I'm trying to build my first CRUD application, and I don't understand if I should use an object containing getters and setters separated.
Considering that we have the Zend Framework Quick Start tutorial with a Model structure containing:
Gateway
DataMapper
Domain Object (model class)
If the Domain Object (as presented on Zend Quick Start Tutorial) consists of only getters and setters, is that an anti-pattern? In a sense, we are unecessarily dividing the domain object with a transaction script?
Please advise.
The Anemic Domain Model is an Anti-Patern ONLY IF you are trying to build a true Domain Model (aka Domain Model from Domain Driven Design) and end up with entities with only state and without behavior.
For a simple CRUD application an anemic domain model is probably a best practice, especially when you have framework that makes your job very easy.
See Martin Fowler's article about Anemic Domain Model and also Greg Young's Article.
The domain objects are seperated from the business logic of the software. This is an important idea of procedural programming.
However this pattern is considered to be a candidate for an anti-pattern by some developers which means that it might be a ineffective practice.
In fact you could consider disadvantages
your model is less expressive, getters and setters aren't really good to describe the model
code is harder to reuse, you get dublicated code among your transactional scripts
you have to use wrappers which hide the actual data structure (so maybe not really OOP)
there is always a global access to entities
I think the most interesting point to consider is that domain model's objects cannot assure their correctness at any time. Because their mutation takes place in seperated layers.
I worked on a CRUD application with zend framework too. The clear separation between logic and data is really great but when you progress you realize that the amount of layers and mappers gets bigger and bigger. Try to reuse your code as much as you can and avoid dublication.
Related
I am developing a Symfony-Application but I guess my question is mostly independent of
framework-usage.
This is the situation:
I got a very lightweight entity that is actually not even managed by the ORM as it is just used to aggregate and communicate information of another entity.
I read that we should keep business-logic out of the model but I also thought that it would
proof quite useful to me if I had a fromXYZ-method in my lightweight-entity to create a SPOC for instanciation based off of the "big entity".
But technically this from-method performs logic which contradicts what I read to be good practice.
So would this be an antipattern? How would I resolve this in a more suitable way, if yes?
What are you talking about is static factory method. It's pretty ok to use it, but it shouldn't do anything fancy because it will be hard to test your code since you can't mock it.
If you feel that your method won't create any problems in testing, you can create it without any doubt. Otherwise, you can encapsulate your creation logic in a separate factory.
Your "lightweight entity" could be referred to as a DTO.
One way to keep the terminology straight.
Entities are managed by the ORM.
DTOs are unmanaged. Often used to convey a subset or aggregation of entities.
Constructing one object from another by copying its fields is not something I would consider business logic, unless there are complex transformations prescribed by business requirements.
The real problem with fromXYZ methods is that they implement a dependency on XYZ. You have to be careful about which direction such dependencies are pointing. For example, if you construct a DTO by passing it an Entity, that DTO now has a dependency on the persistence layer of your application. If you then use that DTO in a service, your service layer now has a (transitive) dependency on your persistence layer. This is referred to as tight coupling.
One way to keep business logic decoupled from persistence, view, etc., is to point dependencies in the other direction: into the services rather than out of them.
I've read in the Patterns, Principles and Practices of DDD book that if you want to fully encapsulate your domain model you can make its properties private and use the Memento pattern to read them. There was also an example that a Repository gets a Snapshot of the domain model, then maps to a database model and saves its changes to the db. Also it retrieves the database model from db, maps it to the Snapshot and then uses the Factory pattern to reconstitute the Domain model from the Snapshot and work with it.
I am a bit confused about how much work is done here when you can just map the database model to the domain model, without using a factory or snapshots. Isn't this much easier?
If we are going to follow the book, is it the correct way to return the Domain model snapshot from the service layer to the presentation layer and then map it to a View Model? Or to create the snapshot in the presentation layer and pass it to service layer, using the factory pattern to reconstitute it there and then pass the domain model to the repository where it will again take its snapshot to map to the database model and save to the db....
Can you give some example when you need to use such complicated mapping architecture?
It really feels like you are writing complicated code when it can be done much simpler.
UPDATE
I can put code examples if it will be easier to understand what i am asking. ;)
A Repository's job is just to save and rehydrate domain entities from a persistent store. Any design pattern beyond that is just technical details, usually a way to work around ORM flaws - but it is not part of Repository (i.e. DDD's fundamental means of storage) per se.
I suppose the Memento pattern in that book is used to solve the "ORM / encapsulation conflict", i.e. an ORM needs write access to all of an entity's fields to be able to rehydrate it, which forces you to expose them and breaks encapsulation.
