Comment by motorest
9 months ago
> It was always like that. And Fowler the same thing with his criticism of anemic domain model.
What leads you to disagree with the fact that anemic domain models are an anti-pattern?
https://martinfowler.com/bliki/AnemicDomainModel.html
I think it's obvious that his critique makes sense if you actually take a moment to try learn and understand what he says and where he comes from. Take a moment to understand what case he makes: it's not object-oriented programming. That's it.
See,in a anemic domain model, instead of objects you have DTOs that are fed into functions. That violates basic tenners of OO programming. It's either straight up procedural programming or, if you squint hard enough, functional programming. If you focus on OO as a goal, it's clearly an anti-pattern.
His main argument is summarized in the following sentence:
> In essence the problem with anemic domain models is that they incur all of the costs of a domain model, without yielding any of the benefits.
Do you actually argue against it?
Listen, people like Fowler and Uncle Bob advocate for specific styles. This means they have to adopt a rethoric style which focuses on stressing the virtues of a style and underlining the problems solved by the style and created by not following the style. That's perfectly fine. It's also fine if you don't follow something with a religious fervor. If you have a different taste, does it mean anyone who disagrees with you is wrong?
What's not cool is criticizing someone out of ignorance and laziness, and talking down on someone or something just because you feel that's how your personal taste is valued.
"It's not object oriented programming" is only a good case to make if you think object oriented programming is synonomous with good. I don't think that's true. It's sometimes good, often not good.
Why would focusing on OO be a goal? The goal is to write good software that can be easily maintained. Nobody outside of book writers are shipping UML charts
Why would you not focus on writing OO code in an OO language for example? Would you start writing OO code in a functional langugage? No you wouldn't, because it would be pointless. There are programming paradigms for a reason
> Why would you not focus on writing OO code in an OO language for example?
Often people do this to deliver higher quality software. Most languages still have some OO features, and people don't use them because they know they lead to bad code. Inheritence (a core OO feature) comes to mind. Most professionals nowadays agree that it should not be used.
OO designs are often over-abstracted which makes them hard to understand and hard to change. They lack "locality of behavior". Trivial algorithms look complicated because parts of them are strewn across several classes. This is why more modern langues tend to move away from OOP.
My guess is that im the long term, what we will keep from OO is the possibility to associate methods with structs.
> Why would you not focus on writing OO code in an OO language for example?
That's circular logic. I wouldn't focus on writing OO code because I know from experience that the result is usually worse. If I had to use a language that was oriented towards writing OO code, I'd still try to limit the damage.
> There are programming paradigms for a reason
Nah. A lot of them are just accidents of history.
> Why would you not focus on writing OO code in an OO language for example? Would you start writing OO code in a functional language? No you wouldn't, because it would be pointless. There are programming paradigms for a reason
I'm paid for efficiently solving business problems with software, not using a particular paradigm. If an FP solution is more appropriate and the team can support it, then that's what I'll use.
> Why would you not focus on writing OO code in an OO language
It should be the best solution to the problem direct whether or not use of OO is best, not the language.
> "It's not object oriented programming" is only a good case to make if you think object oriented programming is synonomous with good. I don't think that's true. It's sometimes good, often not good.
See, this is the sort of lazy ignorance that adds nothing of value to the discussion, and just reads as spiteful adhominems.
Domain models are fundamentally an object-oriented programming concept. You model the business domain with classes, meaning you specify in them the behavior that reflects your business domain. Your Order class has a collection of Product items, but you can update an order, cancel a order, repeat an order, etc. This behavior should be member functions. In Domain-Driven design, with its basis on OO, you implement these operations at the class level, because your classes model the business domain and implement business rules.
The argument being made against anemic domain models is that a domain model without behavior fails to meet the most basic requirements of a domain model. Your domain model is just DTOs that you pass around as if the were value types, and have no behavior at all. Does it make sense to have objects without behavior? No, not in OO and elsewhere as well. Why? Because a domain model without behavior means you are wasting all development effort building up a structure that does nothing and adds none of the benefits, and thus represents wasted effort. You are better off just doing something entirely different which is certainly not Domain-Driven design.
In fact, the whole problem with the blend of argument you are making is that you are trying to push a buzzword onto something that resembles none of it. It's like you want the benefit of playing buzzword bingo without even bothering to learn the absolute basics of it, or anything at all. You don't know what you're doing, and somehow you're calling it Domain-Driven design.
