All 15 Clustering & Worker Threads questions
More in Clustering & Worker Threads
What is the os.cpus() function used for when setting up clustering in a Node.js application? How does PM2's cluster mode compare to manually using Node's built-in cluster module directly in application code? How do you pass data between the main thread and a worker thread using postMessage(), and what are the limits on what can be sent? What is the difference between the 'sticky' and default load-balancing strategies when using Node's cluster module, and when does it matter? What is the difference between the number of CPU cores and the optimal number of cluster workers for a given Node.js application? What is the difference between spawning a Worker Thread and spawning a Child Process for running a CPU-intensive JavaScript function? How do you gracefully terminate a worker thread, and what's the difference between worker.terminate() and letting it exit naturally? What happens to in-memory state (like an in-memory cache) when using the Node.js cluster module, and how do you handle this correctly? How do you use SharedArrayBuffer to share memory directly between Node.js worker threads, and what are the safety considerations? What is a worker thread pool, and why would you build one rather than creating a new Worker for every task?
Advertisement
Node.js › Clustering & Worker Threads

Why does Node.js provide a cluster module, given that a single Node.js process is single-threaded?

Intermediate

Answer

A single process runs JavaScript on one main thread, using only one CPU core -- the cluster module forks multiple worker processes (typically one per core), each an independent copy of the app sharing the same listening port, letting a Node.js app utilize all available cores.
const cluster = require('node:cluster');
const os = require('node:os');
if (cluster.isPrimary) {
  for (let i = 0; i < os.cpus().length; i++) cluster.fork();
} else {
  require('./server');
}
Real-world example An API deployed to an 8-core server using only one core adds cluster.fork() to spawn eight workers, roughly increasing throughput eight-fold under CPU-bound load.

Common follow-ups: How does the cluster module's shared-port mechanism distribute connections across workers?;What state can be shared between cluster workers, given they're separate OS processes?

Advanced Node.js;Performance Optimization & Profiling
All 15 Clustering & Worker Threads questions