Programmer Musings

The One, Right Place

Many years ago, I spent a lot of time training entry-level programmers. One of the problems that the more junior programmers had was duplicating code and information in the code. Some of our senior programmers at the time began talking about the concept of the one, right place. Later, I read the book The Programmatic Programmer and saw their concept of Don't Repeat Yourself (DRY). For a while, I switched to their definition. I've finally come to the conclusion that the one, right place and DRY are two similar, but not quite identical ideas.

The one, right place refers to the one place in a piece of code where a piece of information belongs. When this information is stored in many places, the code is harder to understand and maintain. The key point here is that there is a right place for the information. There may be other places where the information can be stored, but those places may not increase the understandability or maintainability of the code.

The DRY principle covers the concept that a piece of information should only be stored in one place. But no mention is made of whether any place is better than another. In particular, Hunt and Thomas state that

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system

Although you could gather that there is assume that there is a right place for this representation, that requirement is not explicitly stated. As such, someone could claim to be following the DRY principle when storing a piece of knowledge in a one, wrong place. This person might resist changing the authoritative reference because of the DRY principle.

One way to tell if you have the one, right place or a wrong place is if the information ends up being duplicated during maintenance. I've seen this where the authoritative copy of the information cannot be referenced without generating references to a large amount of unrelated information. In order to decrease the unnecessary overhead, a new copy is made. At that point both DRY and the one, right place have been violated.

The one, right place extends DRY to include some design relating to the location of that information. We should not need to include dozens of headers (in a C++ program) in order to get the size of a single data structure. We also should not need to include references to a GUI subsystem to access a thread function. When these kinds of knowledge are coupled together unnecessarily, we may be following the letter of DRY, but we are violating the spirit of the principle.

The one, right place is the spirit of the DRY principle.