Introduction

NodeJS server is single threaded in nature as such depending on the application there is only a finite amount of requests that can be handled by a single instance of the application.
If the application is CPU intensive then the number of requests that can be handled by a single instance of the application will be even lower.

Hence there are use cases where you would need to horizontally scale the application to handle more requests.

In this article we will cover how to horizontally scale a NextJS application using PM2.

What is horizontally scaling?

Horizontal scaling, often referred to as scaling out, is the process of adding more machines or nodes to a system to improve performance and capacity.

In our case we will be adding more instances of a NextJS application using PM2 process manager.

What is PM2?

PM2 is a daemon process manager that will help you manage and keep your application online 24/7.
It has a lot of features that will help you in the process of deploying and maintaining your application.

Additionally PM2 allows you to run a nodeJS server in cluster mode.
Which dramatically improves the performance of your application and allows you to scale your application horizontally.

Implementation

Setup a NextJS project

npx create-next-app@latest my-next-app

Install PM2 globally.

npm install pm2 -g

Create a ecosystem.config.js file in the root of the project.

pm2 ecosystem

Setup your ecosystem file as follows.

module.exports = {
    apps: [
        {
            script: 'npm run start',
            instances: 3,
            name: 'my-next-app',
        },
    ],
}

Further ecosystem configuration options can be found here.

Start the application using PM2.

pm2 start

You can check the status of the application using the following command.

pm2 monit

SSG + ISR

One last caveat to keep in mind is that if you are using SSG with ISR
then you will need to configure NextJS to stop using in memory cache and use file system cache instead.

Add the following in next.config.js file.

module.exports = {
    experimental: {
        // Defaults to 50MB
        isrMemoryCacheSize: 0,
    },
}

Conclusion

In this article we covered how to horizontally scale a NextJS application using PM2.
We also covered how to configure NextJS to use file system cache instead of in memory cache when using SSG with ISR.

Also we configured pm2 to run 3 instances of the application.
Similarly you can configure pm2 to run as many instances as you need.

Further reading

1. PM2
2. NextJS
3. SSG
4. ISR
5. Cluster mode
6. Ecosystem configuration
7. Self hosting ISR