> Why would focusing on OO be a goal?
You are adopting a OO concept, which the most basic traits is that it models business domains with objects. Do you understand the absurdity of this sort of argument?
> Domain models are fundamentally an object-oriented programming concept.
They are not.
> You model the business domain with classes, meaning you specify in them the behavior that reflects your business domain.
I have better tools for doing that.
> In Domain-Driven design, with its basis on OO, you implement these operations at the class level, because your classes model the business domain and implement business rules.
You're still not explaining the "why". You're just repeating a bunch of dogma.
> a domain model without behavior means you are wasting all development effort building up a structure that does nothing and adds none of the benefits, and thus represents wasted effort.
I know from experience that this is completely false.
> You don't know what you're doing, and somehow you're calling it Domain-Driven design.
I don't call it domain-driven. You can call it domain-driven if you want, or not if you don't want. I don't care what it's called, I care whether it results in effective, maintainable software with low defect rates.
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> Your Order class has a collection of Product items, but you can update an order, cancel a order, repeat an order, etc. This behavior should be member functions.
This is how to fuck up OO and give it a bad name:
What else could Order know about? Maybe give it a JSON renderer .toJson(), a pricing mechanism .getCost(), discounting rules .applyDiscount(), and access to customer bank accounts for .directDebit(); Logging and backup too. And if a class has 10+ behaviours you've probably forgotten 5 more.
An Order is a piece of paper that arrived in your mailbox. You can't take a sharpie to it, you can't tell it to march itself into the filing cabinet. It's a piece of paper which you.read() so that you.pack() something into a box and take it to the post office. You have behaviours and the post office has behaviours. The Order and the Box do not. At best they have a few getters() or some mostly-static methods for returning aggregate data - but even then I'd probably steer clear. For instance: if the Order gave me a nice totalPrice() method, it simplifies things for later right? Well no, because in TaxCalculator (not order.calculateTax()) I will want to drill down into the details, not the aggregate. Likewise for DiscountApplier.
> Does it make sense to have objects without behavior? No, not in OO and elsewhere as well.
It does, just like in the Domain (real-world Orders). Incidentally, I believe objects-without-behaviours is one of the core Clojure tenets.
Since this is HN's monthly UB-bashing thread, I should point out that I learnt most of this stuff from him. (It's more from SOLID though, I don't think I have much to say on about cleanliness.)
The above examples violate SRP and DI.
"Single reason to change": If order.cancel(..) knows about email, then this is code I have to change if the cancellation rules change or if the email system changes. What if we don't notify over email anymore? Order has to become aware of SMS or some other tech which will cause more reasons for change.
"Dependency inversion": People know what Orders are, regardless of technical competence. They can exist without computers or any particular implementation. They are therefore (relative to other concerns here) high-level and abstract. Orders are processed using a database, Kafka and/or a bunch of other technologies (or implementation details). DI states that abstract things should not depend on concrete things.
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> Domain models are fundamentally an object-oriented programming concept
They are absolutely not. In fact, they are not even specific to even just programming, let alone OOP.
I really don't understand this fixation on domain modelling. It looks like a lot of UML mixed with a "*DD" (life-pro tip: pretty much any X Driven Development is something experienced programmers rarely care about. You can borrow good ideas from almost any methodology without becoming obsessed with its primary subject. Being obsessed with the One True Way is a great way to waste a lot of brain cells). Also nobody sane touches UML. Or makes big official charts of classes and their relationships. It's a massive waste of time. You might come up with some core concepts and relationships, like a B-REP, but you don't need some jargon-heavy official way to do this.
> The argument being made against anemic domain models is that a domain model without behavior fails to meet the most basic requirements of a domain model. Your domain model is just DTOs that you pass around as if the were value types, and have no behavior at all. Does it make sense to have objects without behavior? No, not in OO and elsewhere as well. Why? Because a domain model without behavior means you are wasting all development effort building up a structure that does nothing and adds none of the benefits, and thus represents wasted effort. You are better off just doing something entirely different which is certainly not Domain-Driven design.
I have barely any idea what you're saying, but I will agree that I'm probably better off without DDD.
> You are adopting a OO concept, which the most basic traits is that it models business domains with objects. Do you understand the absurdity of this sort of argument?
Except I'm not, because I don't care about DDD? My argument is simply: caring how much your code adheres to some third party methodology doesn't matter, what matters is if you're writing good code or not.
