---

name: logging-best-practices description: Structured logging with proper levels, context, PII handling, centralized aggregation. Use for application logging, log management integration, distributed tracing, or encountering log bloat, PII exposure, missing context errors.

Logging Best Practices

Implement secure, structured logging with proper levels and context.

Log Levels

Structured Logging (Winston)

Copyconst winston = require('winston');

const logger = winston.createLogger({
  level: process.env.LOG_LEVEL || 'info',
  format: winston.format.combine(
    winston.format.timestamp(),
    winston.format.json()
  ),
  defaultMeta: { service: 'api-service' },
  transports: [
    new winston.transports.Console(),
    new winston.transports.File({ filename: 'error.log', level: 'error' })
  ]
});

// Usage
logger.info('User logged in', { userId: '123', ip: '192.168.1.1' });
logger.error('Payment failed', { error: err.message, orderId: '456' });

Request Context

Copyconst { AsyncLocalStorage } = require('async_hooks');
const storage = new AsyncLocalStorage();

app.use((req, res, next) => {
  const context = {
    requestId: req.headers['x-request-id'] || uuid(),
    userId: req.user?.id
  };
  storage.run(context, next);
});

function log(level, message, meta = {}) {
  const context = storage.getStore() || {};
  logger.log(level, message, { ...context, ...meta });
}

PII Sanitization

Copyconst sensitiveFields = ['password', 'ssn', 'creditCard', 'token'];

function sanitize(obj) {
  const sanitized = { ...obj };
  for (const field of sensitiveFields) {
    if (sanitized[field]) sanitized[field] = '[REDACTED]';
  }
  if (sanitized.email) {
    sanitized.email = sanitized.email.replace(/(.{2}).*@/, '$1***@');
  }
  return sanitized;
}

Best Practices

• Use structured JSON format
• Include correlation IDs across services
• Sanitize all PII before logging
• Use async logging for performance
• Implement log rotation
• Never log at DEBUG in production

Additional Implementations

See references/advanced-logging.md for:

• Python structlog setup
• Go zap high-performance logging
• ELK Stack integration
• AWS CloudWatch configuration
• OpenTelemetry tracing

Never Do

• Log passwords or tokens
• Use console.log in production
• Log inside tight loops
• Include stack traces for client errors