Should services always return DTOs, or can they also return domain models?

Question

I'm (re)designing large-scale application, we use multi-layer architecture based on DDD.

We have MVC with data layer (implementation of repositories), domain layer (definition of domain model and interfaces - repositories, services, unit of work), service layer (implementation of services). So far, we use domain models (mostly entities) accross all layers, and we use DTOs only as view models (in controller, service returns domain model(s) and controller creates view model, which is passed to the view).

I'v read countless articles about using, not using, mapping and passing DTOs. I understand that there's no any definitive answer, but I'm not sure if it's ok or not returning domain models from services to controllers. If I return domain model, it's still never passed to the view, since controller always creates view-specific view model - in this case, it seem legit. On the other hand, it doesn't feel right when domain model leaves business layer (service layer). Sometimes service needs to return data object that wasn't defined in the domain and then we either have to add new object to the domain that isn't mapped, or create POCO object (this is ugly, since some services return domain models, some effectively return DTOs).

The question is - if we strictly use view models, is it ok to return domain models all the way to controllers, or should we always use DTOs for communication with service layer? If so, is it ok to adjust domain models based on what services need? (Frankly I don't think so, since services should consume what domain has.) If we should strictly stick to DTOs, should they be defined in service layer? (I think so.) Sometimes it's clear that we should use DTOs (e.g., when service performs lot of business logic and creates new objects), sometimes it's clear that we should use just domain models (e.g., when Membership service returns anemic User(s) - it seems it wouldn't make much sense to create DTO that is the same as domain model) - but I prefer consistency and good practices.

Article Domain vs DTO vs ViewModel - How and When to use them? (and also some other articles) is very similar to my problem, but it doesn't answer this question(s). Article Should I implement DTOs in repository pattern with EF? is also similar, but it doesn't deal with DDD.

Disclaimer: I don't intend to use any design pattern only because it exists and is fancy, on the other hand, I'd like to use good design patterns and practices also because it helps designing the application as a whole, helps with separation of concerns, even tohugh using particular pattern isn't "necessary", at least at the moment.

As always, thank you.

Answer 1

it doesn't feel right when domain model leaves business layer (service layer)

Makes you feel like you are pulling the guts out right? According to Martin Fowler: the Service Layer defines the application's boundery, it encapsulates the domain. In other words it protects the domain.

Sometimes service needs to return data object that wasn't defined in the domain

Can you provide an example of this data object?

If we should strictly stick to DTOs, should they be defined in service layer?

Yes, because the response is part of your service layer. If it is defined "somewhere else" then the service layer needs to reference that "somewhere else", adding a new layer to your lasagna.

is it ok to return domain models all the way to controllers, or should we always use DTOs for communication with service layer?

A DTO is a response/request object, it makes sense if you use it for communication. If you use domain models in your presentation layer (MVC-Controllers/View, WebForms, ConsoleApp), then the presentation layer is tightly coupled to your domain, any changes in the domain requires you to change your controllers.

it seems it wouldn't make much sense to create DTO that is the same as domain model)

This is one of the disadvantage of DTO to new eyes. Right now, you are thinking duplication of code, but as your project expands then it would make much more sense, specially in a team environment where different teams are assigned to different layers.

DTO might add additional complexity to your application, but so are your layers. DTO is an expensive feature of your system, they don't come free.

Why use a DTO

This article provides both advantage and disadvantage of using a DTO, http://guntherpopp.blogspot.com/2010/09/to-dto-or-not-to-dto.html

Summary as follows:

When to Use

• For large projects.
• Project lifetime is 10 years and above.
• Strategic, mission critical application.
• Large teams (more than 5)
• Developers are distributed geographically.
• The domain and presentation are different.
• Reduce overhead data exchanges (the original purpose of DTO)

When not to Use

• Small to mid size project (5 members max)
• Project lifetime is 2 years or so.
• No separate team for GUI, backend, etc.

Arguments Against DTO

• Duplication of code.
• Cost of development time, debugging. (use DTO generation tools http://entitiestodtos.codeplex.com/)
• You must synchronize both models all the time.
• Cost of developement: Additional mapping is necessary. (use auto mappers like https://github.com/AutoMapper/AutoMapper)
• Why are Data Transfer Objects an anti-pattern?

Arguments With DTO

• Without DTO, the presentation and the domain is tightly coupled. (This is ok for small projects.)
• Interface/API stability
• May provide optimization for the presentation layer by returning a DTO containing only those attributes that are absolutely required. Using linq-projection, you don't have to pull an entire entity.
• To reduce development cost, use code-generating tools