Sunday, 27 January 2013

MVC projects encourage poorly organised domain namespaces

The MVC framework has been a huge breath of fresh air having usurped Webforms as the standard framework for web applications. Everyone knows the MVC pattern wasn't created by Microsoft, and the current implementation of MVC has some fundamental differences to that of the original pattern found in SmallTalk. But, it has still provided a more lightweight, transparent, extendible and testable platform to build on then Webforms ever was. And its feels so much better suited to the Web. So, regardless of how much it was influenced by Ruby on Rails, I'm grateful that Microsoft were able to bring MVC under the umbrella.

With that said, I have a small gripe with with MVC. It's a very small gripe and one that can really only be loosely attributed with it, but its the most obvious place to point the finger for this sake of this article: I think the framework encourages developers to organise domain namespaces poorly.

First, let me explain what I think a good domain namespace organisation should look like - largely learned from Domain-Driven-Design. Domain namespaces (essentially, folders in the domain project) should expose the purpose of the system. The names should closely and honestly reflect the primary concepts of the domain model. The language of the domain model should be reflected in code - this includes the namespaces the code resides in. If we are designing an application based on a domain model described by the Ubiquitous Language we have unearthed in communication between developers and business experts, then our design must share this language. A well organised namespace should allow the Business experts working on the project to look at the root of the domain and should be able to recognise the key concepts in the names of the root level folders.

In any domain, you will always need some classes as part of your design that do not come directly from the model. These classes commonly come as part of a design pattern being implemented. Their presence makes the communication of the pattern clearer, usually by encapsulating away the details, such as in the Fa├žade Pattern. Such classes usually come with a suffix to describe their purpose, such as: Provider, Factory, Repository, Builder or Manager. It is when domain namespaces become oriented around these classes that are external to the model that organisation has failed. A domain namespace should not describe the purpose of the classes it contains. We should never end up with folders in our domain project called Providers, Factories, Repositories, Builders, or Managers because this is never what our domain is about. Yet this is exactly what MVC encourages.

Because MVC is a Framework (and not a library) it gives us handles to work it. MVC is useful largely because it gives us answers to questions like "where do I put a new Controller?" or "where do my Scripts go?". With MVC, you know where to add new Models because there is a folder called Models. It doesn't matter that those (view) models may relate to completely distinct concepts, they can still exist in the same folder - and they're only view models anyway, right? They don't tie directly to the domain model, they're just there to make the presentation layer work.

Right. However, the problem is that I see developers make the mistake of following this convention in the domain project. The model is ignored and folders are created to categorise a sub collection of classes that all happen to be "Factories" or other such classes external to the model. The domain project then fails to be decipherable to a business expert and no longer describes itself with the Ubiquitous Language.

Presumably developers do this because Microsoft gave us the template MVC project as is and because its just easier to put that new Factory in the Factory folder with the others. It'll be easy to find later. But there-in lies the danger: If we are not forced to think hard about how to arrange the namespace then we risk designing something that is not true to the model. If we have to think before we add a class, it makes the choice to add a new class tougher. We will only then be able to authorise the creation of classes that have an appropriate place in the namespace of our domain - keeping the design as close to the model as possible. If we unearth a purpose for a class that doesn't appear in the model (and also isn't an artefact of a design pattern) then we must revisit our model and communicate the idea with our business experts and incorporate the idea in the Ubiquitous Language.

In defence of MVC, it couldn't really work any other way. MVC is a presentation framework - its not supposed to be a domain framework (if there is such a thing?). It has license to organise the way that suits this purpose. It is the developer's responsibility to organise that the domain suits its purpose (according to the model). Just because MVC has a layout that groups together its classes by their purpose, does not mean any other project should. The real issue is to make sure that developers are in tune with the domain and to ensure that it is exposed in the design, and not use the MVC framework as a guiding tool for how to do that.

Even inside the domain, perhaps a layer or two in from the root, there is a place for arranging classes that are external to the model together. If the development team is comfortable that nested inside folders (named according to the model) that it is appropriate to name a folder 'Factories' - then so be it. But I would expect to find such folders a rarity. (Just how many Factories would really be needed in one corner of the domain?).

But maybe there is something to learn from MVC in how we organise the domain? With MVC3 came the concept of Areas. The goal of an area is to provide logical Url Routing and help organise the presentation layer classes and assets according to the routing rules. If you have a number of Areas in your MVC3 project, then the Areas folder starts to look like a well organised domain. All the root folders will speak in terms of the domain model, while sub folders will house Models, Views & Controllers and be named as such. So, if there is anything MVC can do to encourage better domain namespace organisation, it is to include Areas in all template MVC projects.