In my Repository, I'm calling external REST API and properly retrieve response which I wrap in ResponseEntity object as below:
ResponseEntity<ExternalModelResponse> response = restTemplate.getForEntity(baseUrl + "/api/externalObject", ExternalModelResponse.class);
However, ExternalModelResponse doesn't follow my Domain model so I want to introduce mapping of the ExternalModelResponse -> Domain model.
What would be the correct place to introduce such mapping? Should:
Repository method already return Domain object? That would imply mapping in the same method which fetched object from external REST API.
Repository method return ExternalModelResponse and let Service handle the mapping?
What is the most common place of such mapping?:
added as toDomainEntity method on ExternalModelResponse?
added as fromExternalModelResponse method on Domain Entity?
added as a method on Repository/Service ?
The service layer should use only domain objects, so I've done this mapping in the controllers.
I use SpringBoot and I have some Reactive APIs (Controller call a service which call a repository based on MongoReactiveRepositories)
As I don't want the user to directly see my models objects, I would like to have DTOs.
One DTO can be based on the information of several models.
Ex: Given two Model Items
ItemA:
infoA:String
ItemB:
infoB:String
I want to create a single DTO item
ObjectDTO:
infoA: String
infoB: List<ItemB>
What is the proper way to reactively map data between my objects model and a DTO?
Just use Mono.zip(https://projectreactor.io/docs/core/release/api/reactor/core/publisher/Mono.html#zip-reactor.core.publisher.Mono-reactor.core.publisher.Mono-java.util.function.BiFunction-) to combine 2 results into one.
return Mono.zip(repo1.getItemA(), repo2.getItemB(),
(itemA, itemB) -> new ObjectDTO(itemA.getInfoA(), itemB.getInfoB())
);
The methods returning itemA and itemB should ofcourse return a Mono.
I'm using spring mvc.
I have created the controller, view, pojo, dao.
Now I have the need to create an object composted from multiple objects pojo, is the case of creating a DTO?
If you're looking to build a composite kind of Object for view purposes only, then there is a good argument for a DTO. If the composite is just an aggregation of the POJOs you can use org.springframework.ui.Model and just add attributes inside your Controller. If there is logic and business rules that need to be applied, it is probably best to do this in a Service layer that sits between your Controller and your DAO.
If you mean that you need to access properties of few POJOs on the client side and you want to reduce amount of calls from
client to server then yes. It is better to create a DTO object where place only necessary properties from POJOs that you will
use on client side. And return this DTO as a result of a single call from client to server.
I have several thoroughly unit-tested and finely crafted rich DDD model classes, with final immutable invariants and integrity checks. Object's instantiation happens through adequate constructors, static factory methods and even via Builders.
Now, I have to provide a Spring MVC form to create new instances of some classes.
It seems to me (I'm not an expert) that I have to provide empty constructor and attribute's setters for all form's backing classes I want to bind.
So, what should I do ?
Create anemic objects dedicated to form backing and transfer the informations to my domain model (so much for the DRY principle...) calling the appropriate methods / builder ?
Or is there a mecanisms that I missed that can save my day ? :)
Thank you in advance for your wisdom !
The objects that are used for binding with the presentation layers are normally called view models and they are DTOs purposed toward displaying data mapped from domain objects and then mapping user input back to domain objects. View models typically look very similar to the domain objects they represent however there are some important differences:
Data from the domain objects may be flattened or otherwise transformed to fit the requirements of a given view. Having the mapping be in plain objects is easier to manage than mappings in the presentation framework, such as MVC. It is easier to debug and detect errors.
A given view may require data from multiple domain objects - there may not be a single domain object that fits requirements of a view. A view model can be populated by multiple domain objects.
A view model is normally designed with a specific presentation framework in mind and as such may utilize framework specific attributes for binding and client side validation. As you stated, a typical requirement is for a parameterless constructor, which is fine for a view model. Again, it is much easier to test and manage a view model than some sort of complex mapping mechanism.
View models appear to violate the DRY principle, however after a closer look the responsibility of the view model is different, so with the single responsibility principle in mind it is appropriate to have two classes. Also, take a look at this article discussing the fallacy of reuse often lead by the DRY principle.
Furthermore, view models are indeed anemic, though they may have a constructor accepting a domain object as a parameter and a method for creating and updating a domain object using the values in the view model as input. From experience I find that it is a good practice to create a view model class for every domain entity that is going to be rendered by the presentation layer. It is easier to manage the double class hierarchy of domain objects and view models than it is to manage complex mapping mechanisms.
