Design Pattern: Kitchen Sink

These are the notes on a design pattern I noticed in several contexts.

Suppose, metaphorically, you have a neatly organized bookcase which categorizes the books by their topics. And now, suppose you’ve got a new book, which doesn’t fit clearly into any existing category. What would you do?

Here are some common solution I’ve seen:

  • Put the book somewhere in the bookcase.

  • Start rearranging the shelves until you have a proper topic for this new book. Maybe introduce a dedicated topic just for this single book.

  • Don’t store the book in the bookcase, keep it on the bedside table.

Here’s the “kitchen sink pattern” solution for this problem: have the “Uncategorized” shelf for books which don’t clearly fit into the existing hierarchy.

The idea here is that the overall organization becomes better, if you explicitly designate some place as “stuff that doesn’t fit goes here by default”. Let’s see the examples.

First, the Django web framework has a shortcuts module with contains conveniences functions, not fitting model/view separation. The get_object_or_404 function lookups an object in the database and returns HTTP404 if it is not found. Models (SQL) and views (HTTP) don’t know about each other, so the function doesn’t belong to either of these modules. Placing it in shortcuts allows this separation to be more crisp.

Second, I have two tricks to keep my home folder organized. I have a script that clears ~/downloads on every reboot, and I have a ~/tmp as my dumping ground. Before ~/tmp, various semi-transient things polluted my otherwise perfectly organized workspace.

Third, I asked my colleague recently about some testing infrastructure. They replied that they have a extensive document for it in their fork, because it’s unclear what’s the proper place for it in the main repo. In this case the absence of a “dumping ground” prevented useful work for no good reason.

Fourth, in rust-analyzer we have a ast::make module which is intended to contain the minimal orthogonal set of constructors for AST nodes. Historically, people kept adding non-minimal, non-orthogonal constructors there as well. Useful work was done, but it muddied the design. This was fixed by adding a dedicated ast::make::ext submodule for convenient shortcuts.

Fifth, for big projects I like having stdext modules, which fill-in missing batteries for the standard library. Without it, various modules tend to accumulate unrelated, and often slightly duplicated, functionality.

To sum up, many classifications work best if there is an explicit “can’t classify this” category. If there’s no obvious place to put things which don’t fit, a solid design might erode with time. Note that for this pattern to be useful, an existence of a good solid design is per-requisite, lest all the code ends up in an utils module.