Worker thread vs child process in Node.js - Quick Revision & Key Differences

Recall & Review

beginner

What is a Worker Thread in Node.js?

A Worker Thread is a way to run JavaScript code in parallel within the same Node.js process. It shares memory with the main thread but runs independently to perform CPU-intensive tasks without blocking the main event loop.

beginner

What is a Child Process in Node.js?

A Child Process is a separate process created by the main Node.js process. It runs independently with its own memory and event loop, allowing you to run external programs or scripts without blocking the main process.

intermediate

How do Worker Threads and Child Processes differ in memory usage?

Worker Threads share memory with the main thread using SharedArrayBuffer, which allows faster communication. Child Processes have separate memory spaces, so communication happens via inter-process messaging, which is slower.

intermediate

When should you use a Worker Thread instead of a Child Process?

Use Worker Threads for CPU-heavy JavaScript tasks that need to run in parallel but share data efficiently. They are lighter and faster for tasks within Node.js. Use Child Processes when you need to run external programs or isolate tasks completely.

advanced

What is a key limitation of Worker Threads compared to Child Processes?

Worker Threads run in the same process, so if a Worker Thread crashes, it can affect the main process. Child Processes are isolated, so a crash in a child does not crash the main process.

Which Node.js feature runs JavaScript code in parallel within the same process?

A Event Loop

B Child Process

C Worker Thread

D Callback Function

Worker Threads run JavaScript code in parallel inside the same Node.js process.

Which runs in a separate memory space and process?

A Worker Thread

B Child Process

C Promise

D Async Function

Child Processes run in separate processes with their own memory.

Which is better for running external programs from Node.js?

A Child Process

B setTimeout

C Worker Thread

D Event Loop

Child Processes are designed to run external programs or scripts.

Which communicates faster due to shared memory?

A Worker Thread

B File System

C HTTP Request

D Child Process

Worker Threads share memory, enabling faster communication.

What happens if a Worker Thread crashes?

A Only the Worker Thread stops, main process continues

B Nothing happens

C Child Process restarts automatically

D Main process crashes too

Since Worker Threads share the same process, a crash can affect the main process.

Explain the main differences between Worker Threads and Child Processes in Node.js.

Think about memory, process isolation, and use cases.

When would you choose a Child Process over a Worker Thread in a Node.js application?

Consider isolation and running non-JS code.

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