Master and worker processes in Node.js - Performance & Optimization

Performance: Master and worker processes

HIGH IMPACT

This concept affects how Node.js handles CPU-intensive tasks and concurrent requests, impacting server responsiveness and throughput.

Handling CPU-intensive tasks in a Node.js server

Node.js

const cluster = require('cluster');
const http = require('http');
const numCPUs = require('os').cpus().length;

if (cluster.isMaster) {
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }
} else {
  http.createServer((req, res) => {
    // Worker handles request without blocking master
    let count = 0;
    for (let i = 0; i < 1e9; i++) count += i;
    res.end('Done ' + count);
  }).listen(3000);
}

Spreads workload across multiple worker processes, preventing any single event loop from blocking and improving concurrency.

📈 Performance GainReduces event loop blocking per process, improves INP by parallelizing CPU work across cores.

Handling CPU-intensive tasks in a Node.js server

Node.js

const http = require('http');
http.createServer((req, res) => {
  // Heavy computation blocking event loop
  let count = 0;
  for (let i = 0; i < 1e9; i++) count += i;
  res.end('Done ' + count);
}).listen(3000);

Heavy computation blocks the single main event loop, causing slow or unresponsive server during processing.

📉 Performance CostBlocks event loop for hundreds of milliseconds per request, causing high INP and poor responsiveness.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Single process heavy CPU task	N/A	N/A	N/A	[X] Bad
Master with multiple worker processes	N/A	N/A	N/A	[OK] Good

Rendering Pipeline

Master process delegates tasks to worker processes, which run independently. This avoids blocking the main event loop and keeps the server responsive.

→Event Loop

→Task Scheduling

→CPU Utilization

⚠️ BottleneckSingle-threaded event loop blocking due to CPU-heavy tasks

Core Web Vital Affected

INP

This concept affects how Node.js handles CPU-intensive tasks and concurrent requests, impacting server responsiveness and throughput.

Optimization Tips

1Offload CPU-intensive tasks to worker processes to keep the main event loop free.

2Use as many worker processes as CPU cores for best parallelism.

3Avoid blocking the event loop in the master process to improve server responsiveness.

Performance Quiz - 3 Questions

What is the main performance benefit of using worker processes in Node.js?

A They improve CSS rendering speed in the browser.

B They reduce the size of the JavaScript bundle sent to the browser.

C They prevent the main event loop from blocking by parallelizing CPU work.

D They eliminate the need for asynchronous code.

DevTools: Performance

How to check: Record a CPU profile while sending requests to the server. Look for long blocking tasks in the main thread.

What to look for: Long tasks blocking the event loop indicate poor use of worker processes; multiple smaller tasks across processes indicate good parallelism.

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