Changing your perspective even a small amount can have profound effects on how you approach your system.
Let’s say you’re writing a web application in Java. In the system you deal with orders, customers and products. As a web application, your classes include staples like PersonController, PersonRepository, CustomerController and OrderService. How do you organize your classes into packages?
There are two fundamental ways to structure your packages. Either you can focus on the logical tiers, like com.brodwall.myapp.controllers, com.brodwall.myapp.domain or perhaps com.brodwall.myapp.services.customer. Or you can focus on the domain contexts, like com.brodwall.myapp.customer, com.brodwall.myapp.orders and com.brodwall.myapp.products. The first approach is by far the most prevalent. In my view, it’s also the least helpful.
Here are some ways your thinking changes if you structure your packages around domain concepts, rather than technological tiers:
First, and most fundamentally, your mental model will now be aligned with that of the users of your system. If you’re asked to implement a typical feature, it is now more likely to be focused around a strict subset of the packages of your system. For example, adding a new field to a form will at least affect the presentation logic, entity and persistence layer for the corresponding domain concept. If your packages are organized around tiers, this change will hit all over your system. In a word: A system organized around features, rather than technologies, have higher coherence. This technical term means that a large percentage of a the dependencies of a class are located close to that class.
Secondly, organizing around domain concepts will give you more options when your software grows. When a package contains tens of classes, you may want to split it up in several packages. The discussion can itself be enlightening. “Maybe we should separate out the customer address classes into a com.brodwall.myapp.customer.address package. It seems to have a bit of a life on its own.” “Yeah, and maybe we can use the same classes for other places we need addresses, such as suppliers?” “Cool, so com.brodwall.myapp.address, then?” Or maybe you decide that order status codes and payment status codes deserve to be in the “com.brodwall.myapp.order.codes” package.
On the other hand, what options do you have for splitting up com.brodwall.myapp.controllers? You could create subpackages for customer, orders and products, but these subpackages may only have one or possibly two classes each.
Finally, and perhaps most intriguingly, using domain concepts for packages allows you to vary the design according on a case by case basis. Maybe you really need a OrderService which coordinates the payment and shipping of an order, while ProductController only needs basic create-retrieve-update-delete functionality with a repository. A ProductService would just get in the way. If ProductService is missing from the com.brodwall.myapp.services package, this may be confusing or at the very least give you a nagging feeling that something is wrong. On the other hand, if there’s no Controller in the com.brodwall.myapp.product package, it doesn’t matter much.
Also, most systems have some good parts and some not-so-good parts. If your Services package is not working for you, there’s not much you can do. But if the Products package is rotten, you can throw it out and reimplement it without the whole system being thrown into a state of chaos.
By putting the classes needed to implement a feature together with each other and apart from the classes needed to implement other features, developers can be pragmatic and innovative when developing one feature without negatively affecting other features.
The flip side of this is that most developers are more comfortable with some technologies in the application and less comfortable with other technologies. Organizing around features instead of technologies force each developer to consider a larger set of technological challenges. Some programmers take this as a motivating challenge to learn, while others, it seems, would rather not have to learn something new.
If it were my money being spend to create features, I know what kind of developer I would want.
Trivial changes can have large effects. By organizing your software around features, you get a more coherent system that allows for growth. It may challenge your developers, but it drives down the number of hand-offs needed to implement a feature and it challenges the developers to improve the parts of the application they are working on.
See also my blog post on Architecture as tidying up.