Scaling Node — cluster, worker threads, child processes and PM2 — Node.js — Runtime Internals, Express, APIs and SQL Server

10. Scaling Node — cluster, worker threads, child processes and PM2

Advanced★ Must knowLesson 10 of 11

Tool	Use
cluster module / PM2 cluster mode	Run one Node process per CPU core sharing the port — scale an API on one machine
worker_threads	Run CPU-heavy JavaScript in parallel threads (separate V8 isolates) without blocking the event loop
child_process (`spawn`, `exec`, `fork`)	Run other programs/scripts as separate processes
Horizontal scaling	Many containers behind a load balancer (Kubernetes, App Service, ECS) — keep the app stateless

// CPU-heavy work in a worker thread
import { Worker } from 'node:worker_threads';
function hashInWorker(data) {
  return new Promise((resolve, reject) => {
    const w = new Worker(new URL('./hash-worker.js', import.meta.url), { workerData: data });
    w.once('message', resolve); w.once('error', reject);
  });
}

bash

pm2 start src/server.js -i max --name shop-api    # cluster mode, one process per core, auto-restart
pm2 logs; pm2 reload shop-api                      # zero-downtime reload

Performance tips: never block the event loop (sync fs/crypto in request paths, huge JSON.stringify), use streams for big data, connection pooling, caching (Redis), compression, UV_THREADPOOL_SIZE for heavy fs/crypto, measure with clinic doctor/--prof, watch event-loop lag and heap usage, avoid memory leaks (global caches, listeners never removed — MaxListenersExceededWarning).