Cluster Module

Node is single threaded. No matter how many cores the machine have, node only uses a single core of CPU.

This is fine for I/O operations but if the code has long running and CPU intensive operations, our application might struggle from a performance point of view.

To help with this scenario, node introduced Cluster module.

The cluster module enables the creation of child processes (also called workers) that run simultaneoulsy.

All created workers share the same server port.

Working
Master is only in charge of the workers.

Workers are in charge of handling incoming requests, reading files etc.

Each worker gets its own event loop, memory, and V8 instance.

Common methods:

• cluster.fork(): Creates a new worker process.
• cluster.on(event, callback): Listens to events like fork, online, exit, etc.

Example:

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

if (cluster.isMaster) {
  console.log(`Master process ${process.pid} is running`);
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }
  cluster.on('exit', (worker, code, signal) => {
    console.log(`Worker ${worker.process.pid} died`);
  });
} else {
  http.createServer((req, res) => {
    res.writeHead(200);
    res.end('Hello from Worker!\n');
  }).listen(8000);
  console.log(`Worker ${process.pid} started`);
}

Worker Threads Module

The worker_threads module enables the use of threads that execute JS in parallel.

Code executed in a worker thread runs in a separate child process, preventing it from blocking our main application.

The cluster module can be used to run multiple instances of Node.js that can distribute workloads.

While the worker_threads module allows running multiple application threads within a single Node.js instance.

When process isolation is not needed, that is, no separate instances of V8, event loop and memory are needed, we should use worker_threads module.

main.js

js
const { Worker } = require('worker_threads');

const worker = new Worker('./worker.js', { workerData: 5 });

worker.on('message', (msg) => console.log(`Result: ${msg}`));
worker.on('error', (err) => console.error(err));
worker.on('exit', () => console.log('Worker exited'));

worker.js

js
const { parentPort, workerData } = require('worker_threads');

let result = workerData * 2;
parentPort.postMessage(result);