Debugging Strategies

Name: debugging-strategies
Rating: 92
Author: ThinkfleetAI

Systematic approaches to finding and fixing bugs.

Stack Trace Analysis

Read the trace bottom-up

The bottom of a stack trace is the most recent call — start there.

bash

# Node.js — get full stack traces
NODE_OPTIONS="--stack-trace-limit=50" node app.js

# Python — verbose traceback
python3 -u -X tracemalloc app.py

Common patterns

•TypeError: Cannot read property of undefined → Check the object one level up in the chain
•ECONNREFUSED → Service isn't running or wrong port
•ENOMEM → Memory exhaustion, check for leaks
•Segfault → Native addon or buffer overflow

Log-Based Debugging

bash

# Tail logs with filtering
tail -f app.log | grep -E "ERROR|WARN"

# Search recent logs for pattern
grep -n "error" app.log | tail -20

# Correlate by request ID
grep "req-abc123" app.log

# Docker container logs
docker logs --since 5m --follow myapp 2>&1 | grep -i error

# Kubernetes pod logs
kubectl logs -l app=myapp --since=10m | grep -i "exception\|error\|fatal"

# Multiple pods
kubectl logs -l app=myapp --all-containers --prefix --since=5m

Git Bisect (Find the Breaking Commit)

bash

# Start bisect
git bisect start

# Mark current as bad
git bisect bad

# Mark known good commit
git bisect good v1.0.0

# Test each commit git checks out, then mark:
git bisect good  # or
git bisect bad

# Automate with a test script
git bisect run npm test

# When done
git bisect reset

# View bisect log
git bisect log

Memory Leak Detection

Node.js

bash

# Heap snapshot
node --inspect app.js
# Open chrome://inspect → Take heap snapshot → Compare snapshots

# Track memory over time
node -e "
setInterval(() => {
  const mem = process.memoryUsage();
  console.log(JSON.stringify({
    rss: (mem.rss / 1024 / 1024).toFixed(1) + 'MB',
    heap: (mem.heapUsed / 1024 / 1024).toFixed(1) + 'MB',
    external: (mem.external / 1024 / 1024).toFixed(1) + 'MB'
  }));
}, 5000);
require('./app.js');
"

# Generate heap dump on OOM
node --max-old-space-size=512 --heap-prof app.js

Python

bash

# Track object counts
python3 -c "
import tracemalloc
tracemalloc.start()
# ... run code ...
snapshot = tracemalloc.take_snapshot()
for stat in snapshot.statistics('lineno')[:10]:
    print(stat)
"

# objgraph for reference cycles
python3 -c "
import objgraph
objgraph.show_most_common_types(limit=10)
"

Network Debugging

bash

# DNS resolution
dig example.com +short
nslookup example.com

# Port check
nc -zv localhost 3000

# HTTP timing
curl -s -o /dev/null -w "DNS: %{time_namelookup}s\nConnect: %{time_connect}s\nTTFB: %{time_starttransfer}s\nTotal: %{time_total}s\n" https://example.com

# Watch connections
lsof -i :3000
netstat -an | grep 3000

Reproduce and Isolate

bash

# Minimal reproduction
# 1. Create minimal test case
# 2. Remove code until bug disappears
# 3. Last removal is the culprit

# Environment comparison
node --version && npm --version && echo "OS: $(uname -s)"
env | grep -iE "node|npm|path|home" | sort

# Check for race conditions — run multiple times
for i in $(seq 1 10); do
  npm test 2>&1 | tail -1
done

Notes

•Reproduce first, fix second. If you can't reproduce it, you can't verify the fix.
•git bisect run is underused — it automates finding the exact commit that broke something.
•Memory leaks show as gradually increasing RSS over time. Take snapshots at intervals and compare.
•When stuck, explain the bug to someone (or write it down). Rubber duck debugging works.
•Check the obvious first: is the service running? Are env vars set? Is the config correct?