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

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

### In Services
```javascript
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
```javascript
// 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:
```javascript
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