Forking workers per CPU core in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when forking workers equal to CPU cores?

Consider this Node.js code snippet that forks workers equal to the number of CPU cores. What will be logged to the console when the master process runs?

Node.js

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

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

A Master process running with 4 CPUs\nWorker 1234 started\nWorker 1235 started\nWorker 1236 started\nWorker 1237 started

B Worker 1234 started\nWorker 1235 started\nWorker 1236 started\nWorker 1237 started

C Master process running with 4 CPUs

D Master process running with 1 CPU\nWorker 1234 started

💡 Hint

Remember that the master process logs first, then forks workers equal to CPU cores, each logging their start.

The master process logs the number of CPUs, then forks one worker per CPU. Each worker logs its start message. So the console shows the master message followed by one message per worker.

📝 Syntax

intermediate

Which option correctly forks workers per CPU core?

Which code snippet correctly forks a worker process for each CPU core using Node.js cluster module?

A import cluster from 'cluster'; import os from 'os'; if (cluster.isMaster) { for (let i = 0; i < os.cpus().length; i++) cluster.fork(); }

B import cluster from 'cluster'; import os from 'os'; if (cluster.isPrimary) { for (let i = 0; i < os.cpus().length; i++) cluster.fork(); }

C const cluster = require('cluster'); const os = require('os'); if (cluster.isMaster) { for (let i = 0; i < os.cpus().length; i++) cluster.fork(); }

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

💡 Hint

Check the correct property name for the primary process in Node.js cluster module in recent versions.

In Node.js 20+, the cluster module uses 'isPrimary' instead of 'isMaster'. Also, ES module import syntax is preferred. Option B uses correct import and 'isPrimary'.

🔧 Debug

advanced

Why does this cluster code fork only one worker despite multiple CPUs?

Given this code, why does it fork only one worker even if the machine has multiple CPU cores? import cluster from 'cluster'; import os from 'os'; if (cluster.isPrimary) { cluster.fork(); } else { console.log('Worker started'); }

Node.js

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

if (cluster.isPrimary) {
  cluster.fork();
} else {
  console.log('Worker started');
}

A Because cluster.isPrimary is false, so the master never forks workers.

B Because os.cpus() is not used, so the code errors out and forks no workers.

C Because the code calls cluster.fork() only once, so only one worker is created regardless of CPU count.

D Because cluster.fork() is asynchronous and needs a callback to fork multiple workers.

💡 Hint

Count how many times cluster.fork() is called in the master process.

The code calls cluster.fork() only once inside the if block, so only one worker is created. To fork per CPU core, a loop over os.cpus().length is needed.

❓ state_output

advanced

What is the value of workers count after forking per CPU core?

In this code, what is the value of Object.keys(cluster.workers).length after the master forks workers equal to CPU cores?

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();
  }
  console.log(Object.keys(cluster.workers).length);
}

A Always 1, because cluster.workers only tracks one worker.

B Undefined, because cluster.workers is not a valid property.

C 0, because cluster.workers is empty until workers send a message.

D The number of CPU cores on the machine (e.g., 4 or 8).

💡 Hint

cluster.workers holds all forked worker processes indexed by their ids.

After forking once per CPU core, cluster.workers contains one entry per worker. So its keys length equals the CPU count.

🧠 Conceptual

expert

What happens if a worker crashes in a cluster with workers per CPU core?

In a Node.js cluster setup where one worker is forked per CPU core, what is the typical behavior when a worker process crashes unexpectedly?

A The master process detects the exit event and automatically forks a new worker to replace the crashed one.

B All workers shut down immediately and the master process exits.

C The crashed worker restarts itself without master intervention.

D Nothing happens; the crashed worker stays dead and no new workers are created.

💡 Hint

Think about how cluster module handles worker exits by default.

The cluster module emits an 'exit' event on the master when a worker dies. The master can listen and fork a new worker to maintain the desired number.