Are all the classes containing business logic, domain objects?

Question

So I have few doubts regarding calling something as domain object (and eventually placing the class under domain package) or not.

I have a micro-service whose responsibility is to do some calculations (without getting into actual business requirements, all it does is calculate some return of intereset based on given request). Now to achieve the calculations there are certain sub-calculations which need to take place and hence are composed in different classes respectively. But yes, these calculation do not need to be persisted in DB , and neither they have an ID (so definitely not an Entity or Aggregate). However these individual calculator classes (for the lack of terminology) do contain some complex business logic. Now, my question is, do these individual classes still qualify/classify as domain objects or should they be referred to as services ?

Feel free to ask for more clarifications around use case if need be.

Cheers !

Answer 1

Now, my question is, do these individual classes still qualify/classify as domain objects or should they be referred to as services

From the DDD point of view, in the Domain layer, there are the following terms that could be implemented using classes: Domain entities, Aggregate roots (a type of Domain entity), Value objects and Domain services.

Because your things don't have an Identity they cannot be Domain entities or Aggregate roots. Calculations could be done inside Value objects or Domain services. Value objects contain specific behavior related to values so most probable your calculations are implemented using Domain services.

From my experience, not every domain class must be a DDD building block, they could be just classes extracted for better maintainability, Single responsibility principle (SOLID in general) etc.

Answer 2

A simple test could be asking below questions -

Does your “calculator” (as you refer it) hold the calculation result as an immutable state? — if the answer is yes then it is a Value Object.

Is the “Calculator” stateless and only exposes a “calculate” behaviour which accepts a request & returns a calculation result? — if the answer is yes then it is a Service, however, the “result” returned by this service may be classified as Value Object.