Monitoring & Logging

# Monitoring & Logging

Systematic monitoring, logging, and alerting for production applications.

## Overview

This Skill enforces:
- Structured logging (JSON format)
- CloudWatch integration
- Error tracking with Sentry
- Distributed tracing
- Alert configuration
- Dashboard setup
- Log aggregation
- Metrics collection

Apply when setting up monitoring, tracking errors, or debugging production issues.

## Structured Logging

### JSON Logging Format

```ts
// ✅ GOOD: Structured logs with context
const logger = {
  info: (message: string, context?: Record<string, any>) => {
    console.log(JSON.stringify({
      timestamp: new Date().toISOString(),
      level: 'info',
      message,
      requestId: context?.requestId,
      userId: context?.userId,
      duration: context?.duration,
      ...context
    }));
  },
  error: (message: string, error?: Error, context?: Record<string, any>) => {
    console.error(JSON.stringify({
      timestamp: new Date().toISOString(),
      level: 'error',
      message,
      errorName: error?.name,
      errorMessage: error?.message,
      errorStack: error?.stack,
      requestId: context?.requestId,
      ...context
    }));
  },
  warn: (message: string, context?: Record<string, any>) => {
    console.warn(JSON.stringify({
      timestamp: new Date().toISOString(),
      level: 'warn',
      message,
      requestId: context?.requestId,
      ...context
    }));
  }
};

// Usage
logger.info('User created', { userId: user.id, email: user.email });
logger.error('Database connection failed', error, { 
  host: 'db.example.com',
  port: 5432 
});
```

### Middleware for Request Logging

```ts
// middleware/logging.ts
import { NextRequest, NextResponse } from 'next/server';

export function middleware(request: NextRequest) {
  const requestId = crypto.randomUUID();
  const start = Date.now();

  const response = NextResponse.next();
  response.headers.set('x-request-id', requestId);

  // Log after response
  const duration = Date.now() - start;
  console.log(JSON.stringify({
    timestamp: new Date().toISOString(),
    level: 'info',
    message: 'HTTP Request',
    requestId,
    method: request.method,
    path: request.nextUrl.pathname,
    status: response.status,
    duration,
    userAgent: request.headers.get('user-agent')
  }));

  return response;
}
```

## CloudWatch Integration

### AWS SDK Logging

```ts
// lib/cloudwatch.ts
import { CloudWatchLogsClient, PutLogEventsCommand } from '@aws-sdk/client-cloudwatch-logs';

const client = new CloudWatchLogsClient({ region: 'us-east-1' });

export async function logToCloudWatch(
  logGroup: string,
  logStream: string,
  message: string
) {
  const command = new PutLogEventsCommand({
    logGroupName: logGroup,
    logStreamName: logStream,
    logEvents: [
      {
        message: JSON.stringify({
          timestamp: new Date().toISOString(),
          message
        }),
        timestamp: Date.now()
      }
    ]
  });

  try {
    await client.send(command);
  } catch (error) {
    console.error('Failed to log to CloudWatch:', error);
  }
}
```

### Log Agent Setup

```bash
# Install CloudWatch Logs agent
sudo apt install awslogs -y

# Configure /etc/awslogs/awslogs.conf
[/var/log/nodejs/app.log]
log_group_name = /aws/nodejs/myapp
log_stream_name = {instance_id}
file = /var/log/nodejs/app.log
datetime_format = %Y-%m-%d %H:%M:%S

# Restart agent
sudo systemctl restart awslogsd
```

## Structured Logging in Code

```ts
// lib/logger.ts
import winston from 'winston';

const logger = winston.createLogger({
  level: process.env.LOG_LEVEL || 'info',
  format: winston.format.combine(
    winston.format.timestamp(),
    winston.format.errors({ stack: true }),
    winston.format.json()
  ),
  defaultMeta: { 
    service: 'myapp',
    environment: process.env.NODE_ENV
  },
  transports: [
    new winston.transports.Console(),
    new winston.transports.File({ 
      filename: 'logs/error.log', 
      level: 'error' 
    }),
    new winston.transports.File({ 
      filename: 'logs/combined.log' 
    })
  ]
});

export default logger;

// Usage
logger.info('User logged in', { userId: user.id });
logger.error('Database error', { error: err.message });
```

## Error Tracking with Sentry

### Setup Sentry

```bash
npm install @sentry/nextjs
```

### Configure

```ts
// sentry.client.config.ts
import * as Sentry from '@sentry/nextjs';

Sentry.init({
  dsn: process.env.NEXT_PUBLIC_SENTRY_DSN,
  environment: process.env.NODE_ENV,
  tracesSampleRate: 0.1,  // 10% of transactions
  beforeSend(event) {
    // Filter out certain errors
    if (event.exception) {
      const error = event.exception.values?.[0];
      if (error?.value?.includes('ResizeObserver')) {
        return null;  // Don't send
      }
    }
    return event;
  }
});
```

### Capture Errors

```ts
// Manual error capture
try {
  riskyOperation();
} catch (error) {
  Sentry.captureException(error, {
    tags: {
      section: 'user-profile'
    },
    contexts: {
      user: {
        userId: user.id,
        email: user.email
      }
    }
  });
}

// Automatic error capture
Sentry.withScope((scope) => {
  scope.setContext('user', { userId: user.id });
  Sentry.captureMessage('User action completed');
});
```

## Metrics Collection

### Custom Metrics

```ts
// lib/metrics.ts
import { CloudWatchClient, PutMetricDataCommand } from '@aws-sdk/client-cloudwatch';

const client = new CloudWatchClient({ region: 'us-east-1' });

export async function putMetric(
  metricName: string,
  value: number,
  unit: 'Count' | 'Seconds' | 'Milliseconds' = 'Count'
) {
  const command = new PutMetricDataCommand({
    Namespace: 'MyApp',
    MetricData: [
      {
        MetricName: metricName,
        Value: value,
        Unit: unit,
        Timestamp: new Date()
      }
    ]
  });

  try {
    await client.send(command);
  } catch (error) {
    console.error('Failed to put metric:', error);
  }
}

// Usage
await putMetric('UserCreated', 1, 'Count');
await putMe