Master and worker processes in Node.js - Mini Project: Build & Apply

Master and Worker Processes in Node.js

📖 Scenario: You are building a simple Node.js application that uses multiple processes to handle tasks efficiently. The main process (master) will create worker processes to perform work in parallel.

🎯 Goal: Create a Node.js script that sets up a master process and two worker processes using the cluster module. The master will send a message to each worker, and each worker will respond back.

📋 What You'll Learn

Use the cluster module to create worker processes

Create exactly two worker processes

Master process sends a message to each worker

Each worker sends a message back to the master

Use process.on('message') to handle messages

💡 Why This Matters

🌍 Real World

Using master and worker processes helps build fast and scalable Node.js servers that can handle many tasks at once by using multiple CPU cores.

💼 Career

Understanding process management and inter-process communication is important for backend developers working with Node.js in production environments.

1

Setup the cluster module and check if current process is master

Write code to import the cluster module and the os module. Then create a condition that checks if cluster.isMaster is true.

Node.js

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

// Check if current process is master
if (cluster.isMaster) {
  // Your code here
}

Hint

Use require('cluster') and require('os'). Then check cluster.isMaster to know if the current process is the master.

2

Create two worker processes inside the master block

Inside the if (cluster.isMaster) block, write code to create exactly two worker processes using cluster.fork() twice.

Node.js

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

if (cluster.isMaster) {
  // Create two worker processes
  // Your code here
}

Hint

Call cluster.fork() two times inside the master block to create two workers.

3

Send a message from master to each worker and listen for messages from workers

Inside the if (cluster.isMaster) block, add a for loop that iterates over Object.values(cluster.workers). For each worker, send a message { cmd: 'start' } using worker.send(). Also, add a listener on each worker for 'message' events that logs the received message with console.log.

Node.js

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

if (cluster.isMaster) {
  cluster.fork();
  cluster.fork();

  // Send message to each worker and listen for messages
  // Your code here
}

Hint

Use Object.values(cluster.workers) to get all workers. Send a message with worker.send() and listen with worker.on('message').

4

Add worker process code to receive message and reply to master

Outside the if (cluster.isMaster) block, write code for the worker processes. Use process.on('message') to listen for messages. When a message with cmd === 'start' is received, send back a message to the master with process.send({ reply: 'Worker is ready' }).

Node.js

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

if (cluster.isMaster) {
  cluster.fork();
  cluster.fork();

  for (const worker of Object.values(cluster.workers)) {
    worker.send({ cmd: 'start' });
    worker.on('message', (msg) => {
      console.log(`Master received: ${msg.reply}`);
    });
  }
}

// Worker process code
// Your code here

Hint

Use else for worker code. Listen with process.on('message') and reply with process.send().

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