layer exactly in Spring framework?
I am looking for theoretical answer.
There is no distinction as far as Spring is concerned. By convention you can mark DAO classes with #Repository and services with #Service. Also the former does some persistence layer exception translation.
Since you are asking theoretically: DAO should perform raw database operations and translate them to some higher level constructs (objects, collections). Services should call DAOs and perform business operations. Typically transactions demarcation is performed on service layer to span several DAO calls.
Finally DAO should abstract business logic from persistence details, ideally allowing to switch persistence layer without business logic (services) changes. This is hardly ever possible due to leaking abstraction of persistence providers (e.g. lazy loading).
DAO - data access object, are object to handle connection to your data storage (typicaly database). You have here your queries and DAO provides data to your services.
Services should contain all your logic. If you have logic separete you can theoretically change your UI layer or DAO layer without you affected it.
It gives decoupling benefits. When source of data changes the way you process data in Service for all service users (mobile client, web-client) does not change. But you need to change the way you extract data from data source.
DAO(Data Access Object) is a design pattern, which consists on creating for each table on your database a class,it provides a technique for separating object persistence and data access logic