Ad-Hoc Rules for Maintainable Software

2014-12-23 by Quinlan Pfiffer

I recently read The Lava Layer Anti-Pattern and it inspired me to try to write down some of the rules discovered over my short software career. Most of the time I joke about having no dependencies on a project, but a lot of the time it makes sense. There are a few things that I've been seeing repeated over the past year or so that I think are good ideas, so I'm going to attempt to write them down.

1. Avoid side-effectful code

"You overloaded plus? What does plus do?"
This one comes straight from the functional programming world, but can easily be applied to imperative languages as well. Side-effects are the bane of anyone trying to reason about your program, and reasoning about a large program you're trying to maintain is important. If you don't know what I mean by side-effects, see #5.

2. Assume all code will outlive you, program for longevity and maintainability

It is not uncommon to hear of decade-year old codebases that are still chugging in the background, critical yet unmaintainable. Rewrites would result in a huge expense and downtime that would be unavoidable. It is important to write your code in such a way that it can be easily maintained or replaced, if the need arises.

Hacks should be avoided. Every TODO and temporary fix you leave in the codebase is a stain on your permanent record. This is kind of an open-ended item, because it can effect all parts of the stack, from database (relational? K/V?) choice to programming language (Shorter dev cycles? Faster execution speed? Portability?) so I leave the interpretation up to you.

3. Typed inputs and outputs on functions

Another one out of the FP world. This is purely for the people reading your code (this could be you!). Functions that have no side-effects and clear signatures can be easily reasoned about. Pass stuff in, get stuff out. It also makes testing and linting much, much easier.

4. Immute as much as possible

Put const on everything. Touch only as much memory as you have to. Return new objects instead of modified ones. This is one of those things that can help in tracking down or eliminating bugs. The fewer functions that can modify state, the better. Which leads me to...


I first read this in the Out of the Tar Pit paper by Moseley and Marks is probably one of the most important papers you'll ever read. The go over state, the false security of unit testing, why state is the enemy of the programmer and all sorts of other little tidbits that will help you write better software.

Mutating state is the number one cause of problems, because it effects how everything else in the program that is involved with that state acts. The Tarpit paper goes over this much better than I ever could.

6. Avoid dependencies as much as possible

This is sort of a political philosphy, and most people will fall on the other side of the fence than I, but I've seen too many Django packages fail to work with the software stack I have after a handful of years because they haven't been kept up to date.

Dependencies, like every other piece of software, have to be kept up to date. Most dependencies do not follow rule #2, however, and will eventually fall apart. New language runtimes, new APIs, your language's weird package manager is down again, all sorts of different things can change and render your dependencies broken.

You're probably going to be maintaining those dependencies in the long run anyway, so you might as well start early.

7. If dependencies cannot be avoided, pick stable ones vs. cutting edge ones

This one is simple, don't pick something unproven unless it doesn't really matter. That hot-shit compiles-to-CSS dont-call-it-CSS-language-of-the-month may not be there two years from now. This is, again, heavily tied into #2.

So these are my rules, for now. We'll see how I feel about them five or six years down the line.