Master and worker processes in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the cluster module in Node.js.

Node.js

const cluster = require('[1]');

Drag options to blanks, or click blank then click option'

Acluster

Bhttp

Cfs

Dos

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'http' or 'os' instead of 'cluster' to import the cluster module.

The cluster module is imported using require('cluster') to create master and worker processes.

2fill in blank

medium

Complete the code to check if the current process is the master process.

Node.js

if (cluster.[1]) {
  console.log('This is the master process');
}

Drag options to blanks, or click blank then click option'

AisMaster

BisPrimary

CisMain

DisWorker

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'isWorker' or 'isMaster' instead of 'isPrimary'.

The property cluster.isPrimary is true if the current process is the primary (master) process in recent Node.js versions.

3fill in blank

hard

Fix the error in the code to fork a worker process.

Node.js

if (cluster.isPrimary) {
  cluster.[1]();
}

Drag options to blanks, or click blank then click option'

Afork

Bstart

Cspawn

Dcreate

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'start' or 'spawn' which are not cluster methods.

The method cluster.fork() creates a new worker process.

4fill in blank

hard

Fill both blanks to listen for 'message' events in the master and worker processes.

Node.js

if (cluster.isPrimary) {
  const worker = cluster.fork();
  worker.[1]('message', (msg) => {
    console.log('Master received:', msg);
  });
} else {
  process.[2]('message', (msg) => {
    console.log('Worker received:', msg);
  });
}

Drag options to blanks, or click blank then click option'

Aon

Bsend

Cemit

Dlisten

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'send' or 'emit' to listen for messages instead of 'on'.

Both the worker and master listen for messages using the on method.

5fill in blank

hard

Fill all three blanks to send a message from the master to the worker and reply back.

Node.js

if (cluster.isPrimary) {
  const worker = cluster.fork();
  worker.[1]({ cmd: 'start' });
  worker.[2]('message', (msg) => {
    console.log('Master got reply:', msg);
  });
} else {
  process.[3]('message', (msg) => {
    if (msg.cmd === 'start') {
      process.send({ reply: 'done' });
    }
  });
}

Drag options to blanks, or click blank then click option'

Asend

Bon

Cemit

Dlisten

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'emit' or 'listen' instead of 'send' or 'on'.

The master sends a message with send, listens with on, and the worker listens with on.

Practice

1. What is the main role of the master process in Node.js cluster module?

easy

A. To create and manage worker processes

B. To handle HTTP requests directly

C. To run the application code

D. To listen on network ports

Solution

1. Step 1: Understand the master process role

   The master process is responsible for creating and managing worker processes in the cluster module.

2. Step 2: Differentiate master from worker tasks

   Workers run the app code and handle requests, while the master only manages them.

3. Final Answer:

   To create and manage worker processes -> Option A

4. Quick Check:

   Master manages workers [OK]

Hint: Master only manages workers, does not run app code [OK]

Common Mistakes:

• Thinking master handles requests directly
• Confusing master with worker process
• Assuming master runs app code

2. Which of the following is the correct way to check if the current process is the master in a Node.js cluster?

easy

A. if (process.isWorker) { ... }

B. if (cluster.isWorker) { ... }

C. if (process.isMaster) { ... }

D. if (cluster.isMaster) { ... }

Solution

1. Step 1: Recall cluster module properties

   The cluster module provides isMaster and isWorker boolean properties to identify process roles.

2. Step 2: Identify correct syntax

   To check if current process is master, use cluster.isMaster, not process properties.

3. Final Answer:

   if (cluster.isMaster) { ... } -> Option D

4. Quick Check:

   Use cluster.isMaster to check master process [OK]

Hint: Use cluster.isMaster, not process properties [OK]

Common Mistakes:

• Using process.isMaster which does not exist
• Confusing isWorker with isMaster
• Using wrong object for the check

3. Consider this Node.js cluster code snippet:

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

if (cluster.isMaster) {
  cluster.fork();
  cluster.fork();
} else {
  http.createServer((req, res) => {
    res.end('Worker ' + process.pid);
  }).listen(8000);
}

What will happen when you visit http://localhost:8000?

medium

A. You get response 'Worker <pid>' from one of the two workers

B. You get response 'Worker <pid>' from the master process

C. The server crashes because master listens on port

D. No response because no server is created

Solution

1. Step 1: Identify which process creates the server

   The else block runs in worker processes, which create the HTTP server listening on port 8000.

2. Step 2: Understand request handling

   Requests are handled by one of the two worker processes forked by the master, each responding with its process ID.

3. Final Answer:

   You get response 'Worker <pid>' from one of the two workers -> Option A

4. Quick Check:

   Workers handle requests, master does not listen [OK]

Hint: Only workers create servers and respond [OK]

Common Mistakes:

• Thinking master handles requests
• Assuming master listens on port
• Believing no server is created

4. Given this code snippet:

const cluster = require('cluster');
if (cluster.isMaster) {
  cluster.fork();
  cluster.fork();
} else {
  console.log('Worker started');
}

Why might the workers never start properly?

medium

A. Because the master process forgot to require('http')

B. Because the workers have no code to keep them alive

C. Because cluster.fork() is called inside the else block

D. Because cluster.isMaster is always false

Solution

1. Step 1: Analyze worker code behavior

   The workers only log 'Worker started' and then exit immediately because no server or event loop keeps them alive.

2. Step 2: Understand cluster.fork usage

   cluster.fork() is correctly called in master block, so workers start but exit quickly.

3. Final Answer:

   Because the workers have no code to keep them alive -> Option B

4. Quick Check:

   Workers exit if no server or event loop runs [OK]

Hint: Workers need active code (like server) to stay alive [OK]

Common Mistakes:

• Thinking missing http require stops workers
• Confusing cluster.fork placement
• Assuming cluster.isMaster is always false

5. You want to create a Node.js cluster that uses all CPU cores and restarts any worker that crashes. Which approach correctly implements this?

hard

A. Create workers manually without cluster module and restart them with setInterval

B. Use cluster.fork() once and rely on master to restart automatically

C. Use cluster.fork() for each CPU core and listen to 'exit' event to fork a new worker

D. Use cluster.isWorker to fork new workers inside each worker process

Solution

1. Step 1: Use all CPU cores with cluster.fork()

   Fork one worker per CPU core by looping over the number of CPUs.

2. Step 2: Restart crashed workers by listening to 'exit'

   Listen to the 'exit' event on cluster to detect worker crashes and fork a new worker to replace it.

3. Final Answer:

   Use cluster.fork() for each CPU core and listen to 'exit' event to fork a new worker -> Option C

4. Quick Check:

   Fork per CPU + restart on exit [OK]

Hint: Fork per CPU and restart on 'exit' event [OK]

Common Mistakes:

• Forking only once and expecting auto-restart
• Restarting workers inside workers themselves
• Not handling worker crashes properly