tilbudgivern/backend/LOGGING.md

Structured Logging & Correlation Tracking

This backend implements structured logging with correlation IDs for end-to-end request tracing.

Features

Correlation IDs

• Every request gets a unique correlationId (UUID v4)
• Propagated through middleware, services, and logs
• Returned in response headers as X-Correlation-ID
• Supports client-provided IDs via X-Correlation-ID or X-Request-ID headers

Structured Logs

• JSON format in production
• Colorized console with correlation ID in development
• All logs include: timestamp, level, context, correlationId, message
• Database persistence with full request context

Log Levels

• error: System errors, exceptions
• warn: Slow requests (>1s), deprecated features
• info: Request/response cycles, business events
• debug: Detailed troubleshooting info

Usage

In Middleware

const logger = require('./src/utils/logger');

app.use((req, res, next) => {
  logger.logInfo('MyContext', 'Processing request', { 
    correlationId: req.correlationId,
    req 
  });
  next();
});

In Services

const logger = require('./src/utils/logger');

async function myService(req, data) {
  try {
    logger.logInfo('MyService', 'Starting operation', {
      correlationId: req.correlationId,
      metadata: { itemId: data.id }
    });
    
    // ... operation
    
  } catch (error) {
    logger.logError('MyService', error, {
      correlationId: req.correlationId,
      errorCode: 'SERVICE_ERROR'
    });
    throw error;
  }
}

Client-Side Tracking

// Send correlation ID from frontend
axios.get('/api/endpoint', {
  headers: {
    'X-Correlation-ID': clientGeneratedId
  }
});

// Read it from response
const correlationId = response.headers['x-correlation-id'];

Azure App Insights Ready

Logs are structured for easy integration with Azure Application Insights:

• Correlation IDs map to operation_Id
• Contexts map to customDimensions
• Performance tracking via response times

To enable:

const appInsights = require('applicationinsights');
appInsights.setup(process.env.APPINSIGHTS_INSTRUMENTATIONKEY);
appInsights.defaultClient.context.tags[appInsights.defaultClient.context.keys.cloudRole] = 'tilbudgivern-backend';
appInsights.start();

Performance

• Async logging doesn't block requests
• Connection pooling for database logs
• Automatic cleanup on process termination