Do Domain Objects Have To Be Persisted?

Question

Does a domain object have to be persisted, or does this violate some convention about domain objects?

For example, let's say I'm using an object called AuthenticationState to represent authentication state in the application. This object has a boolean field isLoggedIn. I want to force the user to re-login each time so when I start the application again, I just create a new instance of the model with isLoggedIn set to false, instead of attempting to load one from local device/browser storage.

I feel like I may be overthinking things, but every example I've found online always has its domain objects use some sort of persistence. Is using domain objects in this way still acceptable?

Answer 1

I would say that the domain object has no relation to the persistence model you use in your application.

It's true that some of the web frameworks (like Rails for instance) would assume that every domain-level object could be persisted. But it's has nothing to do with the underlying principles of DDD. DDD in general defines the concept of Repository that is responsible for retrieving and storing the domain object. But the actual implementation might just create an object every time you want to retrieve it or read it from memory, or database. It's completely up to the developer.

Moreover, AuthenticationState could be hardly considered as a domain object. Please look at this SO answer:

Domain object an instance of a class that is related to your domain. I would probably exclude any satellite or utility objects from domain object, e.g. in most cases, domain objects do not include things like logging, formatting, serialisation, encryption etc - unless you are specifically building a product to log, serialise, format or encrypt respectively.