> it's not object-oriented programming. That's it.
Yes, exactly. And this "classical" object-oriented programming is an anti-pattern itself.
(That being said, OOP is not well defined. And, for example, I have nothing against putting related data structures and functionality into the same namespace. But that's not what OOP means to him here)
I'll reply here with a very quick example why the anemic domain model is superior in general, no matter if you do OOP or anything else.
You used the example of an "order" yourself, so I'll built upon it.
I would never combine functionality to update an order with the data and structure of the an order. The reason is simple: the business constraints don't always live inside the order.
Here's an example why such an approach inevitably must fail: if the business says that orders can only be made until 10000 items have been ordered in a month, then you cannot model that constraint inside of the order class. You must move it outside - to the entity that knows about all the orders in the system. That would be the OrderRepository or however you want to call it.
Remember, here is what you said in your other post:
> Your Order class has a collection of Product items, but you can update an order, cancel a order, repeat an order, etc. This behavior should be member functions.
So your Order should have a repeat function? But how can the order know if it can be repeated? It might violate the max-monthly-items constraint. The only way for the Order to do it is to hold a reference to the OrderRepository.
And this is a big problem. You have now entangled the concept of an OrderRepository and of an Order. In fact, Orders could totally live without an OrderRepository alltogether, for example when you build an OrderSimulation where no orders are actually being executed/persisted. But to do so, now you have this OrderRepository, even if you don't need it.
The rule of the thumb is: if the business says "we don't need feature A anymore, remove it" then you should be able to remove that feature from the code without touching any unrelated feature. If you now remove the OrderRepository and cause a bug in the Order class due to your code changes, the business will probably wonder how that could be, because while the OrderRepository cannot exist without Orders, Orders can exist without an OrderRepository.
And if that seems a bit unrealistic, think of users: A user can easily exist without a UserRepository, but not the other way around.
That makes clear, that you the rich domain model is an unsuitable and generally suboptimal solution to modeling the domain of a business. The anemic domain model on the other hand matches it perfectly.
And one more thing: even natural language disagrees with the rich domain model. Does an order repeat itself? No! An order is repeated and that is, it is repeated by something or someone. This alone makes clear that there is an entity beyond the Order that is responsible for such action. And again, the anemic domain model is a great solution for expressing this in code.
But if you disagree, I'd like you to explain what you believe the disadvantages of the anemic domain model are.
You made a great example here and I absolutely agree with you.
In fact I find this type of accidental / unneeded coupling is the number one cause of problems, bugs and limitations of re-use and thus development velocity in any software product. Concepts where a single way dependency is turned into a cycling dependency are really hard to evolve, maintain, test and understand.
In fact I'd go as far as to say that as a general rule of thumb if you have a situation where your class A depends on class B that depends on class A you've made a big doo doo and you should really seriously re-consider your design.
(Adjacent to this rule is that classes that exist in the same level of of the software hierarchy and are thus siblings should also not know about each other).
In fact when you structure your code so that the dependencies only go one way you end up with a neat lasagna code base and everything can easily slotted in. (Combined with this a secondary feature which is to eliminate all jumps upwards in the stack, i.e. callbacks)
> I would never combine functionality to update an order with the data and structure of the an order. The reason is simple: the business constraints don't always live inside the order.
> Here's an example why such an approach inevitably must fail: if the business says that orders can only be made until 10000 items have been ordered in a month, then you cannot model that constraint inside of the order class. You must move it outside - to the entity that knows about all the orders in the system. That would be the OrderRepository or however you want to call it.
It's not that hard.
If the constraint of your example is a domain constraint, and so it's *always* valid, then when you hydrate an Order entity, other than the Order data itself, you also need to provide the total number of orders.
```
// orders is the repository, and it hydrates the entity with the total number of orders for this month
order = orders.new()
// Apply the check
order.canBeCreated()
```
Where the `canBeCreated` method is as simple as:
```
if this.ordersInAMonth > TOTAL_NUMBER_OF_ORDER_IN_A_MONTH ...
```
Fixed.
It's the same as using the OrdersRepository to query the number of orders directly before creating one, but here, the logic is just in the class.
Now, your example is pretty stupid, so I know it must not be taken literally but...
PS: I'm all but not a DDD advocate.
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Links to any existing articles on this topic would be greatly appreciated.
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