Beyond top-down design

Here at Los Techies, we talk a lot about SOLID principles and design.  Two of the principles that have always resonated with me are the Single Responsibility Principle and the Dependency Inversion Principle.  With the two together, the need for some kind of inversion of control almost becomes required.  When I first started using the DIP, I didn’t really feel the pain of locating dependencies, I always provided some no-argument constructor or the various construction patterns to get my dependencies in play.

The combination of test-driven development and SOLID led me towards a sort of default top-down design, as Derick describes as a cloud of objects.  My objects take, for the vast majority, the following shape:

This is great and all, but in some cases, the level of nesting would get completely out-of-hand.  Small classes are great, but I had projects where the level of nesting hit double digits, making it quite difficult to figure out what was going on, and to understand the shape of the application.  At that point, I couldn’t understand what actually happened for a given request by looking at the code, and all of this traced back to top-down design.

But it is still a vast improvement upon the big-ball-of-mud architecture I created before, yet something still bothers me.  Is this all there is to good design?  Top-down, TDD, with SRP and DIP creating top-level controllers, down to a bunch of supporting classes that have a single responsibility but are hard to understand in the whole?

Another issue I run into with top-down design is that I often have vertical slices of functionality, but absolutely zero re-use.  I fully understand that re-use is not the motivator for creating small classes, but I still felt my application partitioned by top-level features.  Sure, there might be some similarities between disparate areas of the application, but I can’t help but feeling there are other, higher-level patterns to better architect an application than purely top down design.

Right now, I have no answers, and really no idea where to look for them.  I have a feeling there are some other crazy ideas out there, beyond top-down design, that I’m just not aware of.  So my question is: am I imagining things, or is there something else beyond top-down design and architecture?

My only inclination of a larger picture as of this moment is Ayende’s post on concepts and features.  Much of this post (and corresponding talk at Seattle) resonated quite a bit for me, and looks like one of those ideas that can get rid of much of the orthogonal duplication amongst various features that I don’t really see.

And no, I don’t think there is a magical chapter in the Big Blue Book on this one…