DDD: Always Valid Model Objects

One of the rules of DDD (Domain Driven Design) is that your model objects should never be in an inconsistent state. For example, if you have a Contact object and Last Name and Social Security Number are required attributes, DDD says you can not create a Contact without specifying Last Name and SSN.

When I first heard this rule it caused me to dismiss DDD right out of hand. My thinking was, “How dumb! How am I going to bind the Contact to the UI if the Contact can’t be inconsistent? After all, the UI will be inconsistent all the time.”

It turns out the answer to my question is pretty simple, you don’t bind the Contact to the UI. If you’re following the Model View View-Model pattern, you bind a View-Model to the UI, then when the user clicks the save button you use the properties on the View-Model to build a Contact. If there is anything wrong an exception is thrown and you display a validation error through the UI.

In a way this kind of sucks. You have more hoops to jump through now.

1. You have to “duplicate” the properties of your Model in your View-Model
2. You have to “map” between the View-Model and Model properties
3. Constructing a Model object can become somewhat difficult

Eric Evans addresses #3 with the Factory Pattern. Unfortunately he doesn’t say anything about #1 and #2. Interestingly regarding #1 and #2 though, if you were developing for the web you would be forced into this situation regardless because your Model is on the server while your UI is on the client. There would be no way to “reuse” your Model in the UI.

This lack of “reuse” certainly will cause more code to have to be written, but I would like to take a paragraph to say I think this is really a good thing. For one thing, to data bind to your UI you may have to make certain accommodations like changing data types or the structure of your properties. You don’t want concerns like this affecting the design of your Model. Another downside to this “reuse” is you are tightly coupling your UI to the Model. If refactorings happen in the Model they will affect all your views. If you add the View-Model layer only the View-Models will have to be updated (depending on what the refactoring was of course…).

The real question is are these hoops worth it? What’s so good about making sure your Model is always consistent? Simply put, this is an effective way to manage complexity. If your rules for “consistency” are simple, this can seem overkill. But as those rules get more and more complicated, especially as they begin to apply to more than one object in your model, making sure you’re consistent becomes drastically more difficult. By requiring all objects to always be valid, you’re taking a huge load of uncertainty off the shoulders of the Application Developer and you’re making a clear and consistent rule that all Model Developers must follow.

Thus once again bringing that DOMAIN complexity into check, even if it does require more code in the Application layer.