A Philosophy of Software Design — Distilled Guide

Source: John Ousterhout, A Philosophy of Software Design Central thesis: The core challenge of software design is managing complexity.

I. Complexity: Know the Enemy

Definition

Complexity is anything related to the structure of a software system that makes it hard to understand and modify. Complexity is not the same as system size — a small system can be complex, and a large well-designed system can be manageable.

Three Symptoms of Complexity

1. Change Amplification: A seemingly simple change requires code modifications in many different places.
2. Cognitive Load: Developers must absorb a large amount of information to complete a task safely. Note: fewer lines of code ≠ simpler — sometimes more code is actually simpler because it reduces cognitive load.
3. Unknown Unknowns: It's unclear which code must be modified or what information is needed to complete a task. This is the most dangerous symptom.