Logging Best Practices — Comprehensive Guide to Production Logging

// Logging best practices implementation

// 1. Define clear log level guidelines
const logLevelGuide = {
  FATAL: 'Application crash, data corruption, security breach. Immediate action required.',
  ERROR: 'Request failed, service degraded, external dependency failure. Needs investigation.',
  WARN: 'Unexpected but handled. Rate limiting, fallback used, deprecated API called.',
  INFO: 'Business events: scan started, user registered, file processed. Standard operations.',
  DEBUG: 'Detailed diagnostic: query parameters, intermediate values, state transitions.',
  TRACE: 'Step-by-step execution: function entry/exit, loop iterations, micro-timings.'
};

// 2. Never log sensitive data
const sensitiveFields = ['password', 'secret', 'token', 'authorization', 'creditCard', 'ssn'];

function sanitizeLogInput(data) {
  if (typeof data === 'object') {
    const sanitized = { ...data };
    for (const key of Object.keys(sanitized)) {
      if (sensitiveFields.some(f => key.toLowerCase().includes(f))) {
        sanitized[key] = '[REDACTED]';
      }
    }
    return sanitized;
  }
  return data;
}

// 3. Include sufficient context
logger.info('Scan processed', {
  scanId: scan.id,
  fileName: scan.fileName,
  fileSize: scan.fileSize,
  duration: scan.durationMs,
  threats: scan.threats.length,
  userId: req.user?.id,
  correlationId: req.correlationId
});

// 4. One log entry per meaningful event
// BAD:
logger.debug('Starting scan');
logger.debug('File size: ' + fileSize);
logger.debug('Scan type: ' + scanType);
logger.debug('Scan completed');

// GOOD:
logger.info('Scan completed', { scanId, fileSize, scanType, duration, threats });

// 5. Use structured data, not string concatenation
// BAD:
logger.info(`User ${userId} scanned file ${fileName} with result ${result}`);

// GOOD:
logger.info('Scan processed', { userId, fileName, result, duration });

// 6. Log at the right level
// ERROR for failures, WARN for fallbacks, INFO for business events, DEBUG for troubleshooting