Currently i'm part of a project (spring with JPA) that has the following layering:
service layer (checks for authorization and delegates actions to
the manager layer)
manager layer - does lots of buisiness logic
and then calls: entityManager.save(object)
As you can see there is no DAO layer in this project and as a DAO layer fanatic i'm starting to fall in love with this setup instead.
what are the pros \ cons of this setup?
Should we bother with dao?
Separation of concerns is the word that you should think before falling in love with this approach. In general practice business logic and dao logic should be kept separate as in case if in future you decide to change your database you need not go through every class searching for change the way to access new database but in DAO layer approach you need to change only your DAO layer as business layer remains intact.
Related
I understand from documentation the distinction between entity and service creation. I'm tempted to simply decorate my services with #RestController/#RequestMapping/#[http-method]Mapping in the same way that /web/rest/resources/* are decorated to expose new rest api methods... maybe using "/api/srv/" to distinguish service-level api resources? It seems to work... when I do it the decorated service methods show up in swagger, etc. I guess I'm just looking for a sanity check that I'm not violating framework convention or misunderstanding the intended use of /service classes.
This really comes down to separation of responsibility. You can expose any class as #RestController no matter it is real Controller or a Service or even a Dao.
However, most of the time, we want each layer to do its own thing.
Controller layer handles the http request/response(request param/body, response code/body etc...) and dispatch to different Service layer methods.
Service layer takes care of real business logic and make calls to the DAO layer to retrieve data needed for business logic calculation.
Dao layer eventually has the responsibility access database with query/hibernate/jpa specifics.
As you can see, each layer is trying to do its own thing and isolate the dependencies from other layers. Like controller layer handles http related stuff so that other layers do not have to carry Servlet dependencies. Same thing for DAO, so other layers need not know any detail about data persistence.
As for swagger, it just analyze your code and annotations and expose whatever in the controller layer. :)
All in all, IMHO, it is still recommended to annotate your real Controller classes with #RestController and let your Service handle the real business.
I am using spring 3.0 with annotation which i am creating dao interface , then dao implementation class,
then creating service interface and then creating service implementation class.
and using object of service implementation into controller. this process is too lengthy.
can i use only dao class and directly use it in controller?
#service
#controller
public class MyController { ... }
You can but you really really really shouldn't. If you have any plans of putting this code into production, then you need to keep your layers well defined. This is because each layer has a different responsibility.
Your DAO layer is responsible for all database access. It doesn't care what object you want, what you want to do with it, and when you want it done. It only cares about how it's done.
Your Service layer is responsible for what you want to do to your objects. It contains all of your business logic, convenience methods and may use multiple DAO's inside a single service. Your service layer is where you handle your database transactions as well.
Your controller layer is responsible for how you want to show your data to the user. It only contains services and minimal logic concerned with how to display the data returned by your service layer.
I think there are 2 distinct questions being asked here :
Is it really necessary to have all three layers : web layer, service layer, and DAO ?
and :
Do Spring beans necessarily need to implement and interface and to bve (auto)wired by interface rather thant concrete class ?
The answer to both is the same : it is not a necessity, but it is strongly recommended for anything other thant trivial projects.
For the first question, yes it is better so separate concerns in different layers. However I must admint that my controllers sometimes access the DAOs directly for simple read-only tasks in some projects. Sometimes the service layer seems like too much when all it does is to map the DAO methods. However I always use a service layer for transactional business logic that actually modifies the data.
For the second question : Spring can, without problem, instantiate and inject beans that don't implement any interface. However you will start to have problems if you use more advanced stuff linked to aspect-oriented programming, the most common being the #Transactional annotation. In this case Spring has to create proxies to your objects, and it is all simpler to create proxies of interfaces. Ohterwise it has to manipulate bytecode and it introduces limitations.
Perhaps more importantly, "programming to an interface" is a good practice regardless if you are using Spring or not, or even Java or anothrer language. A simple google search will bring up many good articles on this, or SO questions such as this one.
So again, while you can live well without both things in the short term, both will make your life much better in the long term.
I have a project in which I am using NHibernate and ASP.Net MVC. The application is intended to allow users to track certain data and then produce views of statistics based upon the data entered. The structure of my application thus far looks something like this:
NHibernate Layer: Contains Repository<T> and UnitOfWork classes, as well as entity mapping definitions.
Core/Service Layer: Contains generic EntityService class. At the moment, this simply defines transaction scope via IUnitOfWork and interfaces with IRepository to provide higher-level data access services.
Presentation Layer (MVC Application): Not yet implemented, but contains the usual stuff plus dependency injection.
I have a couple of questions:
Is it poor design to allow my MVC application to handle dependency injection for ALL layers? For example, as well as dependency injection of EntityService instances into controllers, it will handle the dependency injection of IRepository into the EntityService classes. Should the service layer handle this itself, even though this would mean performing dependency injection in two distinct places?
