Forking workers per CPU core in Node.js

Introduction

Forking workers per CPU core helps your Node.js app use all the computer's power. It makes your app faster and can handle more users at the same time.

When your app needs to handle many requests at once without slowing down.

When you want to use all CPU cores to improve performance.

When you want to keep your app running even if one worker crashes.

When you build a server that should be fast and reliable.

Syntax

Node.js

import cluster from 'cluster';
import os from 'os';

if (cluster.isPrimary) {
  const cpuCount = os.cpus().length;
  for (let i = 0; i < cpuCount; i++) {
    cluster.fork();
  }

  cluster.on('exit', (worker, code, signal) => {
    console.log(`Worker ${worker.process.pid} died. Restarting...`);
    cluster.fork();
  });
} else {
  // Worker code here, e.g., start server
}

cluster.isPrimary checks if the current process is the main one that controls workers.

cluster.fork() creates a new worker process.

Examples

This example forks one worker per CPU core and each worker prints its process ID.

Node.js

import cluster from 'cluster';
import os from 'os';

if (cluster.isPrimary) {
  const cpuCount = os.cpus().length;
  for (let i = 0; i < cpuCount; i++) {
    cluster.fork();
  }
} else {
  console.log(`Worker ${process.pid} started`);
}

This example creates an HTTP server in each worker to handle requests in parallel.

Node.js

import cluster from 'cluster';
import os from 'os';
import http from 'http';

if (cluster.isPrimary) {
  const cpuCount = os.cpus().length;
  for (let i = 0; i < cpuCount; i++) {
    cluster.fork();
  }
} else {
  http.createServer((req, res) => {
    res.writeHead(200);
    res.end(`Handled by worker ${process.pid}`);
  }).listen(8000);
}

Sample Program

This program uses the cluster module to fork one worker per CPU core. The primary process logs its start and forks workers. Each worker runs an HTTP server that responds with its process ID. If a worker dies, the primary process restarts it automatically.

Node.js

import cluster from 'cluster';
import os from 'os';
import http from 'http';

if (cluster.isPrimary) {
  const cpuCount = os.cpus().length;
  console.log(`Primary process ${process.pid} is running`);
  console.log(`Forking ${cpuCount} workers`);

  for (let i = 0; i < cpuCount; i++) {
    cluster.fork();
  }

  cluster.on('exit', (worker, code, signal) => {
    console.log(`Worker ${worker.process.pid} died. Restarting...`);
    cluster.fork();
  });
} else {
  http.createServer((req, res) => {
    res.writeHead(200);
    res.end(`Hello from worker ${process.pid}`);
  }).listen(8000);
  console.log(`Worker ${process.pid} started and listening on port 8000`);
}

Important Notes

Each worker is a separate process, so they do not share memory directly.

Use cluster to improve performance on multi-core machines.

Restarting workers on exit helps keep your app reliable.

Summary

Forking workers lets your app use all CPU cores.

The primary process controls workers and restarts them if needed.

Workers can run servers or other tasks in parallel.

Practice

1. What is the main reason to fork workers equal to the number of CPU cores in a Node.js app?

easy

A. To make the app run slower for debugging purposes

B. To use all CPU cores and improve performance by running tasks in parallel

C. To reduce memory usage by limiting the number of processes

D. To avoid using the cluster module

Solution

1. Step 1: Understand CPU cores and parallelism

   Each CPU core can run one process at a time, so using all cores means better performance.

2. Step 2: Role of forking workers

   Forking workers equal to CPU cores lets Node.js run multiple tasks simultaneously, improving speed.

3. Final Answer:

   To use all CPU cores and improve performance by running tasks in parallel -> Option B

4. Quick Check:

   Fork workers = use all cores = better performance [OK]

Hint: More workers = more CPU cores used = faster app [OK]

Common Mistakes:

• Thinking forking reduces memory usage
• Believing forking slows the app
• Confusing cluster module usage

2. Which of the following is the correct way to get the number of CPU cores in Node.js for forking workers?

easy

A. const cores = require('os').cpus().length;

B. const cores = require('cluster').cpuCount;

C. const cores = require('os').cpuCount();

D. const cores = require('os').cpus.length;

Solution

1. Step 1: Check Node.js os module usage

   The os module has a method cpus() that returns an array of CPU core info.

2. Step 2: Correct syntax to get core count

   Using cpus().length gives the number of CPU cores available.

