Worker thread vs child process in Node.js - Hands-On Comparison

Worker Thread vs Child Process in Node.js

📖 Scenario: You are building a Node.js application that needs to perform heavy calculations without blocking the main program. You want to learn how to use worker_threads and child_process modules to run tasks in parallel.

🎯 Goal: Create a simple Node.js app that uses a worker thread and a child process to run the same calculation separately. You will see how to set up both and send messages between the main program and these parallel workers.

📋 What You'll Learn

Create a worker thread using the worker_threads module

Create a child process using the child_process module

Send a number from the main thread to both the worker thread and child process

Receive the squared result back from both and log it

Use exact variable and function names as instructed

💡 Why This Matters

🌍 Real World

Node.js apps often need to run heavy tasks without freezing the main program. Worker threads and child processes help by running code in parallel.

💼 Career

Understanding these parallel processing methods is important for backend developers to build efficient, responsive Node.js applications.

1

Set up the main data and imports

Create a variable called number and set it to 7. Import Worker from worker_threads and fork from child_process.

Node.js

const { Worker } = require('worker_threads');
const { fork } = require('child_process');

const number = 7;
// Your code here

Hint

Use const number = 7; and import Worker and fork exactly as shown.

2

Create a worker thread to calculate square

Create a new Worker instance called worker that runs a file named 'worker.js'. This worker will receive the number and calculate its square.

Node.js

const { Worker } = require('worker_threads');
const { fork } = require('child_process');

const number = 7;

const worker = new Worker('./worker.js');
// Your code here

Hint

Use new Worker('./worker.js') to create the worker thread.

3

Create a child process to calculate square

Create a child process called child using fork to run a file named 'child.js'. This child process will also receive the number and calculate its square.

Node.js

const { Worker } = require('worker_threads');
const { fork } = require('child_process');

const number = 7;

const worker = new Worker('./worker.js');

const child = fork('./child.js');
// Your code here

Hint

Use fork('./child.js') to create the child process.

4

Send number and receive results from worker and child

Send the number to both worker and child. Listen for messages from both using worker.on('message', ...) and child.on('message', ...). Log the results with console.log using the exact text: "Worker result: " and "Child result: " followed by the squared number.

Node.js

const { Worker } = require('worker_threads');
const { fork } = require('child_process');

const number = 7;

const worker = new Worker('./worker.js');

const child = fork('./child.js');

worker.postMessage(number);
worker.on('message', (result) => {
  // Your code here
});

child.send(number);
child.on('message', (result) => {
  // Your code here
});

Hint

Use postMessage and send to send data. Use on('message') to receive and console.log to print results.

Practice

1. Which statement best describes the difference between worker threads and child processes in Node.js?

easy

A. Worker threads run in the same process sharing memory, while child processes run in separate processes with separate memory.

B. Worker threads run separate programs, child processes share the same memory space.

C. Both worker threads and child processes run in the same process and share memory.

D. Child processes run inside worker threads to improve performance.

Solution

1. Step 1: Understand worker threads behavior

   Worker threads run JavaScript code in parallel but inside the same Node.js process and share memory.

2. Step 2: Understand child processes behavior

   Child processes run completely separate programs with their own memory space and communicate via messages.

3. Final Answer:

   Worker threads run in the same process sharing memory, while child processes run in separate processes with separate memory. -> Option A

4. Quick Check:

   Worker threads share memory, child processes do not [OK]

Hint: Remember: threads share memory, processes do not [OK]

Common Mistakes:

• Confusing memory sharing between threads and processes
• Thinking child processes share memory
• Assuming worker threads run separate programs

2. Which of the following is the correct way to create a worker thread in Node.js?

easy

A. const worker = spawn('worker.js');

B. const worker = fork('worker.js');

C. const worker = createThread('worker.js');

D. const worker = new Worker('worker.js');

Solution

1. Step 1: Recall worker thread creation syntax

   Worker threads are created using the Worker class from the worker_threads module.

2. Step 2: Identify correct constructor usage

   The correct syntax is new Worker('filename'). The fork and spawn methods are for child processes.

3. Final Answer:

   const worker = new Worker('worker.js'); -> Option D

4. Quick Check:

   Worker threads use new Worker() [OK]

Hint: Use new Worker() for threads, fork/spawn for processes [OK]

Common Mistakes:

• Using fork() to create worker threads
• Using spawn() for worker threads
• Using non-existent createThread() function

3. Consider this Node.js code snippet using a child process:

const { fork } = require('child_process');
const child = fork('child.js');
child.on('message', msg => console.log('Parent got:', msg));
child.send('Hello');

What will the parent process output if child.js sends back { reply: 'Hi' } on receiving a message?

medium

A. Parent got: Hello

B. Parent got: { reply: 'Hi' }

C. Parent got: Hi

D. No output, error occurs

Solution

1. Step 1: Understand child process communication

   The parent sends 'Hello' to the child. The child responds with an object { reply: 'Hi' } via process.send().

2. Step 2: Analyze parent's message event handler

   The parent's child.on('message') receives the object and logs it as Parent got: { reply: 'Hi' }.

3. Final Answer:

   Parent got: { reply: 'Hi' } -> Option B

4. Quick Check:

   Child sends object, parent logs object [OK]

Hint: Child sends object, parent logs exact message [OK]

Common Mistakes:

• Assuming string 'Hi' instead of object
• Expecting parent's message to be 'Hello'
• Confusing child and parent message directions

4. What is wrong with this code snippet that tries to create a worker thread?

const { Worker } = require('worker_threads');
const worker = Worker('worker.js');

medium

A. The file 'worker.js' must be a JSON file

B. Wrong module imported, should be 'child_process'

C. Missing new keyword before Worker constructor

D. Worker threads cannot be created with a filename

Solution

1. Step 1: Check Worker thread creation syntax

   The Worker class must be instantiated with the new keyword.

2. Step 2: Identify error in code

   The code calls Worker('worker.js') without new, causing a TypeError.

3. Final Answer:

   Missing new keyword before Worker constructor -> Option C

4. Quick Check:

   Use new Worker() to create threads [OK]

Hint: Always use new with Worker() constructor [OK]

Common Mistakes:

• Forgetting new keyword
• Importing wrong module for threads
• Thinking worker.js must be JSON

5. You want to perform a CPU-heavy task in Node.js without blocking the main event loop. Which approach is best and why?

hard

A. Use a child process to run the task in a separate process communicating via messages.

B. Use a worker thread to run the task sharing memory with the main thread.

C. Run the task directly in the main thread asynchronously.

D. Use setTimeout to delay the task execution in the main thread.

Solution

1. Step 1: Understand CPU-heavy task impact

   CPU-heavy tasks block the main event loop if run directly, causing app unresponsiveness.

2. Step 2: Compare worker threads and child processes for heavy tasks

   Worker threads share memory but still run in the same process, which can cause contention. Child processes run in separate processes, isolating CPU load and preventing blocking.

3. Step 3: Evaluate other options

   Running asynchronously in main thread or delaying with setTimeout does not prevent blocking for CPU-heavy tasks.

4. Final Answer:

   Use a child process to run the task in a separate process communicating via messages. -> Option A

5. Quick Check:

   Heavy CPU tasks best isolated in child processes [OK]

Hint: Heavy CPU tasks? Use child processes to avoid blocking [OK]

Common Mistakes:

• Assuming worker threads fully isolate CPU load
• Thinking async or setTimeout avoids CPU blocking
• Ignoring message communication overhead