Where should I produce my statistics? This business logic doesn't seem to belong in my service layer, which, at present, only contains entity type definitions and an interface for modifying and accessing entity properties. I have a few thoughts on this, but I'm not sure which I like best:
Keep my service layer as is and create a separate Statistics project - this is completely independent of the entity types for which it will be used, meaning my MVC controllers will have to pass raw numerical information between my business entities and my (presumably static) statistics classes. This is quite a neat separation but potentially means a lot of business logic still remaining in the presentation layer.
Create a Statistics project; however, create a tight coupling between the classes in this project and my business entities. For example, instead of passing a Reading object's values into a method, I will pass the entire object (or define them as extension methods). This will shift business logic out of my MVC app but the tight coupling seems a bit messy.
Keep all of my business logic inside my service layer. Define strongly-typed subclasses of EntityService, so my services contain both entity-specific business methods and data storage methods, while keeping the entity classes themselves as pure data containers. Create a separate Statistics project for any generic statistical processing and call its methods via my derived service classes. My service classes effectively merge business functions with the storage functionality provided created by IRepository<T>.
I am erring toward the third option but does anyone have any thoughts? Alternative suggestions?
Thanks in advance!
Preliminary observation:
I like the way in which you described your project, I just didn't get why your Data Access Layer (DAL) is called NHibernate Layer: it is odd with all the rest in which you didn't use technology name to describe a logical layer (correctly). So I suggest you to rename it DAL, and use it to abstract your app from NHibernate.
My opinions about your questions:
Absolutely no. It is good to apply Dependency Injection to All Layers. A couple or reasons for which it is good:
1.1 Testing: you can mock DAL interfaces and do unit test Service Layer w/o DAL using another DI config file. In the same way you can mock Service for Web Controllers layer and so on.
1.2 Different DAL implementations: suppose you need different DAL implementation (NOSQL, SQL or LINQ instead of NHibernate, etc..) technologies for different deployment of you project or to scale in the future. You can do that easily maintaining different DI config files.
You can have the same layer deployed in different projects. In the same way you can have a project containing different layers. I think their relation is orthogonal: project is describing a physical (development time and run time) implementation. Layers are logical. So initially I would keep it simple with the third option.
I just don't understand why you saying the following regarding this option:
Create a separate Statistics project for any generic statistical
processing and call its methods via my derived service classes. My
service classes effectively merge business functions with the storage
functionality provided created by IRepository.
I see Statistics as one or more services so you can implement it as namespace with classes inside your Service Layer. And, as any other service, you can inject DAL Repository classes. And, as any other Service/DAL, the Model classes can be shared between different Services and DAL classes.
StatsService.AverageReadingFor(Person p, DateTime start, DateTime end) sounds good.
There are several implementation options:
Using underlying repository features (for example: SQL avg function)
Using Observer Pattern which is implementable also using Dependency Injection
Using Aspect Oriented Programming. See that Spring.Net chapter as an example.
If you have more than one Service Layer instance (more than one server) than 2 and 3 must be adapted for out of process communication using a messaging system.
Just an update - Regarding my second question, I have decided to define an IStatsService<T> which expects an IEntityService<T> to be passed into its constructor. I'll use this for generic statistical processing of business entities and create further interfaces that implement IStatsService<T> where I need more type-specific information.
Hopefully this will help someone who has been scratching their head about a similar problem!
Folks,
Apologies if this has been covered in another thread, but I have searched ddd and mvc articles and have not found a straightforward answer.
I am hoping to apply a DDD approach to the architecture of my MVC projects. Please correct me where I am wrong.
All MVC controller actions that involve hitting the domain model will initially hit
and application service layer.
The application service layer here acts as a facade between presentation and the domain.
Any requests from the application service later that clearly involve discrete domain aggregates will perform fetch or modify operations on aggregate roots using repositories. Each aggregate root will have its own repository.
so the application service layer must be injected with any/all repositories required by the domain.
Where an operation may involve multiple aggregates or requires logic that does not fit neatly into one aggregate, the application service will call a domain service to carry out operations across aggregates.
This does not seem right to me.
My confusion is that from a DDD perspective Im not sure whether for example aggregate roots should perform their own persistance i.e. the aggregate gets injected with a repository and then persists/fetches itself or whether as above the application service layer uses repositories to act on or fetch aggregates?
Also if the application service layer is injected with all repositories, does the domain service that the application service layer calls also need repositories injected?
Im keeping CQRS out of this at this point. I want to get the layering and the relationship between services and aggregates sorted out first.
Thanks for any advice.
