dvcrn Debug Methodology
Systematic approach to debugging and problem-solving. Distilled from real production incidents and industry best practices.
⚠️ The Root Cause Imperative
Every fix MUST target the root cause. Workarounds are forbidden unless explicitly approved.
Before proposing ANY solution, pass the Root Cause Gate:
┌─────────────────────────────────────────────┐
│ ROOT CAUSE GATE │
│ │
│ 1. What is the ACTUAL problem? │
│ 2. WHY does it happen? (not just WHAT) │
│ 3. Does my fix eliminate the WHY? │
│ YES → proceed │
│ NO → this is a workaround → STOP │
│ │
│ Workaround test: │
│ "If I remove my fix, does the bug return?" │
│ YES → workaround (fix the cause instead)│
│ NO → genuine fix ✅ │
└─────────────────────────────────────────────┘
The 5 Whys — Mandatory for Non-Obvious Problems
Problem: API returns 524 timeout
Why? → Cloudflare cuts connections >100s
Why? → The API call takes >100s
Why? → Using non-streaming request, server holds connection silent
Why? → Code uses regular fetch, not streaming
Fix: → Use streaming (server sends data continuously, Cloudflare won't cut)
❌ WRONG: Switch to faster model (workaround — avoids the timeout instead of fixing it)
✅ RIGHT: Use streaming API (root cause — Cloudflare needs ongoing data)
Common Workaround Traps
| Problem | Workaround (❌) | Root Cause Fix (✅) |
|---|---|---|
| API timeout | Switch to faster model | Use streaming / fix the slow query |
| Data precision loss | Search by name instead of ID | Fix BigInt parsing |
| Search returns nothing | Try different search strategy | Fix the search implementation |
| Dependency conflict | Downgrade / pin version | Use correct environment (venv) |
| Feature doesn't work | Remove the feature | Debug why it fails |
Self-check question: "Am I solving the problem, or avoiding it?"
Phase 1: STOP — Assess Before Acting
Before ANY fix attempt:
□ What is the EXACT symptom? (error message, behavior, screenshot)
□ When did it last work? What changed since then?
□ How is the service running? (process, env, startup command)
For running services:
ps -p <PID> -o command= # How was it started?
ls .venv/ venv/ env/ # Virtual environment?
which python3 && python3 --version
which node && node --version
NEVER restart a service without first recording its original startup command.
Phase 2: Hypothesize — Form ONE Theory
Priority order:
- Did I change something? → diff/revert first
- Did the environment change? → versions, deps, configs
- Did external inputs change? → API responses, data formats
- Genuine new bug? → only after ruling out 1-3
Phase 3: Test — One Change at a Time
Change X → Test → Works? → Done
→ Fails? → REVERT X → new hypothesis
Do NOT stack changes.
Phase 4: Patch-Chain Detection
2 fix attempts failed → STOP. Revert ALL. Back to Phase 1.
You are likely:
- Fixing symptoms of a wrong fix
- In the wrong environment entirely
- Misunderstanding the architecture
Phase 5: Post-Fix Verification
After any fix, verify:
□ Does it solve the ORIGINAL problem? (not just silence the error)
□ Did I introduce new issues? (regression check)
□ Would removing my fix bring the bug back? (confirms causality)
□ Is the fix in the right layer? (not patching symptoms upstream)
Anti-Patterns
🚨 Workaround Addiction (NEW — Most Common!)
Bypassing the problem instead of fixing it. "It's slower but works" / "Use a different approach". → Ask: "Am I solving or avoiding?" If avoiding → find the real fix. → Workarounds are ONLY acceptable when: (1) explicitly approved by user, (2) clearly labeled as temporary, (3) a TODO is created for the real fix.
🚨 Drunk Man Anti-Pattern
Randomly changing things until the problem disappears. → Each change needs a hypothesis.
🚨 Streetlight Anti-Pattern
Looking where comfortable, not where the problem is. → "Is this where the bug IS, or where I KNOW HOW TO LOOK?"
🚨 Cargo Cult Fix
Copying a fix without understanding why it works. → Understand the mechanism first.
🚨 Ignoring the User
User says "it broke after you changed X" → immediately diff X. → User observations are the most valuable data.
Environment Checklist
□ Runtime: system or venv/nvm?
□ Dependencies: match expected versions?
□ Config: .env, config.json — recent changes?
□ Process manager: PM2/systemd — restart method?
□ Logs: tail -f before reproducing
□ Backup: snapshot before any change
Deployment Safety (Hardened SCP Flow)
Iron Rule: NEVER edit files directly on the server. NEVER overwrite server files without backup.
Standard deployment (every time, no exceptions):
1. PULL scp server:/opt/apps/项目/ ./local-项目/
(pull the files you need + related files)
2. EDIT Make changes locally
(complex multi-line → write full file, never sed)
3. VERIFY node -c *.js # syntax check
node -e "require('./file')" # module load check
(STOP if verification fails — do not proceed)
4. BACKUP ssh server "cp file file.bak.$(date +%s)"
5. PUSH scp ./local-file server:/opt/apps/项目/file
6. RESTART pm2 restart <app>
(use SAME method as original — check ps/pm2 show first)
7. HEALTH curl -s http://localhost:<port>/health
pm2 logs <app> --lines 5 --nostream
(if unhealthy → revert backup immediately)
Pull Scope Rules
Changing 1 file → pull that file + its imports/importers
Changing routes → also pull server.js (check mount points)
Changing frontend → also pull index.html (check script tags)
Changing config → also pull code that reads the config
Unsure what to pull → pull the whole project directory
What NOT to Do
❌ sed -i for multi-line code on server
❌ Skip node -c after editing .js
❌ pm2 restart before syntax verification
❌ Tell user to refresh before health check passes
❌ Push without backup
🚨 Server Code Modification Rules
Every code change on a server MUST be syntax-verified before restart/reload.
After editing .js files:
□ node -c <file> # Syntax check
□ node -e "require('./<file>')" # Module load check (for route files)
□ FAIL → DO NOT restart. DO NOT tell user to refresh. Fix first.
After editing .html files:
□ Check critical tag closure (div/script/style)
□ grep -c '<div' file && grep -c '</div' file # Count match
Complex multi-line changes:
□ Write complete file locally → scp upload
□ NEVER use sed for multi-line code insertion (newlines get swallowed)
□ If sed is unavoidable → verify with node -c immediately after
Restart sequence:
□ node -c *.js passes → pm2 restart <app>
□ Check pm2 logs --lines 5 for startup errors
□ curl health endpoint to confirm service is up
Why: sed -i multi-line insertion silently corrupts JS (newlines become single line), causing syntax errors that break the entire page with no visible error to the user.
Decision Tree
Problem appears
├─ I just edited something? → DIFF → REVERT if suspect
├─ Service won't start? → CHECK startup command + env
├─ New error after fix? → STOP (patch chain!) → Revert → Phase 1
├─ User reports regression? → DIFF before/after
├─ Tempted to work around? → ROOT CAUSE GATE → fix the real issue
└─ Intermittent? → CHECK logs + external deps + timing