How cluster module works in Node.js - Practice Exercises

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a worker process dies in Node.js cluster?

Consider a Node.js cluster setup where the master forks several worker processes. What is the default behavior when one worker process unexpectedly exits?

A The master restarts the entire cluster including all workers.

B The master shuts down all other workers and exits.

C The master ignores the dead worker and does not replace it.

D The master automatically forks a new worker to replace the dead one.

💡 Hint

Think about how Node.js cluster maintains availability by managing worker processes.

By default, the cluster module listens for 'exit' events on workers. When a worker dies, the master process forks a new worker to keep the cluster size consistent.

📝 Syntax

intermediate

Identify the correct way to create a cluster master and fork workers

Which code snippet correctly creates a cluster master that forks 2 worker processes?

A

const cluster = require('cluster');
if (cluster.isWorker) {
  cluster.fork();
  cluster.fork();
} else {
  console.log('Master running');
}

B

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

C

const cluster = require('cluster');
if (cluster.isMaster) {
  cluster.start(2);
} else {
  console.log('Worker running');
}

D

const cluster = require('cluster');
if (cluster.isMaster) {
  cluster.createWorkers(2);
} else {
  console.log('Worker running');
}

💡 Hint

Check the official cluster API properties and methods for forking workers.

The cluster module uses cluster.isMaster to check if the current process is the master. The master forks workers using cluster.fork().

🔧 Debug

advanced

Why does this cluster code cause workers not to start?

Given the code below, why do no worker processes start? const cluster = require('cluster'); if (cluster.isMaster) { for (let i = 0; i < 2; i++) { cluster.fork; } } else { console.log('Worker started'); }

Node.js

const cluster = require('cluster');

if (cluster.isMaster) {
  for (let i = 0; i < 2; i++) {
    cluster.fork;
  }
} else {
  console.log('Worker started');
}

A cluster.isMaster is deprecated and should be replaced with cluster.isPrimary.

B The else block should be inside the for loop to start workers.

C cluster.fork is missing parentheses, so the function is not called.

D The cluster module requires an explicit call to cluster.start() to begin.

💡 Hint

Look carefully at how functions are called in JavaScript.

The code uses cluster.fork without parentheses, so it references the function but does not execute it. Workers are never created.

❓ state_output

advanced

What is the output of this cluster worker count code?

What will be printed when running this code? const cluster = require('cluster'); if (cluster.isMaster) { console.log('Master process'); cluster.fork(); cluster.fork(); console.log('Workers count:', Object.keys(cluster.workers).length); } else { console.log('Worker process'); }

Node.js

const cluster = require('cluster');

if (cluster.isMaster) {
  console.log('Master process');
  cluster.fork();
  cluster.fork();
  console.log('Workers count:', Object.keys(cluster.workers).length);
} else {
  console.log('Worker process');
}

A

Master process
Workers count: 0
Worker process
Worker process

B

Master process
Worker process
Worker process
Workers count: 2

C

Master process
Workers count: 2
Worker process
Worker process

D

Master process
Workers count: 2

💡 Hint

Consider when the workers are fully registered in the cluster.workers object.

The cluster.fork() calls are asynchronous. Immediately after calling fork twice, cluster.workers is still empty, so length is 0. Worker processes print their messages separately.

🧠 Conceptual

expert

How does Node.js cluster module distribute incoming connections?

In a Node.js cluster with multiple worker processes, how are incoming TCP connections distributed among workers by default?

A The master process uses a round-robin algorithm to distribute connections evenly to workers.

B Each worker listens on the same port independently and the OS load balances connections.

C The first worker accepts all connections until it crashes, then the next worker takes over.

D Connections are distributed randomly without any specific order or balancing.

💡 Hint

Think about how Node.js cluster manages load balancing internally.

Node.js cluster module uses a round-robin approach by default on most platforms, where the master distributes incoming connections evenly to workers to balance load.

