Logging & Monitoring

Structured Logging

import pino from 'pino';

const logger = pino({
  level: process.env.LOG_LEVEL || 'info',
});

// Good: Structured with context
logger.info({
  event: 'user_created',
  userId: user.id,
  email: user.email,
  duration: 150
}, 'User created successfully');

// Bad: Unstructured
console.log('User ' + user.id + ' created');

Log Levels

What to Log

DO Log

• Request/response (without sensitive data)
• Business events (user signup, purchase)
• Errors with stack traces
• Performance metrics
• Security events (login, failed auth)

DON'T Log

• Passwords or tokens
• PII without consent
• Credit card numbers
• Health check endpoints (too noisy)

Request Tracing

// Add request ID to all logs
app.use((req, res, next) => {
  req.requestId = uuid();
  logger.child({ requestId: req.requestId });
  next();
});

Metrics to Track

• Request rate (requests/second)
• Error rate (errors/total)
• Response time (p50, p95, p99)
• CPU/Memory usage
• Database query time
• Queue depth

Alerting Rules

• Error rate > 1% for 5 minutes
• Response time p99 > 2s
• CPU > 80% for 10 minutes
• Disk space < 20%