Interrogation Debugging

The Interrogation: Systematic Debugging

Overview

Random fixes waste time and create new bugs. Quick patches let the real rat walk free.

Core principle: ALWAYS find the root cause before attempting fixes. Symptom fixes are invalid.

Violating the letter of this process is violating the spirit of debugging.

The Iron Law

NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST

If you haven't completed The Brief, you cannot propose fixes. This is Omerta.

When to Use

Use for ANY technical issue:

• Test failures
• Bugs in production
• Unexpected behavior
• Performance problems
• Build failures
• Integration issues

Use this ESPECIALLY when:

• Under time pressure (emergencies make guessing tempting)
• "Just one quick fix" seems obvious
• You've already tried multiple fixes
• Previous fix didn't work
• You don't fully understand the issue

Don't skip when:

• Issue seems simple (simple bugs have root causes too)
• You're in a hurry (rushing guarantees rework)
• The Don wants it fixed NOW (systematic is faster than thrashing)

The Four Phases

You MUST complete each phase before proceeding to the next.

The Brief

BEFORE attempting ANY fix:

1. Read the Evidence

   • Don't skip past errors or warnings
   • They often contain the exact solution
   • Read stack traces completely
   • Note line numbers, file paths, error codes

2. Reproduce the Crime

   • Can you trigger it reliably?
   • What are the exact steps?
   • Does it happen every time?
   • If not reproducible → gather more data, don't guess

3. Check Recent Activity

   • What changed that could cause this?
   • Git diff, recent commits
   • New dependencies, config changes
   • Environmental differences

4. Gather Evidence at Every Boundary

   WHEN system has multiple components (CI → build → signing, API → service → database):

   BEFORE proposing fixes, add diagnostic instrumentation:

   For EACH component boundary:

   • Log what data enters the component
   • Log what data exits the component
   • Verify environment/config propagation
   • Check state at each layer

   Run once to gather evidence showing WHERE it breaks. THEN analyze evidence to identify the failing component. THEN investigate that specific component.

5. Trace the Data Flow

   WHEN error is deep in the call stack:

   • Where does the bad value originate?
   • What called this with the bad value?
   • Keep tracing backward until you find the source
   • Fix at source, not at symptom

Cross-Examination

Find the pattern before fixing:

1. Find Working Examples

   • Locate similar working code in the same codebase
   • What works that's similar to what's broken?

2. Compare Against References

   • If implementing a pattern, read the reference implementation COMPLETELY
   • Don't skim — read every line
   • Understand the pattern fully before applying

3. Identify Differences

   • What's different between working and broken?
   • List every difference, however small
   • Don't assume "that can't matter"

4. Understand Dependencies

   • What other components does this need?
   • What settings, config, environment?
   • What assumptions does it make?

The Theory

Scientific method:

1. Form Single Hypothesis

   • State clearly: "I think X is the root cause because Y"
   • Write it down
   • Be specific, not vague

2. Test Minimally

   • Make the SMALLEST possible change to test the hypothesis
   • One variable at a time
   • Don't fix multiple things at once

3. Verify Before Continuing

   • Did it work? Yes → The Hit
   • Didn't work? Form NEW hypothesis
   • DON'T add more fixes on top

4. When You Don't Know

   • Say "I don't understand X"
   • Don't pretend to know
   • Ask for help or research more

The Hit

Fix the root cause, not the symptom:

1. Create Failing Test Case

   • Simplest possible reproduction
   • Automated test if possible
   • One-off test script if no framework
   • MUST have before fixing
   • Use gangsta:drill-tdd for writing proper failing tests

2. Implement Single Fix

   • Address the root cause identified
   • ONE change at a time
   • No "while I'm here" improvements
   • No bundled refactoring

3. Verify Fix

   • Test passes now?
   • No other tests broken?
   • Issue actually resolved?
   • Use gangsta:sweep-verification to verify before claiming success

4. If Fix Doesn't Work

   • STOP
   • Count: How many fixes have you tried?
   • If < 3: Return to The Brief, re-analyze with new information
   • If ≥ 3: STOP and escalate (Step 5 below)
   • DON'T attempt Fix #4 without architectural discussion

5. If 3+ Fixes Failed: Escalate to the Don

   Pattern indicating architectural rot:

   • Each fix reveals new shared state/coupling/problem in a different place
   • Fixes require "massive refactoring" to implement
   • Each fix creates new symptoms elsewhere

   STOP and question fundamentals:

   • Is this pattern fundamentally sound?
   • Are we sticking with it through sheer inertia?
   • Should we refactor architecture vs. continue fixing symptoms?

   Discuss with the Don before attempting more fixes.

   This is NOT a failed hypothesis — this is a wrong architecture.

Red Flags — STOP and Follow Process

If you catch yourself thinking:

ALL of these mean: STOP. Return to The Brief.

The Don's Signals You're Doing It Wrong

Watch for these redirections:

• "Is that not happening?" — You assumed without verifying
• "Will it show us...?" — You should have added evidence gathering
• "Stop guessing" — You're proposing fixes without understanding
• "Think harder" — Question fundamentals, not just symptoms
• "We're stuck?" (frustrated) — Your approach isn't working

When you see these: STOP. Return to The Brief.

Common Rationalizations

Quick Reference

When Investigation Reveals No Root Cause

If systematic investigation reveals the issue is truly environmental, timing-dependent, or external:

1. You've completed the process
2. Document what you investigated
3. Implement appropriate handling (retry, timeout, error message)
4. Add monitoring/logging for future investigation

But: 95% of "no root cause" cases are incomplete investigation.

Related Skills

• gangsta:drill-tdd — For creating failing test case (The Hit, Step 1)
• gangsta:sweep-verification — Verify fix worked before claiming success

Omerta Compliance

• Rule of Truth: All findings cite specific code, error output, or evidence
• Spec is Law: Fixes trace to diagnosed root cause, not guesswork

Source: kucherenko/gangsta — distributed by TomeVault.