PM2 for process management in Express

Introduction

PM2 helps keep your Express app running smoothly. It restarts your app if it crashes and makes managing multiple app instances easy.

You want your Express server to restart automatically if it stops unexpectedly.

You need to run multiple copies of your app to handle more users.

You want to monitor your app's performance and logs easily.

You want to start your app automatically when your computer reboots.

You want a simple way to manage your app processes from the command line.

Syntax

Express

pm2 start app.js --name my-app
pm2 stop my-app
pm2 restart my-app
pm2 list
pm2 logs my-app

Use pm2 start to launch your app with PM2.

The --name option helps you identify your app in PM2 commands.

Examples

Starts your Express app and names it my-express-app for easy management.

Express

pm2 start app.js --name my-express-app

Restarts the running app named my-express-app without downtime.

Express

pm2 restart my-express-app

Stops the app named my-express-app.

Express

pm2 stop my-express-app

Shows real-time logs of your app to help you see what is happening.

Express

pm2 logs my-express-app

Sample Program

This is a simple Express app that responds with a greeting. You can run it with PM2 to keep it running and manage it easily.

Express

const express = require('express');
const app = express();
const port = 3000;

app.get('/', (req, res) => {
  res.send('Hello from Express with PM2!');
});

app.listen(port, () => {
  console.log(`Server running on http://localhost:${port}`);
});

Important Notes

PM2 can run your app in cluster mode to use multiple CPU cores.

Use pm2 save to save your process list and pm2 startup to enable auto-start on reboot.

Check logs often with pm2 logs to catch errors early.

Summary

PM2 keeps your Express app running without manual restarts.

You can easily start, stop, and restart your app using simple commands.

PM2 helps monitor your app and manage multiple instances for better performance.

Practice

1. What is the main purpose of using PM2 with an Express application?

easy

A. To write the Express app code faster

B. To style the Express app's frontend

C. To keep the app running continuously without manual restarts

D. To create database schemas automatically

Solution

1. Step 1: Understand PM2's role

   PM2 is a process manager that keeps Node.js apps running smoothly by restarting them if they crash or stop.

2. Step 2: Identify its use with Express

   Using PM2 with Express ensures the server stays online without manual restarts after crashes or updates.

3. Final Answer:

   To keep the app running continuously without manual restarts -> Option C

4. Quick Check:

   PM2 manages app uptime = B [OK]

Hint: PM2 keeps your app alive automatically [OK]

Common Mistakes:

• Thinking PM2 helps with frontend styling
• Confusing PM2 with code writing tools
• Assuming PM2 manages databases

2. Which of the following is the correct command to start an Express app named app.js using PM2?

easy

A. pm2 run app.js

B. pm2 start app.js

C. pm2 launch app.js

D. pm2 execute app.js

Solution

1. Step 1: Recall PM2 start command

   The correct PM2 command to launch an app is pm2 start <file>.

2. Step 2: Match the command with options

   Among the options, only pm2 start app.js matches the correct syntax.

3. Final Answer:

   pm2 start app.js -> Option B

4. Quick Check:

   Start app with 'pm2 start' = D [OK]

Hint: Use 'pm2 start' to launch apps [OK]

Common Mistakes:

• Using 'pm2 run' which is invalid
• Confusing 'launch' or 'execute' as PM2 commands
• Omitting the 'start' keyword

3. Given the following PM2 commands run in order:

pm2 start app.js --name myapp
pm2 restart myapp
pm2 stop myapp
pm2 delete myapp

What is the state of the app after these commands?

medium

A. The app is running but without a name

B. The app is stopped but still managed by PM2

C. The app is running with the name 'myapp'

D. The app is removed from PM2 process list and not running

Solution

1. Step 1: Analyze each command effect

   pm2 start app.js --name myapp starts the app named 'myapp'.
   pm2 restart myapp restarts it.
   pm2 stop myapp stops the app but keeps it in PM2 list.
   pm2 delete myapp removes the app from PM2 list and stops it completely.

2. Step 2: Determine final state

   After deletion, the app is no longer managed by PM2 and is not running.

3. Final Answer:

   The app is removed from PM2 process list and not running -> Option D

4. Quick Check:

   Delete removes app from PM2 = A [OK]

Hint: Delete removes app from PM2 and stops it [OK]

Common Mistakes:

• Thinking stop deletes the app from PM2
• Assuming restart keeps app stopped
• Confusing app running state after delete

4. You tried to start your Express app with PM2 using pm2 start app.js, but it immediately stops. Which of the following is the most likely cause?

medium

A. The app.js file has a syntax error causing it to crash

B. PM2 does not support Express apps

C. You need to use pm2 run instead of pm2 start

D. The app.js file is missing the package.json

Solution

1. Step 1: Understand PM2 behavior on crashes

   If the app crashes immediately after start, PM2 will stop it and try to restart repeatedly.

2. Step 2: Identify common crash reasons

   A syntax error in app.js is a common cause for immediate crash and stop.

3. Final Answer:

   The app.js file has a syntax error causing it to crash -> Option A

4. Quick Check:

   Crash causes immediate stop = C [OK]

Hint: Check app.js syntax if PM2 stops immediately [OK]

Common Mistakes:

• Believing PM2 can't run Express apps
• Using wrong PM2 commands like 'run'
• Thinking package.json is required to start app.js

5. You want to run multiple instances of your Express app using PM2 to improve performance on a multi-core server. Which command correctly achieves this?

hard

A. pm2 start app.js -i max

B. pm2 start app.js --instances 1

C. pm2 start app.js --scale 4

D. pm2 start app.js --multi 4

Solution

1. Step 1: Understand PM2 instance options

   PM2 uses -i or --instances to specify how many app instances to run. Using -i max runs as many instances as CPU cores.

2. Step 2: Evaluate options

   --instances 1 runs only one instance.
   --scale and --multi are not valid PM2 options.

3. Final Answer:

   pm2 start app.js -i max -> Option A

4. Quick Check:

   Use '-i max' for multi-core instances = A [OK]

Hint: Use '-i max' to run all CPU cores [OK]

Common Mistakes:

• Using invalid options like --scale or --multi
• Setting instances to 1 for multi-core use
• Confusing PM2 flags for scaling