3. Final Answer:

   const cores = require('os').cpus().length; -> Option A

4. Quick Check:

   os.cpus() returns array, length gives core count [OK]

Hint: Use os.cpus().length to count CPU cores [OK]

Common Mistakes:

• Forgetting parentheses after cpus
• Using non-existent cpuCount method
• Trying to get cores from cluster module

3. Given this code snippet, what will be logged when run on a machine with 4 CPU cores?

const cluster = require('cluster');
const os = require('os');

if (cluster.isPrimary) {
  const numCPUs = os.cpus().length;
  console.log(`Primary process is running`);
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }
} else {
  console.log(`Worker ${process.pid} started`);
}

medium

A. Primary process is running
Worker 1234 started
Worker 1235 started
Worker 1236 started
Worker 1237 started

B. Primary process is running
Worker started
Worker started
Worker started
Worker started

C. Primary process is running
Worker 1234 started

D. SyntaxError due to missing cluster setup

Solution

1. Step 1: Identify primary and worker behavior

   The primary logs once and forks 4 workers (one per CPU core).

2. Step 2: Each worker logs its own process id

   Each worker logs "Worker [pid] started" with its unique process id.

3. Final Answer:

   Primary process is running Worker 1234 started Worker 1235 started Worker 1236 started Worker 1237 started -> Option A

4. Quick Check:

   Primary logs once, 4 workers log with pids [OK]

Hint: Primary logs once; each worker logs with unique pid [OK]

Common Mistakes:

• Assuming workers log without pid
• Thinking only one worker starts
• Confusing primary and worker logs

4. What is wrong with this code snippet that tries to fork workers per CPU core?

const cluster = require('cluster');
const os = require('os');

if (cluster.isPrimary) {
  const numCPUs = os.cpus.length;
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }
} else {
  console.log('Worker started');
}

medium

A. cluster.fork() is not a function

B. No error; code works fine

C. Missing else block for worker process

D. os.cpus.length is undefined; should be os.cpus().length

Solution

1. Step 1: Check os module usage

   os.cpus is a function, so os.cpus.length is undefined and causes error.

2. Step 2: Correct usage

   Use os.cpus().length to get the number of CPU cores correctly.

3. Final Answer:

   os.cpus.length is undefined; should be os.cpus().length -> Option D

4. Quick Check:

   os.cpus() is function, need parentheses [OK]

Hint: Remember os.cpus() is a function, not a property [OK]

Common Mistakes:

• Forgetting parentheses on os.cpus()
• Assuming cluster.fork() is missing
• Thinking else block is required for syntax

5. You want to create a Node.js server that forks one worker per CPU core and restarts any worker if it crashes. Which code snippet correctly implements this behavior?

hard

A. const cluster = require('cluster');
const http = require('http');
const os = require('os');

if (cluster.isPrimary) {
const numCPUs = os.cpus().length;
for (let i = 0; i < numCPUs; i++) {
cluster.fork();
}
} else {
http.createServer((req, res) => {
res.writeHead(200);
res.end('Hello from worker ' + process.pid);
}).listen(8000);
cluster.on('exit', (worker) => {
console.log(`Worker ${worker.process.pid} died`);
});
}

B. const cluster = require('cluster');
const http = require('http');
const os = require('os');

if (cluster.isWorker) {
const numCPUs = os.cpus().length;
for (let i = 0; i < numCPUs; i++) {
cluster.fork();
}
} else {
http.createServer((req, res) => {
res.writeHead(200);
res.end('Hello from worker ' + process.pid);
}).listen(8000);
}

C. const cluster = require('cluster');
const http = require('http');
const os = require('os');

if (cluster.isPrimary) {
const numCPUs = os.cpus().length;
for (let i = 0; i < numCPUs; i++) {
cluster.fork();
}
cluster.on('exit', (worker) => {
console.log(`Worker ${worker.process.pid} died, restarting...`);
cluster.fork();
});
} else {
http.createServer((req, res) => {
res.writeHead(200);
res.end('Hello from worker ' + process.pid);
}).listen(8000);
}

D. const cluster = require('cluster');
const http = require('http');
const os = require('os');

if (cluster.isPrimary) {
cluster.fork();
} else {
http.createServer((req, res) => {
res.writeHead(200);
res.end('Hello from worker ' + process.pid);
}).listen(8000);
}

Solution

1. Step 1: Fork one worker per CPU core in primary process

   In if (cluster.isPrimary), use const numCPUs = os.cpus().length; for (let i = 0; i < numCPUs; i++) cluster.fork();

2. Step 2: Restart workers on exit event

   In primary process, cluster.on('exit', (worker) => { console.log(`Worker ${worker.process.pid} died, restarting...`); cluster.fork(); });

3. Step 3: Worker creates HTTP server

   Workers create the HTTP server listening on port 8000, responding with their pid.

4. Final Answer:

   forks one worker per CPU core and restarts any worker if it crashes -> Option C

5. Quick Check:

   Primary forks os.cpus().length + cluster.on('exit', fork()) + workers create HTTP server [OK]

Hint: Use cluster.on('exit') in primary to restart workers [OK]

Common Mistakes:

• Using cluster.isWorker instead of cluster.isPrimary
• Not restarting workers on exit
• Forking workers inside worker process