Code smell and a lesson learned

September 5 - 2017

In an ideal world there’s always time and resources available to refactor code and to make sure it is as clean as possible. Even though it is well known that refactoring code helps with maintainability and thus reduces development costs in the long run, for various reasons refactoring does not always happen, or at least not until things start to fall apart.

In this post I will tell a little horror story that happened to me recently and that could have been avoided if we had stopped to refactor the code we were working on…

Back story

I recently had to work on an Angular 1 app that was having some issues. The app would display forms based on JSON documents that were usually pretty large. The problem was that sometimes the forms would take a long time to load (especially the really large ones), or changing some of the inputs caused noticeable lag. Some slowness was expected as some of these forms were particularly huge, but at times the app became too frustrating to use, and even when sitting in the background, the CPU usage of the app was incredibly high.

Fixing things little by little

I always found performance problems to be interesting for some reason. I’ve come across some problems with really simple solutions (like applying the Flyweight pattern to avoid unnecessary object creation), while others have proven to be a lot more challenging (such as having to debug and fix polygon merging algorithms).

For this particular case, I decided to go for simple and obvious optimizations and best practices first and delve into more complicated ones later. My first step was to remove functions from expressions such as ng-disable and replace them with attributes on plain objects. This improved performance a lot, as adding functions in expressions like these causes them to be called often. Unfortunately it was still not enough, something else had to be done.

The form rendering code was put together impressively quick and was not too hard to follow, however it was clear that parts of the code had mutated to do different things than originally intended and thus the naming of some of some functions did not reflect 100% what they did anymore. After fixing the HTML it was time to start optimizing the code underneath. One of the most important functions of the View Controller was in charge of figuring out whether the form was ready to be submitted, lets call it setValid. This function was being called each time an input changed which sounds reasonable, the problem was that some inputs could cause other inputs to be disabled triggering another call to the function. These inputs could in turn disable yet more inputs, causing a cascade of calls.

A better solution

Because in most cases inputs would only disable inputs that came after, a simple but effective solution was to avoid calling the function over and over and instead wrap part of the logic in a loop. On each iteration, check if inputs should be disabled (via an already written isDisabled function) and set a flag. This would continue until no inputs were changed. Unless inputs somehow disabled previous inputs (which was very very rare), no more than two iterations would be needed to make sure the form was valid. A very simple version of the algorithm looked like this:

This solution was pretty straight forward, we loaded forms, checked the number of iterations and they were indeed 2. We selected an input that would cause another to be disabled and the number of iterations was also 2! This seemed to work, notwithstanding, performance did not improve much.

More optimizations and the real culprit, code smell

Days passed, and more things were optimized, but performance still was not as expected. While adding some logging statements we accidentally found out that the setValid function was looping many times, as in, thousands of times for some forms, but why? This made no sense! We went through the code, tested things, but couldn’t figure out what was wrong, it made no sense! Until finally… the problem was so obvious! It was in front of us the whole time but we failed to notice it! We were relying on side effects for things to work! The isDisabled function would not only tell us if an input was not disabled it would also actually go and disable it. Because the || operator short circuits only the first input would be disabled, but subsequent inputs were totally ignored. This made it so that only one input was disabled per iteration instead of all (or most) of them. We felt like total fools.

If we had stopped to actually refactor code and make sure functions were properly named and separated before even attempting to optimize, it would have saved us a headache, embarrassment, and most importantly, a lot of time.

Avoiding this in the future

The need to follow best practices and to constantly clean up code is something we all knew but failed to do. This was a kick in the teeth to remind us that it is something that always needs to be done, without exception. Because we let our code rot, we ended up with functions that had side effects we relied on for our logic to work which eventually bit us in the ass, big time.

This is also a lesson learned on why writing pure functions is pretty much always a good idea, as they help make the code easier to understand especially when functions are nested within other functions. Clearly seeing what a function does and what it does not helps a lot when attempting to figure out if a program is correct or not. This does not mean that we should all move to functional programming, but good things from it can most certainly be followed.

Of course this was a very stupid mistake that we failed to notice because we were busy looking at details and not the whole pictures, but these points still stand. It is important to keep your code clean in order to avoid silly mistakes, at least I know that is what I will be doing in the future as much as I can.