Can a “rich domain model” violate the Single Responsibility Principle?

Question

An interesting thread came up when I typed in this question just now. I don't think it answers my question though.

I've been working a lot with .NET MVC3, where it's desirable to have an anemic model. View models and edit models are best off as dumb data containers that you can just pass off from a controller to a view. Any kind of application flow should come from the controllers, and the views handle UI concerns. In MVC, we don't want any behavior in the model.

However we don't want any business logic in the controllers either. For larger applications its best to keep the domain code separate from and independent of the models, views, and controllers (and HTTP in general for that matter). So there is a separate project which provides, before anything else, a domain model (with entities and value objects, composed into aggregates according to DDD).

I've made a few attempts to move away from an anemic model towards a richer one in domain code, and I'm thinking of giving up. It seems to me like having entity classes which both contain data and behavior violates the SRP.

Take for example one very common scenario on the web, composing emails. Given some event, it is the domain's responsibility to compose an EmailMessage object given an EmailTemplate, EmailAddress, and custom values. The template exists as an entity with properties, and custom values are provided as input by the user. Let's also say for the sake of argument that the EmailMessage's FROM address can be provided by an external service (IConfigurationManager.DefaultFromMailAddress). Given those requirements, it seems like a rich domain model could give the EmailTemplate the responsibility of composing the EmailMessage:

public class EmailTemplate
{
    public EmailMessage ComposeMessageTo(EmailAddress to, 
        IDictionary<string, string> customValues, IConfigurationManager config)
    {
        var emailMessage = new EmailMessage(); // internal constructor
                                            // extension method
        emailMessage.Body = this.BodyFormat.ApplyCustomValues(customValues);
        emailMessage.From = this.From ?? config.DefaultFromMailAddress;
        // bla bla bla
        return emailMessage;
    }
}

This was one of my attempts at rich domain model. After adding this method though, it was the EmailTemplate's responsibility to both contain the entity data properties and compose messages. It was about 15 lines long, and seemed to distract the class from what it really means to be an EmailTemplate -- which, IMO, is to just store data (subject format, body format, attachments, and optional from/reply-to addresses).

I ended up refactoring this method into a dedicated class who's sole responsibility is composing an EmailMessage given the previous arguments, and I am much happier with it. In fact, I'm starting to prefer anemic domains because it helps me keep responsibilities separate, making classes and unit tests shorter, more concise, and more focused. It seems that making entities and other data objects "devoid of behavior" can be good for separating responsibility. Or am I off track here?

Answer 1

The argument in favor of a rich domain model instead of an anemic model hinges on one of the value propositions of OOP, which is keeping behavior and data next to each other. The core benefit is that of encapsulation and cohesion which assists in reasoning about the code. A rich domain model can also be viewed as an instance of the information expert pattern. The value of all of these patterns however is to a great extent subjective. If it is more helpful for you to keep data and behavior separate, then so be it, though you might also consider other people that will be looking at the code. I prefer to encapsulate as much as I can. The other benefit of a richer domain model in this case would be the possibility to make certain properties private. If a property is only used by one method on the class, why make it public?

Whether a rich domain model violates SRP depends on your definition of responsibility. According to SRP, a responsibility is a reason to change, which itself calls for a definition. This definition will generally depend on the use-case at hand. You can declare that the responsibility of the template class is to be a template, with all of the implications that arise, one of which is generating a message from the template. A change in one of the template properties can affect the ComposeMessageTo method, which indicates that perhaps these are a single responsibility. Moreover, the ComposeMessageTo method is the most interesting part of the template. Clients of the template don't care how the method is implemented or what properties are present of the template class. They only want to generate a message based on the template. This also votes in favor of keeping data next to the method.

Answer 2

Well, it depends on how you want to look at it.

Another way is: "Can the Single Responsibility Principle violate a rich domain model?"

Both are guidelines. There is no "principle" anywhere in software design. There are, however, good designs and bad designs. Both these concepts can be used in different ways, to achieve a good design.