Node.JS built in cluster or PM2 clustering?

Question

Which one is better?

I have activated Nodejs clustering mode with workers but now I discovered PM2 that does the same thing. I'm using keymetrics to see the stats from my webserver and I have noticed that when I launch my NodeJS node (with a built in cluster) without using PM2 cluster feature, Keymetrics reports 20/30MB of Ram used.

If I deactivate clustering (inside node) and I switch on PM2 cluster, keymetrics reports about 300MB of Ram usage.

Now, which method is better and why with a built in cluster keymetrics reports only 30MB of ram usage?

Chaw

Answer 1

It actually depends on how your Node application works. If your application is stateless then it is easy to use pm2 cluster mode as it does not require much effort (or no effort) in code changes. But if your application uses local data, sessions or using sockets then it is recommended to use Node.js inbuilt cluster module and start your application normally using pm2.

My Node application is using sockets and MQTT so I can't directly use pm2 cluster mode (pm2 start app.js -i max) as same node application will run on every CPU and it was creating multiple socket connection with the client. So I have to manage Clusters and Workers manually using Node cluster and have to use sticky-sessions and socket.io-redis like node packages to setup proper communication flow between all workers. And then starting my node app using simply pm2 start app.js

Below are some links which can be helpful.

• PM2 Clustur mode
• PM2 Recommendation Note
• Node Cluster

Holocene

Couldn't you use something like redis to store your socket sessions, then use only pm2 ? – Tennyson

According to PM2's devs, "your process is never the master so it can't run the master part." This means you'll only be able to pm2 your app without -i, and then your app is responsible for managing the cluster. github.com/Unitech/pm2/issues/2885 – Twandatwang

PM2 provides environment variables that you can use to check if the current process is the main process or one of the workers. So, you could definitely use PM2 cluster mode, you'll just have to implement a simple if (isMain) check in the part of your code that uses sockets. – Belvabelvedere

Answer 2

I use PM2. There are a number of reasons it is better.

1. Unlike using core's clustering, your code needs little to no modification to use PM2. Clustering logic doesn't belong in every app we ever build.
2. It scales from the command line. I can simply run pm2 scale my-app +1 to add another worker in realtime after deployment.
3. You should already be using PM2 anyway to keep the process alive. So clustering comes for free.

I cannot reproduce anything close to your 300MB number. In fact, I recently had a leaky app that I had to use --max-memory-restart on and even in that situation memory usage usually stayed below 100MB. Though it wouldn't surprise me in the slightest if PM2's clustering used more memory, simply because it does a lot for you out-of-the-box.

My suggestion would be to not prematurely optimize. Use PM2 until you genuinely need to squeeze every drop of memory / performance out of your systems (definitely not before you have lots of traffic). At that point you can figure out what the bare minimum is you need from clustering and can re-implement just those parts yourself.

Resources

• Clustering walkthrough: https://keymetrics.io/2015/03/26/pm2-clustering-made-easy/
• PM2 tutorial: https://futurestud.io/tutorials/pm2-cluster-mode-and-zero-downtime-restarts

Belvabelvedere