No, the Memento or Snapshot is for persistence purposes only. Service (or Application) layer maps from the real Entities or uses precomputed read-specific models if you're under CQRS.
I'm designing a layered application where 90% of the business and data access entities have the same properties. Basically it doesn't make sense to create one set of classes for each layer (and map) with the same properties for the sake of separation of concerns. I'm completely aware of automappers but I'd rather not use one in this case as I think it's unecessary. Is it ok to share share the business entities between the business and data access layer in this scenario? We will manage the remaining 10% of the classes by creating adhoc/transformed classes within the same namespace.
Any other design approach?
I think sharing between layers is the whole point of having model classes backed by a data store. I would avoid adding unnecessary architecture unless the code really needs it. If you get to a point where you need to be agnostic of the data store or some other similar situation, I would you might look into the Repository pattern. Simple code = maintainable code.
We are taking a long, hard look at our (Java) web application patterns. In the past, we've suffered from an overly anaemic object model and overly procedural separation between controllers, services and DAOs, with simple value objects (basically just bags of data) travelling between them. We've used declarative (XML) managed ORM (Hibernate) for persistence. All entity management has taken place in DAOs.
In trying to move to a richer domain model, we find ourselves struggling with how best to design the persistence layer. I've spent a lot of time reading and thinking about Domain Driven Design patterns. However, I'd like some advice.
First, the things I'm more confident about:
We'll have "thin" controllers at the front that deal only with HTTP and HTML - processing forms, validation, UI logic.
We'll have a layer of stateless business logic services that implements common algorithms or logic, unaware of the UI, but very much aware of (and delegating to) the domain model.
We'll have a richer domain model which contains state, relationships, and logic inherent to the objects in that domain model.
The question comes around persistence. Previously, our services would be injected (via Spring) with DAOs, and would use DAO methods like find() and save() to perform persistence. However, a richer domain model would seem to imply that objects should know how to save and delete themselves, and perhaps that higher level services should know how to locate (query for) domain objects.
Here, a few questions and uncertainties arise:
Do we want to inject DAOs into domain objects, so that they can do "this.someDao.save(this)" in a save() method? This is a little awkward since domain objects are not singletons, so we'll need factories or post-construction setting of DAOs. When loading entities from a database, this gets messy. I know Spring AOP can be used for this, but I couldn't get it to work (using Play! framework, another line of experimentation) and it seems quite messy and magical.
Do we instead keep DAOs (repositories?) completely separate, on par with stateless business logic services? This can make some sense, but it means that if "save" or "delete" are inherent operations of a domain object, the domain object can't express those.
Do we just dispense with DAOs entirely and use JPA to let entities manage themselves.
Herein lies the next subtlety: It's quite convenient to map entities using JPA. The Play! framework gives us a nice entity base class, too, with operations like save() and delete(). However, this means that our domain model entities are quite closely tied to the database structure, and we are passing objects around with a large amount of persistence logic, perhaps all the way up to the view layer. If nothing else, this will make the domain model less re-usable in other contexts.
If we want to avoid this, then we'd need some kind of mapping DAO - either using simple JDBC (or at least Spring's JdbcTemplate), or using a parallel hierarchy of database entities and "business" entities, with DAOs forever copying information from one hierarchy to another.
What is the appropriate design choice here?
Martin
Your questions and doubts ring an interesting alarm here, I think you went a bit too far in your interpretation of a "rich domain model". Richness doesn't go as far as implying that persistence logic must be handled by the domain objects, in other words, no, they shouldn't know how to save and delete themselves (at least not explicitely, though Hibernate actually adds some persistence logic transparently). This is often referred to as persistence ignorance.
I suggest that you keep the existing DAO injection system (a nice thing to have for unit testing) and leave the persistence layer as is while trying to move some business logic to your entities where it's fit. A good starting point to do that is to identify Aggregates and establish your Aggregate Roots. They'll often contain more business logic than the other entities.
However, this is not to say domain objects should contain all logic (especially not logic needed by many other objects across the application, which often belongs in Services).
I am not a Java expert, but I use NHibernate in my .NET code so my experience should be directly translatable to the Java world.
When using ORM (like Hibernate you mentioned) to build Domain-Driven Design application, one of good (I won't say best) practices is to create so-called application services between the UI and the Domain. They are similar to stateless business objects you mentioned, but should contain almost no logic. They should look like this:
public void SayHello(int id, String helloString)
{
SomeDomainObject target = domainObjectRepository.findById(id); //This uses Hibernate to load the object.
target.sayHello(helloString); //There is a single domain object method invocation per application service method.
domainObjectRepository.Save(target); //This one is optional. Hibernate should already know that this object needs saving because it tracks changes.
