Appendix

Quick Reference: Debugging

Find it. Fix it. Verify it.

The Methodology

Reproduce it. If you can't reproduce it, you can't fix it with confidence.
Isolate it. Narrow down where the problem is.
Understand it. Know why it's happening before you fix it.
Fix it. Smallest change that addresses root cause.
Verify it. Confirm fix works. Confirm nothing else broke.

Reproducing the Problem

[ ] What are the exact steps?
[ ] What's the environment?
[ ] Is it consistent or intermittent?
[ ] Can you write a failing test?
[ ] What data/state is required?

Isolating the Problem

[ ] What changed recently?
[ ] Where does it work? Where does it fail?
[ ] Can you simplify the reproduction case?
[ ] Binary search: eliminate half the possibilities at a time

Before You Touch the Code

[ ] Read the error message. All of it.
[ ] Check the logs.
[ ] Check recent changes (git log, deploy history).
[ ] Search for similar issues.

When You're Stuck

[ ] What assumptions are you making?
[ ] Can you verify each assumption?
[ ] Take a break. Fresh eyes.
[ ] Explain the problem out loud (rubber duck).
[ ] Ask someone else to look.

After You Fix It

[ ] Does the fix address the root cause?
[ ] Did you write a test for it?
[ ] Are there other places this could happen?
[ ] What can you do to prevent this class of bug?

The Mindset

Assume nothing
Verify everything
The bug is always in the last place you look, so look there first