All MVC controller actions that involve hitting the domain model will
initially hit and application service layer. The application service layer here acts as a facade between presentation and the domain.
There's debate over that but I would consider carefully whether that additional layer is needed or not. It adds a lot of boilerplate code and degrades maintainability - as someone pointed out recently, when you separate things that change for the same reasons (ie your service methods and the corresponding domain methods), you have to make changes in many different places in the system.
On the other hand, you could need that service layer to map your domain objects to DTOs but there again, it could be done directly in the Controller and nothing forces you to use DTOs in the presentation layer.
My confusion is that from a DDD perspective Im not sure whether for
example aggregate roots should perform their own persistance i.e. the
aggregate gets injected with a repository and then persists/fetches
itself or whether as above the application service layer uses
repositories to act on or fetch aggregates?
It's usually considered bad practice to have aggregate roots manage their own persistence because it breaks persistence ignorance and violates the Single Responsibility Principle. If you do that, your aggregate root class now has 2 reasons to change, 2 reasons to break the code, it is less maintainable, etc.
You should instead delegate the responsibility of saving the aggregate root in its repository to an object that will be aware of the application execution context (for instance, a Controller or an object in the Application layer).
Also if the application service layer is injected with all
repositories, does the domain service that the application service
layer calls also need repositories injected?
Yes, I think it pretty much makes sense especially if the domain service heavily relies on the repository.
Firstly it is possible that I am asking something that has been asked and answered before but I could not get a search result back. We define transactional annotations on service layer typical spring hibernate crud is usually
Controller->Manager->Dao->Orm .
I now have a situation where I need to choose between the domain model based on client site.
Say client A is using my domain model all is good but then an other client site would give me a web service and not be using our domain model.
Which layer should I be replacing . I believe it has to be DAO which will be getting me data from web service and sending it back.i.e two separately written DAO layers and plugged in based on scenario.
I have now realized that we have been doing tight coupling (if there is such a thing or say not having loose coupling) when we put #Transactional in Service layer. So many brains can not be wrong or are they (I doubt it).
So question is "Where should "#Transactional" be placed Service Layer or DAO ?" and is it service layer downwards I should be replacing.
Eleven years on and still relevant . If I look back at the project somethings were obviously wrong with my understanding of Domain model back then . I was regarding ORM layer as domain model and we wanted to work with ORM and detached entities and no have any data mapping and not have any DTOs. That was the trend those days. These days Domain Model is not the ORM and having a proper Domain model and using ORM or Webservices are datasources take care of this issue. Like many pointed out yes Service is the right place for it and have proper domain model and not regard JPA (ORM) as domain model.
Ideally, Service layer (Manager) represents your business logic and hence it should be annotated with #Transactional.
Service layer may call different DAOs to perform DB operations. Lets assume a situation where you have 3 DAO operations in a service method. If your 1st DAO operation failed, other two may be still passed and you will end up with an inconsistent DB state. Annotating Service layer can save you from such situations.
You are going to want your services to be transactional. If your DAOs are transactional, and you call different DAOs in each service, then you would have multiple transactions, which is not what you want. Make the service calls transactional, and all DAO calls inside those methods will participate in the transactions for the method.
i will suggest to put #Transactional in Service layer methods since we can have multiple DAO implementations. by using this we can made our services will be transactional. refer
best practice is to use A generic BasicService to offer common services.
The Service is the best place for putting #Transactional, service layer should hold the detail-level use case behavior for a user interaction that would logically go in a transaction. in this way we can have maintain separation between web application code and business logic.
There are a lot of CRUD applications that don't have any significant business logic, for them having a service layer that just passes stuff through between the controllers and data access objects is not useful. In these cases we can put transaction annotation on Dao.
So in practice you can put them in either place, it's up to you.
By having multiple calls in your service you need #Transactional in service. different calls to service will execute in different transactions if you put #Transactional in service.
It's of a personal choice based on application types, if application is layerd across many modules and majority of operations are #CRUD based ,then having #transactional annotation at service level makes more sence.. engine type application like schedulers , job servers,#etl report apps, where sessions and user concept does not exists, then propagational transaction at context level is most suitable... we should not end up creating clusterd transactions by putting #transactional every where ending up transactional anti patters...anyways for pragmatic transaction control JTA2 is most suitable answer...again it depends on weather you can use it in a given situations...
You should use #Transactional at service layer, if you want to change the domain model for client B where you have to provide the same data in a different model,you can change the domain model without impacting the DAO layer by providing a different service or by creating a interface and implementing the interface in different model and with the same service populate the model based on the client.This decision is based on the business requirement and the scope of the project.
i have heard in a programming class,that dao layer is responsible for interacting with database, and service is a set of operations might be with dao and therefore database or not and that set of operation is in one transaction, mean is better service be transactional.