Monitoring & Observability

// app.js
const express = require('express');
const client = require('prom-client');

const app = express();

// Enable default metrics (CPU, memory, etc.)
client.collectDefaultMetrics();

// Custom metrics
const httpRequestsTotal = new client.Counter({
  name: 'http_requests_total',
  help: 'Total HTTP requests',
  labelNames: ['method', 'route', 'status']
});

const httpRequestDuration = new client.Histogram({
  name: 'http_request_duration_seconds',
  help: 'HTTP request latency in seconds',
  labelNames: ['method', 'route'],
  buckets: [0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5]
});

// Middleware to track requests
app.use((req, res, next) => {
  const end = httpRequestDuration.startTimer({
    method: req.method,
    route: req.route?.path || req.path
  });
  
  res.on('finish', () => {
    httpRequestsTotal.inc({
      method: req.method,
      route: req.route?.path || req.path,
      status: res.statusCode
    });
    end();
  });
  
  next();
});

// Metrics endpoint
app.get('/metrics', async (req, res) => {
  res.set('Content-Type', client.register.contentType);
  res.send(await client.register.metrics());
});

// logger.js
const pino = require('pino');

const logger = pino({
  level: process.env.LOG_LEVEL || 'info',
  formatters: {
    level(label) {
      return { level: label };
    }
  },
  timestamp: pino.stdTimeFunctions.isoTime,
  // Add service context to all logs
  base: {
    service: 'api-gateway',
    version: process.env.APP_VERSION,
    env: process.env.NODE_ENV
  }
});

// Usage
logger.info({
  event: 'user_login',
  userId: '12345',
  ipAddress: '192.168.1.1',
  durationMs: 45
}, 'User logged in successfully');

logger.error({
  event: 'payment_failed',
  userId: '12345',
  orderId: 'ORD-789',
  err: error  // Pino serializes errors nicely
}, 'Payment processing failed');

// Output (JSON):
// {"level":"info","time":"2024-01-15T...","service":"api-gateway","event":"user_login",...}