How do you scale a Node.js application across cores and machines?

Detailed Answer

Answer: Because your JavaScript runs on one thread, scaling means running more Node processes — on the same machine and across machines.

1. Use all cores on one machine — cluster / PM2:

const cluster = require('cluster');
const os = require('os');
if (cluster.isPrimary) {
  os.cpus().forEach(() => cluster.fork()); // one worker per core
  cluster.on('exit', () => cluster.fork()); // restart crashed workers
} else {
  require('./server'); // each worker runs the HTTP server, sharing the port
}

In practice, PM2 does this for you (pm2 start app.js -i max) and adds restarts, zero-downtime reloads, and monitoring.

2. Scale horizontally — many instances behind a load balancer:

Run N containers/VMs; put Nginx / a cloud LB / Kubernetes in front.
The orchestrator handles health checks, rolling deploys, and autoscaling.

3. The enabling requirement — statelessness:

Don't keep session state, caches, or uploaded files in a single process's memory (another instance won't have it).
Externalize state: sessions/cache → Redis, files → object storage (S3), pub/sub across instances → Redis/message broker.
With stateless instances, any request can hit any instance — the basis of easy horizontal scaling.

4. Other levers:

Offload CPU work to worker_threads or a background job queue so the request path stays responsive.
Cache and a CDN to reduce load.
Graceful shutdown so rolling deploys drop zero requests.

Interview summary: cluster/PM2 for cores, load-balanced stateless instances for machines, Redis/object-storage to externalize state.

How do you scale a Node.js application across cores and machines?

Quick Answer

Detailed Answer