Monitoring Alerting

Monitoring & Alerting

The Golden Signals

Signal	What it measures	Example metric
Latency	Time to response	p50, p95, p99
Traffic	System demand	requests/sec
Errors	Failure rate	error %
Saturation	Capacity headroom	CPU, memory

Prometheus Metrics

import { Registry, Counter, Histogram, Gauge } from 'prom-client';

const registry = new Registry();

// Counter - tracks total count
const httpRequests = new Counter({
  name: 'http_requests_total',
  help: 'Total HTTP requests',
  labelNames: ['method', 'path', 'status'],
  registers: [registry],
});

// Histogram - tracks distributions
const httpDuration = new Histogram({
  name: 'http_request_duration_seconds',
  help: 'HTTP request duration',
  labelNames: ['method', 'path'],
  buckets: [0.01, 0.05, 0.1, 0.5, 1, 5],
  registers: [registry],
});

// Gauge - tracks current value
const activeConnections = new Gauge({
  name: 'active_connections',
  help: 'Number of active connections',
  registers: [registry],
});

// Use in middleware
app.use((req, res, next) => {
  const start = Date.now();

  res.on('finish', () => {
    const duration = (Date.now() - start) / 1000;
    httpRequests.inc({ method: req.method, path: req.route?.path, status: res.statusCode });
    httpDuration.observe({ method: req.method, path: req.route?.path }, duration);
  });

  next();
});