Maintainable Code

Published: 2022-11-15


When we say “maintainable code” what does that mean?

I say that it means code that is easy to change in order to accommodate new features or dependency updates.

What does it mean for code to be easy to change?

I say code that is easy to change is code that is easy to understand.

What does it mean for code to be easy to understand?

I say code that is easy to understand has fast feedback loops.

And fast feedback loops are where we can differentiate.

In short: if I had to pick one property of maintainable code I would say that the code has fast feedback loops.

Fast Feedback Loops

A fast feedback loop can be code that has comprehensive unit tests that run as fast as unit tests should (hundreds or thousands per second). In this form the fast feedback loop is mechanical: the tests themselves guide future change and help ensure correctness. If the tests are well written, comprehensive, and fast then the production code can be allowed to be opaque with necessary complexity.

A fast feedback loop can be code that has well organized and consistent abstractions in the production code. That means you won’t see high level code wrapping CSV output right next to code using string manipulation to build HTML output. In this form the fast feedback loop is mental: you the programmer can quickly parse and understand the code directly without being guided by tests.

A fast feedback loop can be clearly written and understandable documentation that guides the programmer through different areas of the codebase. In this form major responsibilities and considerations of the code are listed directly along with their past issues, present circumstances, and future warnings. The fast feedback loop comes from quickly being able to identify areas of the code that need to change and how to see effects of those changes. In this form the fast feedback loop can be both mechanical and mental. The documentation may explain specifics such as how to manually exercise areas of code or may explain the logic flow clearly enough that a programmer new to the code will be able to mentally follow along.

How do you know if you have fast feedback loops?

For mechanical feedback loops consider the amount of time between changing a line of code and seeing the effects. Consider the complexity of setting up an environment in which to see code changes reflected in a running system. The time between changing a line of code and seeing its effects in some way is the speed of your feedback loops.

For mental feedback loops consider the amount of domain knowledge a programmer needs to have in order to follow along with the code. Consider the chain of responsibility when traced from code entry to code exit. Every jump from one abstraction to another slows down the mental feedback loop of someone tracing through the code.

If your codebase doesn’t have any of the above feedback loops then nothing is more helpful to future developers than working on bringing one or more of those feedback loops to your codebase. Documentation, tests, well organized code: pick all three if you can. Pick at least one if you can’t. If you don’t know what to pick then documentation is applicable to any codebase.


What should you document?

  • List every major responsibility of the code
  • Describe the lifecycles of the code and its abstractions
    • Does it run indefinitely like a web application with child abstractions used during some requests?
    • Does it run for a set unit of work like the ls command that runs, lists files, and exits?
  • List all the files or at least the directories and their responsibilities
  • List problematic areas of the code
  • List areas of the code with pending improvements
  • How do the mechanical feedback loops work? How do you run the tests? How do you see effects of code changes?
  • What are the collaborators of the codebase? (i.e. what other systems does it rely on and how?)

In short: write down everything you know about the code even going as far as having every programmer in the code writing their own docs/ file with running documentation and commentary


A well architected and organized codebase guides new developers to effective contributions following clearly established patterns.

If your code changes frequently sprawl over unanticipated files and side effects then you do not have a maintainable codebase.

If you find several (re)implementations of the same abstraction throughout your codebase then you essentially have the antithesis of maintainable code.

Logging, metrics, telemetry

Is the codebase getting faster or slower? Is it using more or less memory?

You cannot effectively maintain code if you don’t know what it is doing.