Single Responsibility Principle vs Anemic Domain Model anti-pattern

Solution 1:

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.

Solution 2:

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."