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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:

JavaScript
// 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:

JavaScript
// 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

JavaScript
// 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

JavaScript
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

JavaScript
const startTime = Date.now();

// ... operation ...

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

Error Context

JavaScript
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

JavaScript
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

Best Practices

  • Use context for request/operation data: User IDs, request IDs, session data
  • Use metadata for business/technical data: Performance metrics, business values
  • Keep objects flat: Avoid deeply nested structures
  • Use consistent naming: Stick to camelCase or snake_case throughout your app
  • Include relevant data only: Don't log everything, focus on what's useful for debugging

Next Steps

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Global Metadata
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