Logging Best Practices Skill

Logging Best Practices

Implement secure, structured logging with proper levels and context.

Log Levels

| Level | Use For | Production | |-------|---------|------------| | DEBUG | Detailed debugging | Off | | INFO | Normal operations | On | | WARN | Potential issues | On | | ERROR | Errors with recovery | On | | FATAL | Critical failures | On |

Structured Logging (Winston)

const 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

const { 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

const 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

Agent Skills: Logging Best Practices

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