Worker Threads vs. Child Processes in Node.js: When to Use Which?

Early in my career when I first started working with Node.js, I loved how fast and event-driven it was. But then I hit a wall—what happens when my app needs to crunch a lot of data, like processing large files or performing complex calculations? The event loop would freeze, and everything would slow down.

That’s when I discovered Worker Threads and Child Processes—two ways to run tasks in parallel without blocking the main thread. But when should you use one over the other?

In this post, I’ll break down Worker Threads vs. Child Processes, explain when to use each, and give real-world Express.js examples.

Understanding the Problem: Node.js and Concurrency

Node.js is single-threaded, meaning it handles one task at a time in its event loop. This works great for I/O-heavy operations (like reading from a database or making an API call) but falls apart when you throw CPU-heavy tasks at it (like image processing or encryption).

To solve this, we can offload heavy computations using:

  • Worker Threads (multi-threading within the same process).
  • Child Processes (separate processes that communicate via IPC).

Let’s break them down one by one.

Worker Threads: Multi-Threading in Node.js

When I first heard about Worker Threads, I thought, Isn’t Node.js single-threaded? Turns out, Worker Threads allow you to run tasks in parallel within the same process, sharing memory when needed.

When to Use Worker Threads

  • For CPU-intensive computations (e.g., image processing, cryptography).
  • When you need shared memory (via SharedArrayBuffer).
  • When you want lower overhead compared to creating new processes.

Avoid Worker Threads for:

  • Running external scripts (Python, Bash, etc.).
  • Simple tasks that don’t require parallel processing.

Example: Worker Threads in an Express API

Let’s build an API that calculates Fibonacci numbers (a heavy computation) using Worker Threads.

worker.js

const { parentPort } = require('worker_threads');

const computeFibonacci = (num) => {
  if (num <= 1) return num;
  return computeFibonacci(num - 1) + computeFibonacci(num - 2);
};

parentPort.on('message', (num) => {
  const result = computeFibonacci(num);
  parentPort.postMessage(result);
});

server.js

const express = require('express');
const { Worker } = require('worker_threads');

const app = express();
const PORT = 3000;

app.get('/compute', (req, res) => {
  const worker = new Worker('./worker.js');

  worker.postMessage(40); // Calculate Fibonacci(40)

  worker.on('message', (result) => res.json({ result }));
  worker.on('error', (error) => res.status(500).json({ error: error.message }));
});

app.listen(PORT, () => console.log(`Server running on port ${PORT}`));

why above code performs well ?

  • Doesn’t block the main thread, keeping the app responsive.
  • Uses shared memory, making data processing more efficient.
  • Ideal for CPU-heavy tasks that can run in parallel.

Child Processes: Isolated Execution

At some point, I realized that Worker Threads weren’t the answer to everything. Sometimes, I needed complete process isolation—like running a Python script or handling a huge file conversion. That’s where Child Processes come in.

When to Use Child Processes

  • For running external scripts (e.g., Python, Bash, other Node.js scripts).
  • When you need process isolation (so crashes don’t affect the main app).
  • For I/O-heavy tasks (like database backups, large file operations).

When to avoid Child Processes for:

  • Small computations that can be handled in the main thread.
  • Memory-intensive tasks where shared memory is needed.

Example: Child Processes in an Express API Let’s compute the factorial of a number using a Child Process.

child.js

process.on('message', (num) => {
  let result = 1;
  for (let i = 2; i <= num; i++) {
    result *= i;
  }
  process.send(result);
});

server.js

const express = require('express');
const { fork } = require('child_process');

const app = express();
const PORT = 3000;

app.get('/factorial', (req, res) => {
  const child = fork('./child.js');

  child.send(10); // Calculate Factorial(10)

  child.on('message', (result) => res.json({ result }));
  child.on('error', (error) => res.status(500).json({ error: error.message }));
});

app.listen(PORT, () => console.log(`Server running on port ${PORT}`));

This give few benefits

  • Completely separate from the main process—if it crashes, your app keeps running.
  • Perfect for running external scripts or executing heavy I/O operations.
  • Scales well by spawning multiple child processes as needed.

Advice to Senior-Level Developers: Making the Right Choice

As a senior-level programmer, you’re likely working on high-performance, scalable, and resilient systems. Choosing between Worker Threads and Child Processes isn’t just about technical feasibility—it’s about architectural trade-offs that impact scalability, fault tolerance, and maintainability.

Here are some pro tips to keep in mind:

When Using Worker Threads:

  • Minimize Memory Use – Worker Threads share memory, but wasteful use (e.g., excessive cloning of objects) will make it sluggish. Use SharedArrayBuffer if it’s supported.
  • Never Block the Main Thread – Even when you are using Worker Threads, still always precompute heavyweight computation so it doesn’t hit the event loop.
  • Test for Race Conditions – Since Worker Threads share memory, always be cautious about data races when multiple threads are accessing the same resource.
  • Don’t use Worker Threads if you need total process isolation or if your workload has external dependencies like databases or APIs.

When Using Child Processes:

  • Keep Processes Lightweight – Forking a process has overhead, so be sure that you’re not spawning unnecessary processes that hold onto system resources.
  • Use a Process Pool – If your application regularly spawns Child Processes, think about pooling them with libraries such as generic-pool to reuse processes rather than creating new ones every time.
  • Gracefully Handle Failures – Child Processes are isolated, which is excellent for resilience. Always use error handling and restarts to avoid orphaned processes or resource leaks.
  • Do not use Child Processes for operations that require high-speed inter-thread communication. The IPC method may be slower than shared memory in Worker Threads.

That’s it, happy coding and catch you in the next post !!!