What makes software stable

Having been given a new area of responsibility as team lead inspired me to think about aspects of coding beyond just typing it.

I’ve experienced we where chasing our own tail. Trying to close issues “fast” and hurry on to the next in line. But something wasn’t working. Every time we changed a comma in one end, something in the other end would break horribly. Bug fixing was feeling like killing a hydra. I started blaming the “fast” way of doing things.

I needed to figure out what was wrong with our approach and transition into one that would result in “making stable software”. My gut feeling told me that we needed to become better at “decoupling” and “breaking the code into smaller pieces”.

But I was missing the direct link between my gut feeling and how it would translate into “stable software” until I saw a tweet which brought the things together:

Source @frankvilhelmsen

one of the most important quality parameters for stable code is that it not change too much - apart from the essential bugs of course #hint

Okay I could easily follow the logic: Code that doesn’t change much, is usually a sign of stable code (why change it, if it works - right?).

The solution would be to write code that doesn’t need to be changed in the first place, and when “decoupling” and “breaking code into smaller pieces” are done correctly, it does exactly that. Code impacted by changes is greatly reduced.


Imagine a piece of software consisting of a single function. Now every time a change is made (a bug is fixed or a feature is implemented) in that function 100% of the system is changed. If instead it was possible to split this function up into 3 functions then having to change a function would only mean that 33% of the system was changed.

33% beats 100% every time.

And before someone says it I would like to entertain the possibility that the exact same code from the 3 functions was gathered in 1 function. What would be the difference?

The computational results of the two pieces of code right then and there would of course be the same. But the fact that the code isn’t already split up is in my experience a sign of too tightly coupled code. Also having the code split up usually makes it both easier to test and reason about (win/win).

Having to change a long chain of functions to fix a bug or implement a new feature is usually ALSO a sign of too tightly coupled code. Just to point out that having a lot of functions doesn’t necessarily decouple the code :-)

Anyways… for identifying both what and how to “decouple” and “break down” code, I use my “hands on” experience from functional programming and async systems. Also reading about S.O.L.I.D. principles is i good source of inspiration, though nothing beats “hands on”.

Creative Commons License
What makes software stable by Jacob Emcken is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.