Practice

1. What is the main purpose of the cluster module in Node.js?

easy

A. To provide a graphical user interface for Node.js apps

B. To manage database connections efficiently

C. To handle file system operations asynchronously

D. To create multiple worker processes to use all CPU cores

Solution

1. Step 1: Understand the cluster module role

   The cluster module allows Node.js to create multiple worker processes.

2. Step 2: Recognize the benefit

   These workers use all CPU cores to improve performance by handling requests in parallel.

3. Final Answer:

   To create multiple worker processes to use all CPU cores -> Option D

4. Quick Check:

   cluster module = multiple workers for CPU cores [OK]

Hint: Cluster = multiple processes for CPU cores [OK]

Common Mistakes:

• Confusing cluster with database or file system modules
• Thinking cluster manages UI or frontend
• Assuming cluster runs code in a single process

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

easy

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

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

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

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

Solution

1. Step 1: Recall the updated property name

   In recent Node.js versions, cluster.isPrimary replaces cluster.isMaster.

2. Step 2: Identify the correct syntax

   Using cluster.isPrimary correctly checks if the process is the primary (master) process.

3. Final Answer:

   if (cluster.isPrimary) { ... } -> Option B

4. Quick Check:

   Primary process check = cluster.isPrimary [OK]

Hint: Use cluster.isPrimary, not isMaster [OK]

Common Mistakes:

• Using deprecated cluster.isMaster instead of cluster.isPrimary
• Confusing isWorker with isPrimary
• Using non-existent properties like isMain

3. Consider this Node.js cluster code snippet:

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

if (cluster.isPrimary) {
  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 multiple times?

medium

A. You will see responses from different worker process IDs

B. Only one worker will handle all requests

C. The server will crash because of multiple forks

D. You will get a syntax error on startup

Solution

1. Step 1: Understand cluster.fork creates workers

   Two workers are created, each running the HTTP server on port 8000.

2. Step 2: Recognize load balancing behavior

   Requests are distributed between workers, so responses show different process IDs.

3. Final Answer:

   You will see responses from different worker process IDs -> Option A

4. Quick Check:

   Multiple workers share port, respond with different PIDs [OK]

Hint: Multiple forks = multiple workers respond differently [OK]

Common Mistakes:

• Thinking only one worker handles all requests
• Assuming server crashes due to multiple forks
• Expecting syntax errors from this code

4. What is wrong with this cluster code snippet?

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

medium

A. cluster.isPrimary is deprecated, should use isMaster

B. No call to cluster.fork in the worker process

C. Missing server code inside worker

D. No error, code works fine

Solution

1. Step 1: Check cluster usage

   The primary forks one worker, which only logs a message but does not start a server.

2. Step 2: Identify missing functionality

   Without server code, the worker does not handle requests, so the cluster setup is incomplete.

3. Final Answer:

   Missing server code inside worker -> Option C

4. Quick Check:

   Worker must run server code to handle requests [OK]

Hint: Workers need server code to handle requests [OK]

Common Mistakes:

• Confusing isPrimary with deprecated isMaster
• Expecting cluster.fork in worker process
• Assuming code runs without server in worker

5. You want to create a cluster that automatically restarts a worker if it crashes. Which approach correctly implements this behavior?

hard

A. Listen to the 'exit' event on cluster and fork a new worker inside the handler

B. Use setInterval to fork new workers every second

C. Call cluster.fork() only once at startup and never again

D. Use cluster.disconnect() inside the worker to restart itself

Solution

1. Step 1: Understand worker crash handling

   The primary process can listen to the 'exit' event when a worker dies.

2. Step 2: Restart worker on exit

   Inside the 'exit' event handler, calling cluster.fork() creates a new worker to replace the crashed one.

3. Final Answer:

   Listen to the 'exit' event on cluster and fork a new worker inside the handler -> Option A

4. Quick Check:

   Restart crashed workers by handling 'exit' event [OK]

Hint: Use 'exit' event to restart workers automatically [OK]

Common Mistakes:

• Forking workers repeatedly with setInterval causes overload
• Not restarting workers after crash leads to downtime
• Using cluster.disconnect() inside worker does not restart it