Clustering in Node.js

Node.js runs in a single thread by default. To take advantage of multi-core systems, you can use the built-in cluster module to spawn worker processes that share the same server port.

How it Works

The cluster module allows you to create worker processes that handle incoming requests. The master process manages the workers and distributes incoming connections using a round-robin strategy (except on Windows).

Basic Example

const cluster = require('cluster');
const http = require('http');
const numCPUs = require('os').cpus().length;

if (cluster.isMaster) {
  console.log(`Master ${process.pid} is running`);

  // Fork workers.
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  cluster.on('exit', (worker, code, signal) => {
    console.log(`worker ${worker.process.pid} died`);
    // Optionally restart the worker
    cluster.fork();
  });
} else {
  // Workers can share any TCP connection
  // In this case it is an HTTP server
  http.createServer((req, res) => {
    res.writeHead(200);
    res.end('hello world\n');
  }).listen(8000);

  console.log(`Worker ${process.pid} started`);
}

Benefits of Clustering

• Performance: Improved throughput on multi-core machines.
• Availability: If a worker crashes, other workers can continue to serve requests.
• Zero-Downtime Reloads: Workers can be restarted one by one.

When to use Cluster vs. Worker Threads?

• Cluster: Use for scaling I/O heavy tasks (like HTTP servers) across multiple CPU cores. Each worker has its own memory.
• Worker Threads: Use for CPU-intensive tasks where memory sharing is beneficial.