App-level settings in DDD?

Question

Just wanted to get the groups thoughts on how to handle configuration details of entities.

What I\'m thinking of specifically is high level settings which might be admin

Answer 1

I would say that that whether a value is configurable or not is irrelevant from the Domain Model's perspective - what matters is that is is externally defined.

Let's say that you have a class that must have a Name. If the Name is always required, it must be encapsulated as an invariant irrespective of the source of the value. Here's a C# example:

public class MyClass
{
    private string name;

    public MyClass(string name)
    {
        if(name == null)
        {
            throw new ArgumentNullException("name");
        }

        this.name = name;
    }

    public string Name
    {
        get { return this.name; }
        set
        {
            if(value == null)
            {
                throw new ArgumentNullException("name");
            }
            this.name = value;
        }
    }
}

A class like this effectively protects the invariant: Name must not be null. Domain Models must encapsulate invariants like this without any regard to which consumer will be using them - otherwise, they would not meet the goal of Supple Design.

But you asked about default values. If you have a good default value for Name, then how do you communicate that default value to MyClass.

This is where Factories come in handy. You simply separate the construction of your objects from their implementation. This is often a good idea in any case. Whether you choose an Abstract Factory or Builder implementation is less important, but Abstract Factory is a good default choice.

In the case of MyClass, we could define the IMyClassFactory interface:

public interface IMyClassFactory
{
    MyClass Create();
}

Now you can define an implementation that pulls the name from a config file:

public ConfigurationBasedMyClassFactory : IMyClassFactory
{
    public MyClass Create()
    {
        var name = ConfigurationManager.AppSettings["MyName"];
        return new MyClass(name);
    }
}

Make sure that code that needs instances of MyClass use IMyClassFactory to create it instead of new'ing it up manually.