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Complexity is anything related to the structure of a software system that makes it hard to understand and modify the system.
Change amplification: The first symptom of complexity is that a seemingly simple change requires code modifications in many different places.
Cognitive load: The second symptom of complexity is cognitive load, which refers to how much a developer needs to know in order to complete a task.
Sometimes an approach that requires more lines of code is actually simpler, because it reduces cognitive load.
Unknown unknowns: The third symptom of complexity is that it is not obvious which pieces of code must be modified to complete a task, or what information a developer must have to carry out the task successfully.
One of the most important goals of good design is for a system to be obvious. This is the opposite of high cognitive load and unknown unknowns. In an obvious system, a developer can quickly understand how the existing code works and what is required to make a change.
a dependency exists when a given piece of code cannot be understood and modified in isolation; the code relates in some way to other code, and the other code must be considered and/or modified if the given code is changed.
The second cause of complexity is obscurity. Obscurity occurs when important information is not obvious.
In many cases, obscurity comes about because of inadequate documentation; Chapter 13 deals with this topic. However, obscurity is also a design issue. If a system has a clean and obvious design, then it will need less documentation.
Complexity isn’t caused by a single catastrophic error; it accumulates in lots of small chunks. A single dependency or obscurity, by itself, is unlikely to affect significantly the maintainability of a software system. Complexity comes about because hundreds or thousands of small dependencies and obscurities build up over time.