Note also, there are libraries that attempt to simplify the mapping between view models and domain objects, for example AutoMapper for the .NET Framework.
Yes you will need to create Objects for the form to take all the input, and the update the your model with this objects in one operation.
But I wont call this objects anemic (especially if you do DDD). This objects represent one unit of work. So this are Domain Concepts too!
I solved this by creating a DTO Interface:
public interface DTO<T> {
T getDomainObject();
void loadFromDomainObject(T domainObject);
}
public class PersonDTO implements DTO<Person> {
private String firstName;
private String lastName;
public PersonDTO() {
super();
}
// setters, getters ...
#Override
public Person getDomainObject() {
return new Person(firstName, lastName);
}
#Override
public void loadFromDomainObject(Person person) {
this.firstName = person.getFirstName();
this.lastName = person.getLastName();
}
// validation methods, view formatting methods, etc
}
This also stops view validation and formatting stuff from leaking into the domain model. I really dislike having Spring specific (or other framework specific) annotations (#Value, etc) and javax.validation annotations in my domain objects.
What is the difference between a MVC Model object, a domain object and a DTO?
My understanding is:
MVC Model object:
Models the data to be displayed by a corresponding view. It may not map directly to a domain object, i.e. may include data from one or more domain objects.
Client side
May contain business logic. Eg. validations, calculated properties, etc
No persistence related methods
Domain object:
An object that models real-world object in the problem domain like Reservation, Customer, Order, etc. Used to persist data.
Server side
No business logic
DTO (Data Transfer Object):
An object used to transfer data between layers when the layers are in separate processes, e.g. from a DB to a client app. Allows a single transaction across the wire rather than multiple calls when fetching data corresponding to multiple domain objects. A DTO contains just data and accessor methods and there is no logic present. The data is for a particular DB transaction, so it may or may not directly map to a domain object as it may include data from one or more domain objects.
Used on both server and client sides as it is passed between layers
No business logic
No persistence related methods
So, the questions:
Is above understanding correct? Am I missing some key points?
Are there any reasons not to use Domain objects as the MVC Model assuming that the Model objects do not require extra business logic?
Are there any reasons not to use DTOs as the MVC Model assuming that the Model objects do not require extra business logic?
Domain and model objects are essentially the same, and may contain business logic. Depending on implementation, domain and DTO objects may be equivalent if you remove business logic from the model into a service class.
Often a key variant of the DTO is the View Model, which is used purely to transfer data between the domain model and the view, although often a View Model may contain logic, although this should be purely UI logic.
The Domain and DTO can also be your "model" objects - you can have a view to render the details of the "Customer" domain object.
A domain object can have business logic to enforce the properties of the domain entity. validation is one such case. The domain object by itself does not contain persistence related methods, but it can have meta-data (like annotations) to support persistence
the POJO programming model makes it possible to use the same object as your domain, DTO and model objects - essentially, you will not be implemented any extraneous interfaces that will only apply to one layer but does not apply to others.
A DTO = is an object that carries data between processes.
But the most interesting part is that, it does not have any behavior except for storage and retrieval of its own data!!!
Sticking with the MVC methodology...
Domain = subject of your entire application.
Model = contains the (programming languages objects : EX: C# objects) to make up the universe of your application.
They can obvioussly have behaviour and properties(see difference with DTO).
Often an application (a light one) can have one model - case in which your model is exactly your domain.
Another model can be, a totaly different object type, that is processing another one. Both of them, in this case are part of your domain, and are named "domain models - objects".
Hopefully this answer is exhaustive and makes it all clear for you !
My understing (in a big short) is as follows:
(MVC) Model object:
represent some things in a some usage context eg. PersonEditModel, PersonViewModel or just PersonModel
has no business logic
can be subject of some valdation logic etc.
used to provide data from one application layer to another eg. MVC Controller <-> MVC View
Domain object:
represents some business object (real world object in the problem domain)
has business logic
do not allow invalid object state, has methods to properly change object's state
used to encapsulate business logic related to it
have not to be used to persist data (or even should not)
DTO (Data Transfer Object):
similar to Model object but should have flat structure
only simple type properties/fields (strings, numbers, datetimes, booleans)
used to transfer data cross application boundaries eg. between web server and web browser
Any definition for most of the objects is various based on place of using of objects:
Model: is a general definition for using object in client or server.
