majiayu000 Root Cause Tracing
Core Principle
TRACE BACKWARD TO THE BUG'S ORIGIN
Don't patch symptoms - fix the disease
The Backward Trace Method
SYMPTOM
↓ Where does this value come from?
IMMEDIATE CAUSE
↓ What set that to wrong value?
DEEPER CAUSE
↓ Why did that happen?
ROOT CAUSE ← Fix HERE
Practical Steps
1. Start at Symptom
Q: What exactly went wrong?
→ Document the exact symptom
→ Note file, line, error message
2. First Backward Step
Q: Where does this value/state come from?
→ Find the direct source
→ Check: is THIS the bug or just a carrier?
3. Keep Tracing
Q: What set THAT to the wrong value?
→ Follow the data flow backward
→ Each step: is this the origin or just passing it along?
4. Identify Root
SIGNS YOU'VE FOUND ROOT:
- Logic error in requirements interpretation
- Initial state incorrectly set
- Wrong assumption in algorithm
- Missing validation at entry point
- Broken invariant at creation
Stop Conditions
You've found root cause when:
- There's no earlier source to trace
- The error was introduced HERE (not inherited)
- Fixing HERE prevents the bug entirely
- The "why" answer is: "wrong logic/design"
You're still at symptoms if:
- The wrong value came from somewhere else
- Another function passed bad data
- It's a consequence, not origin
- Fixing here = adding a workaround
Common Patterns
| Bug Pattern | Root Usually At |
|---|---|
| Wrong value displayed | Data calculation, not display code |
| Null pointer | Where object should've been created |
| Off-by-one | Loop bounds definition, not body |
| Race condition | Shared state design, not specific access |
| Invalid state | State transition logic, not current handler |
Anti-Patterns (Don't Do This)
❌ Symptom Patching
// Bad: Fix where error happens
if (value === undefined) value = defaultValue
// Good: Find why value is undefined
❌ Guard Stacking
// Bad: Add guards everywhere
if (x) if (y) if (z) doThing()
// Good: Ensure x, y, z correct at source
❌ Quick Fix Pressure
"Just make it work" → Technical debt
Better: "5 more minutes tracing saves 5 hours debugging later"
Verification
After identifying root cause:
- Explain the chain - Symptom ← caused by ← caused by ← ROOT
- Confirm origin - "This is where wrong value first appears"
- Fix at root - Change the origin, not the symptoms
- Verify fix - Original symptom gone + no new bugs
Integration with Checkers
Use with:
assumption-checker→ Was root cause an assumption?dependency-validator→ Was root cause a broken dependency?pre-action-verifier→ Verify root cause fix before applying