Where to put business logic in DDD

Question

I'm trying to figure out the best way to build an easily maintainable and testable architecture. Having gone through several projects, I've seen some pretty bad architectures and I want to avoid making future mistakes on my own projects.

Let's say I'm building a fairly complex three layer application and I want to use DDD. My question is, where should I place my business logic? Some people say it should be placed in services (service layer) and that does make sense. Having a number of services which adhere to Single Responsibility Principle makes sense.

However, some people said that this is an anti pattern and that business logic shouldn't be implemented in the service layer. Why is this?

Let's say we have IAuthenticationService which has a method with bool UsernameAvailable(string username) signature. The method would implement all required logic to check whether the username is available or not.

What is the problem here according to the "this is an antipattern" crowd?

Answer 1

A service layer in itself is not an anti-pattern, it is a very reasonable place to put certain elements of your business logic. However, you do need to apply discretion to the design of the service layer, ensuring that you aren't stealing business logic from your domain model and the objects that comprise it.

By doing that you can end up with a true anti-pattern, an anaemic domain model. This is discussed in-depth by Martin Fowler here.

Your example of an IAuthenticationService isn't perhaps the best for discussing the problem - much of the logic around authentication can be seen as living in a service and not really associated with domain objects. A better example might be if you had some sort of IUserValidationService to validate a user, or even worse a service that does something like process orders - the validation service is stripping logic out of the user object and the order processing service is taking logic away from your order objects, and possibly also from objects representing customers, delivery notices etc...

Answer 2

If you put all your business logic in an (implicitly stateless) service layer you're writing procedural code. By decoupling behavior from data, you're giving up on writing object-oriented code.

That's not always bad: it's simple, and if you have simple business logic there's no reason to invest in a full-fledged object-oriented domain model.

The more complex the business logic (and the larger the domain), the faster procedural code turns into spaghetti code: procedures start calling each other with different pre- and post-conditions (in incompatible order) and they begin to require ever-growing state objects.

Martin Fowler's article on Anemic Domain Models is probably the best starting point for understanding why (and under what conditions) people object to putting business logic in a service layer.