}
Any changes to objects contained by DomainObject (also adding objects to collections) will be handled by Hibernate.
You will also need some kind of AOP to intercept application service method invocations and create Hibernate's session before the method executes and save changes after method finishes with no exceptions.
There is a really good sample how to do DDD in Java here. It is based on the sample problem from Eric Evans' 'Blue Book'. The application logic class sample code is here.
Should entities have behavior? or not?
Why or why not?
If not, does that violate Encapsulation?
If your entities do not have behavior, then you are not writing object-oriented code. If everything is done with getters and setters and no other behavior, you're writing procedural code.
A lot of shops say they're practicing SOA when they keep their entities dumb. Their justification is that the data structure rarely changes, but the business logic does. This is a fallacy. There are plenty of patterns to deal with this problem, and they don't involve reducing everything to bags of getters and setters.
Entities should not have behavior. They represent data and data itself is passive.
I am currently working on a legacy project that has included behavior in entities and it is a nightmare, code that no one wants to touch.
You can read more on my blog post: Object-Oriented Anti-Pattern - Data Objects with Behavior .
[Preview] Object-Oriented Anti-Pattern - Data Objects with Behavior:
Attributes and Behavior
Objects are made up of attributes and behavior but Data Objects by definition represent only data and hence can have only attributes. Books, Movies, Files, even IO Streams do not have behavior. A book has a title but it does not know how to read. A movie has actors but it does not know how to play. A file has content but it does not know how to delete. A stream has content but it does not know how to open/close or stop. These are all examples of Data Objects that have attributes but do not have behavior. As such, they should be treated as dumb data objects and we as software engineers should not force behavior upon them.
Passing Around Data Instead of Behavior
Data Objects are moved around through different execution environments but behavior should be encapsulated and is usually pertinent only to one environment. In any application data is passed around, parsed, manipulated, persisted, retrieved, serialized, deserialized, and so on. An entity for example usually passes from the hibernate layer, to the service layer, to the frontend layer, and back again. In a distributed system it might pass through several pipes, queues, caches and end up in a new execution context. Attributes can apply to all three layers, but particular behavior such as save, parse, serialize only make sense in individual layers. Therefore, adding behavior to data objects violates encapsulation, modularization and even security principles.
Code written like this:
book.Write();
book.Print();
book.Publish();
book.Buy();
book.Open();
book.Read();
book.Highlight();
book.Bookmark();
book.GetRelatedBooks();
can be refactored like so:
Book book = author.WriteBook();
printer.Print(book);
publisher.Publish(book);
customer.Buy(book);
reader = new BookReader();
reader.Open(Book);
reader.Read();
reader.Highlight();
reader.Bookmark();
librarian.GetRelatedBooks(book);
What a difference natural object-oriented modeling can make! We went from a single monstrous Book class to six separate classes, each of them responsible for their own individual behavior.
This makes the code:
easier to read and understand because it is more natural
easier to update because the functionality is contained in smaller encapsulated classes
more flexible because we can easily substitute one or more of the six individual classes with overridden versions.
easier to test because the functionality is separated, and easier to mock
It depends on what kind of entity they are -- but the term "entity" implies, to me at least, business entities, in which case they should have behavior.
A "Business Entity" is a modeling of a real world object, and it should encapsulate all of the business logic (behavior) and properties/data that the object representation has in the context of your software.
If you're strictly following MVC, your model (entities) won't have any inherent behavior. I do however include whatever helper methods allow the easiest management of the entities persistence, including methods that help with maintaining its relationship to other entities.
If you plan on exposing your entities to the world, you're better off (generally) keeping behavior off of the entity. If you want to centralize your business operations (i.e. ValidateVendorOrder) you wouldn't want the Order to have an IsValid() method that runs some logic to validate itself. You don't want that code running on a client (what if they fudge it. i.e. akin to not providing any client UI to set the price on an item being placed in a shopping cart, but posting a a bogus price on the URL. If you don't have server-side validation, that's not good! And duplicating that validation is...redundant...DRY (Don't Repeat Yourself).
Another example of when having behaviors on an entity just doesn't work is the notion of lazy loading. Alot of ORMs today will allow you to lazy load data when a property is accessed on an entities. If you're building a 3-tier app, this just doesn't work as your client will ultimately inadvertantly try to make database calls when accessing properties.
These are my off-the-top-of-my-head arguments for keeping behavior off of entities.