Metadata & Context

Metadata and context allow you to add structured information to your logs, making them more informative and easier to analyze.

Context Property

Use the context property to add structured data to your logs:

// Basic context usage
Log.info('User login successful', {
    context: {
        userId: 123,
        username: 'john.doe',
        ipAddress: '192.168.1.1'
    }
});

// Context with error logging
Log.error('Database connection failed', {
    context: {
        database: 'users',
        host: 'localhost',
        port: 5432,
        retryCount: 3
    }
}, error);

Metadata Property

Use the metadata property for additional structured information:

// Metadata with performance data
Log.info('API request completed', {
    metadata: {
        responseTime: 234,
        statusCode: 200,
        cacheHit: true
    }
});

// Metadata with business context
Log.warn('Order processing delayed', {
    metadata: {
        orderId: 'ORD-123',
        priority: 'high',
        estimatedDelay: '5 minutes'
    }
});

Combining Context and Metadata

// Using both context and metadata
Log.info('Payment processed', {
    context: {
        customerId: 456,
        orderId: 'ORD-789'
    },
    metadata: {
        amount: 99.99,
        currency: 'USD',
        paymentMethod: 'credit_card',
        processingTime: 1.2
    }
});

Express.js Integration

app.use((req, res, next) => {
    // Add request context to all logs
    req.logContext = {
        requestId: req.id,
        method: req.method,
        url: req.url,
        userAgent: req.get('User-Agent')
    };
    next();
});

app.post('/api/users', async (req, res) => {
    Log.info('User creation started', {
        context: req.logContext,
        metadata: {
            email: req.body.email,
            role: req.body.role
        }
    });
    
    try {
        const user = await userService.create(req.body);
        
        Log.info('User created successfully', {
            context: req.logContext,
            metadata: {
                userId: user.id,
                email: user.email
            }
        });
        
        res.json(user);
    } catch (error) {
        Log.error('User creation failed', {
            context: req.logContext,
            metadata: {
                email: req.body.email,
                validationErrors: error.validationErrors
            }
        }, error);
        
        res.status(400).json({ error: 'Failed to create user' });
    }
});

Common Usage Patterns

Performance Monitoring

const startTime = Date.now();

// ... operation ...

Log.info('Operation completed', {
    metadata: {
        duration: Date.now() - startTime,
        operationType: 'database_query',
        recordsProcessed: 150
    }
});

Error Context

try {
    await processPayment(paymentData);
} catch (error) {
    Log.error('Payment processing failed', {
        context: {
            customerId: paymentData.customerId,
            orderId: paymentData.orderId
        },
        metadata: {
            amount: paymentData.amount,
            paymentMethod: paymentData.method,
            attemptNumber: 2
        }
    }, error);
}

Business Events

Log.info('Product purchased', {
    context: {
        customerId: customer.id,
        sessionId: session.id
    },
    metadata: {
        productId: product.id,
        price: product.price,
        category: product.category,
        discountApplied: discount.amount
    }
});

Best Practices

Next Steps

• Global Metadata - Add consistent context to all logs
• Tags - Categorize and organize logs
• Correlation IDs - Track requests across services