Model View : is a object using in client most of the time.
Domain Object : is a object using in server and transfering data to the database.
Data Transfer Object(DTO) : is a object that transfer data from one object to another object, specially in getting data in API Call(for example: in api GET Method call for getting data you must not to give database models to client, for this purpose you use dto).
Notice: the definitions are true most of the time but in some situations arent't practical.
MVC and DDD can be used together. What we call "Models" both in DDD and MVC are virtually the same: abstractions. Using pseudo-code we can illustrate a few examples.
Model View Controller (MVC)
The Model View Controller ARCHITECTURE separates the software into three parts:
The Model Layer
The Model layer from the MVC Architecture is where the logic resides. In this layer we have our models and business logic.
class Car {
String color;
String year;
Cat(color, year) {
this.color = color;
this.year = year;
}
//getters & setters
}
A simple Car abstraction.
class CarService {
save(car) {
if(car.getColor() != null && car.getYear() != null) {
methodToSave(car);
} else {
throwsException();
}
}
find(car) {
return methodToFind(car);
}
update(car) {
assertThatExists(car);
methodToSave(car);
}
delete(car) {
assertThatExists(car);
methodToDelete(car);
}
}
A simple CRUD for Car using a Service
The View Layer
The View layer is where the user interface resides. Here's where the user can interact with the system, which will then trigger the Controllers on actions performed, which will then inform the Model layer and request data. The View Layer can reside either in the client-side of the application or the server-side of the application (ie: JSF (Java Server Faces) as Server-Side, ReactJS as Client-Side). By any means, even if the View layer resides on the client side, the client will need to request the server-side for sending requests. This may be done by HTTP requests for a Web-Based Application.
<theCarPage>
<theCar>
getTheCarOnLoad();
</theCar>
</theCarPage>
A pseudo-page for the Car.
The Controller Layer
The Controller layer basically receives input from the View and then convert and send the data to the Model Layer and vice-versa.
class CarController {
#OnLoadingTheCarPage
getTheCarOnLoad() {
return theCar();
}
}
The method to load the Car.
Domain Driven Design (DDD)
Domain Driven Design is a concept:
DDD lays it's foundations in the concept that classes, class variables and class methods must match it's core business domain.
Domain Driven Design Resides into the "M"
In this case, when MVC is applied, the Domain Driven Design resides into the Model Layer of the MVC Architecture. As explained before, the Model Layer is where the Business Logic of the application resides.
Either if you have entities or not, they still are Models. A Model is just an abstraction of something in the real world. A cat can be a Model if abstracted:
class Cat {
String color;
String age;
Cat(color, age) {
this.color = color;
this.age = age;
}
//getters & setters
}
Simple Cat abstraction. It is a Model of Cat.
DDD Entities
In Domain Driven Design we have Entities, that are also classified as Models. The difference between them is that Entities are identifiable. If you have a class that is identifiable and can be persisted, then it's an Entity. An Entity still, is a Model.
#AnEntity
#ThisCanBePersisted
class Cat {
#ThisIsAnId
#ThisValueIncrementsAutomatically
#PersistentProperty
Long id;
#PersistentProperty
String color;
#PersistentProperty
String age;
Cat(color, age) {
this.color = color;
this.age = age;
}
//getters & setters
}
A simple Entity. An Entity is a Model.
Data Transfer Objects (DTO)
Data Transfer Objects have no logic inside them and the only use to them is to be containers for transferring data from one endpoint to another. Usually Enterprise Entities are not Serializable by nature, so we need a way to send only the data that we need to be sent to a client.
Since a Model could have sensible data or simply data we don't want to share in a request for a fetch, for instance, then considering our Cat Model, we could create a DTO that does not share the Cat's ID:
class CatDTO {
String color;
String age;
//getters & setters
}
A Data Transfer Object for Cat. We only need it's properties and something to get and set the properties. We don't want to share it's ID.
So if we, for instance, had to request a list of all cats from our client using REST, then we would request the endpoint that responds with our CatDTO instead of our Cat Entity:
[
Cat {
"color": "yellow",
"age": "1"
},
Cat {
"color": "black",
"age": "4"
}
]
And that would be all the data that our client could see.
1) No, It is a definition of ViewModel. MVC Model Object and Domain Object both are same.
2) Domain Models (Objects) are always present, business logic is optional
3) If there is no business logic in Domain Object then automatically it becomes DTO.