Single Responsibility Principle vs Anemic Domain Model anti-pattern

Question

I'm in a project that takes the Single Responsibility Principle pretty seriously. We have a lot of small classes and things are quite simple. However, we have an anemic domain model - there is no behaviour in any of our model classes, they are just property bags. This isn't a complaint about our design - it actually seems to work quite well

During design reviews, SRP is brought out whenever new behaviour is added to the system, and so new behaviour typically ends up in a new class. This keeps things very easily unit testable, but I am perplexed sometimes because it feels like pulling behaviour out of the place where it's relevant.

I'm trying to improve my understanding of how to apply SRP properly. It seems to me that SRP is in opposition to adding business modelling behaviour that shares the same context to one object, because the object inevitably ends up either doing more than one related thing, or doing one thing but knowing multiple business rules that change the shape of its outputs.

If that is so, then it feels like the end result is an Anemic Domain Model, which is certainly the case in our project. Yet the Anemic Domain Model is an anti-pattern.

Can these two ideas coexist?

EDIT: A couple of context related links:

SRP - http://www.objectmentor.com/resources/articles/srp.pdf
Anemic Domain Model - http://martinfowler.com/bliki/AnemicDomainModel.html

I'm not the kind of developer who just likes to find a prophet and follow what they say as gospel. So I don't provide links to these as a way of stating "these are the rules", just as a source of definition of the two concepts.

Answer 1

I'd have to say "yes", but you have to do your SRP properly. If the same operation applies to only one class, it belongs in that class, wouldn't you say? How about if the same operation applies to multiple classes? In that case, if you want to follow the OO model of combining data and behavior, you'd put the operation into a base class, no?

I suspect that from your description, you're ending up with classes which are basically bags of operations, so you've essentially recreated the C-style of coding: structs and modules.

From the linked SRP paper: "The SRP is one of the simplest of the principle, and one of the hardest to get right."

Answer 2

I've found following the solid principles did in fact lead me away from DDD's rich domain model, in the end, I found I didn't care. More to the point, I found that the logical concept of a domain model, and a class in whatever language weren't mapped 1:1, unless we were talking about a facade of some sort.

I wouldn't say this is exactly a c-style of programming where you have structs and modules, but rather you'll probably end up with something more functional, I realise the styles are similar, but the details make a big difference. I found my class instances end up behaving like higher order functions, partial functions application, lazily evaluated functions, or some combination of the above. It's somewhat ineffable for me, but that's the feeling I get from writing code following TDD + SOLID, it ended up behaving like a hybrid OO/Functional style.

As for inheritance being a bad word, i think that's more due to the fact that the inheritance isn't sufficiently fine grained enough in languages like Java/C#. In other languages, it's less of an issue, and more useful.

Answer 3

I like the definition of SRP as:

"A class has only one business reason to change"

So, as long as behaviours can be grouped into single "business reasons" then there is no reason for them not to co-exist in the same class. Of course, what defines a "business reason" is open to debate (and should be debated by all stakeholders).

Answer 4

Rich Domain Model (RDM) and Single Responsibility Principle (SRP) are not necessarily at odds. RDM is more at odds with a very specialised subclassof SRP - the model advocating "data beans + all business logic in controller classes" (DBABLICC).

If you read Martin's SRP chapter, you'll see his modem example is entirely in the domain layer, but abstracting the DataChannel and Connection concepts as separate classes. He keeps the Modem itself as a wrapper, since that is useful abstraction for client code. It's much more about proper (re)factoring than mere layering. Cohesion and coupling are still the base principles of design.

Finally, three issues:

• As Martin notes himself, it's not always easy to see the different 'reasons for change'. The very concepts of YAGNI, Agile, etc. discourage the anticipation of future reasons for change, so we shouldn't invent ones where they aren't immediately obvious. I see 'premature, anticipated reasons for change' as a real risk in applying SRP and should be managed by the developer.

• Further to the previous, even correct (but unnecessary anal) application of SRP may result in unwanted complexity. Always think about the next poor sod who has to maintain your class: will the diligent abstraction of trivial behaviour into its own interfaces, base classes and one-line implementations really aid his understanding of what should simply have been a single class?

• Software design is often about getting the best compromise between competing forces. For example, a layered architecture is mostly a good application of SRP, but what about the fact that, for example, the change of a property of a business class from, say, a boolean to an enum has a ripple effect across all the layers - from db through domain, facades, web service, to GUI? Does this point to bad design? Not necessarily: it points to the fact that your design favours one aspect of change to another.

Answer 5

Convert your plain domain objects to ActiveRecord pattern with a common base class to all domain objects. Put common behaviour in the base class and override the behaviour in derived classes wherever necessary or define the new behaviour wherever required.

Answer 6

The quote from the SRP paper is very correct; SRP is hard to get right. This one and OCP are the two elements of SOLID that simply must be relaxed to at least some degree in order to actually get a project done. Overzealous application of either will very quickly produce ravioli code.

SRP can indeed be taken to ridiculous lengths, if the "reasons for change" are too specific. Even a POCO/POJO "data bag" can be thought of as violating SRP, if you consider the type of a field changing as a "change". You'd think common sense would tell you that a field's type changing is a necessary allowance for "change", but I've seen domain layers with wrappers for built-in value types; a hell that makes ADM look like Utopia.

It's often good to ground yourself with some realistic goal, based on readability or a desired cohesion level. When you say, "I want this class to do one thing", it should have no more or less than what is necessary to do it. You can maintain at least procedural cohesion with this basic philosophy. "I want this class to maintain all the data for an invoice" will generally allow SOME business logic, even summing subtotals or calculating sales tax, based on the object's responsibility to know how to give you an accurate, internally-consistent value for any field it contains.

I personally do not have a big problem with a "lightweight" domain. Just having the one role of being the "data expert" makes the domain object the keeper of every field/property pertinent to the class, as well as all calculated field logic, any explicit/implicit data type conversions, and possibly the simpler validation rules (i.e. required fields, value limits, things that would break the instance internally if allowed). If a calculation algorithm, perhaps for a weighted or rolling average, is likely to change, encapsulate the algorithm and refer to it in the calculated field (that's just good OCP/PV).

I don't consider such a domain object to be "anemic". My perception of that term is a "data bag", a collection of fields that has no concept whatsoever of the outside world or even the relation between its fields other than that it contains them. I've seen that too, and it's not fun tracking down inconsistencies in object state that the object never knew was a problem. Overzealous SRP will lead to this by stating that a data object is not responsible for any business logic, but common sense would generally intervene first and say that the object, as the data expert, must be responsible for maintaining a consistent internal state.

Again, personal opinion, I prefer the Repository pattern to Active Record. One object, with one responsibility, and very little if anything else in the system above that layer has to know anything about how it works. Active Record requires the domain layer to know at least some specific details about the persistence method or framework (whether that be the names of stored procedures used to read/write each class, framework-specific object references, or attributes decorating the fields with ORM information), and thus injects a second reason to change into every domain class by default.

My $0.02.