In a system architecture using Domain Driven Design (DDD) you typically find a few typical building blocks (stereotypes) - Entities, Value Objects, Repositories and Domain Services – where the first two are stateful objects and the rest are stateless services implemented either as infrastructure services outside the domain (Repositories) or inside the domain containing only business logic (Domain Services). A common question is “When is it appropriate to design a service instead of placing the business logic in an entity or value object?”
For developers more used to building procedural designs, rather than object-oriented ones, it seems to be more natural to place logic in stateless services and have them operate on objects that are no more than data containers. This is what is commonly referred to as an “anemic domain model”. It is considered an anti-pattern in the DDD community since it decouples data from behavior and thereby produces a much less expressive and knowledge tense domain model.
I'm not a fan of stateless services in the domain model. Instead I try to favor bringing business logic into the Entity or Value Object that holds the information needed - in OO-terms it is called the "information expert". In most cases it isn’t that hard, especially if functionality is decomposed into short methods, each allocated to the object representing the concept at heart of the functionality.
A specific type of functionality that might be trickier to handle is entity creation. Who is to be responsible? For sure, it can’t be the entity itself. In general my experience is that there will be some sort of hierarchy between concepts. E.g. a SalaryPeriod might be connected to an existing RegistrationPeriod, which also contains submitted Timesheets. Then it is a good fit to have the RegistrationPeriod handle creation of the SalaryPeriod. So in general it is almost always possible to find a good place for rules regarding creation of an entity in a parent concept. The same goes for functionality that has to operate over several entities of a given type.
However, there might be cases where no suitable parent concept exists in the domain. That is one of the cases where I find designing a domain service appropriate. And there are others. Here is a small extract from a previous blog post of mine where I briefly describe another situation where I think domain services are a good choice:
"In general I think you could talk about two types of systems, or parts of systems; those mostly concerned with changes in object state and those mostly concerned with processing data streaming through the system. In the first case entities, aggregates and repositories are a natural fit, in the second I think transaction scripts (in DDD context called domain services, since they do only concern domain logic, no infrastructure code [..]) are a nice fit. When the most important feature is to crunch some data, perhaps modify it and then route it further to some recipient (like another system or some persistent store) I think it is the "processing pipeline", i.e. the stateless service code, which should be emphasized. So in those cases the internals of the data isn't very interesting and might be better left in some simple DTO format."
To make the list of appropriate service design complete I’ll end with adding a few lines on external services. These services get injected into the domain. In the domain I would have only an interface describing the service in terms of the domain. This is the way to integrate with surrounding infrastructure or other systems. It is the same pattern as with Repositories, in fact a Repository is just a specialized service.
Another example of an external service is when some part of the business logic, e.g. a calculation is broken out into a separate service, implemented using another programming language or paradigm. From a domain point-if-view it is as if we get that calculation service from another system instead of doing it ourselves. The reason might be performance or it might be that the logic already exists and we want to continue using it instead of re-implementing. However, each time this happens the maintenance burden is increased a bit, so I think it should be done with careful consideration.
To sum up, favor business logic implementations in the domain objects, not in services. It leads to a more elaborate and therefore more useful domain model which is easier to keep consistent than logic spread over disparate services. I’m convinced in the long run this approach makes maintenance of the system easier.