Systematic Debugging

The Iron Law

NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST

The Four Phases

Phase 1: Root Cause Investigation

• Read Error Messages Carefully

• Reproduce Consistently

• Check Recent Changes

• Gather Evidence in Multi-Component Systems

• Trace Data Flow

Phase 2: Pattern Analysis

• Find Working Examples

• Compare Against References

• Identify Differences

• Understand Dependencies

Phase 3: Hypothesis and Testing

• Form Single Hypothesis

• Test Minimally

• Verify Before Continuing

Phase 4: Implementation

• Create Failing Test Case

• Implement Single Fix

• Verify Fix

• If 3+ Fixes Failed: Question Architecture

Red Flags - STOP and Follow Process

• "Quick fix for now, investigate later"

• "Just try changing X and see if it works"

• "I don't fully understand but this might work"

• Proposing solutions before tracing data flow

Quick Reference

Phase Key Activities Success Criteria

1. Root Cause Read errors, reproduce, check changes Understand WHAT and WHY

2. Pattern Find working examples, compare Identify differences

3. Hypothesis Form theory, test minimally Confirmed or new hypothesis

4. Implementation Create test, fix, verify Bug resolved, tests pass