Cluster and Fork mode difference in PM2

Node.js is single-thread.

That means only 1 core of your Intel quad-core CPU can execute the node application.

It called: fork_mode.

We use it for local dev.

pm2 start server.js -i 0 helps you running 1 node thread on each core of your CPU.

And auto-load-balance the stateless coming requests.

On the same port.

We call it: cluster_mode.

Which is used for the sake of performance on production.

You may also choose to do this on local dev if you want to stress test your PC :)

Documentation and sources are really misleading here.

Reading up on this in the sources, the only differences seems to be, that they use either node cluster or child_process API. Since cluster uses the latter, you are actually doing the same. There is just a lot more custom stdio passing around happening inn fork_mode. Also cluster can only be communicated with via strings, not objects.

By default you are using fork_mode. If you pass the the -i [number]-option, you're going into cluster_mode, which you generally aim for w/ pm2.

Also fork_mode instance probably can't listen on the same port due to EADDRINUSE. cluster_mode can. This way you also can structure you app to run on the same port being automatically load balanced. You have to build apps without state then though e.g. sessions, dbs.

The main difference between fork_mode and cluster_mode is that it orders pm2 to use either the child_process.fork api or the cluster api.

What does this means internally?

Fork mode

Take the fork mode as a basic process spawning. This allows to change the exec_interpreter, so that you can run a php or a python server with pm2. Yes, the exec_interpreter is the "command" used to start the child process. By default, pm2 will use node so that pm2 start server.js will do something like:

require('child_process').spawn('node', ['server.js'])

This mode is very useful because it enables a lot of possibilities. For example, you could launch multiple servers on pre-established ports which will then be load-balanced by HAProxy or Nginx.

Cluster mode

The cluster will only work with node as it's exec_interpreter because it will access to the nodejs cluster module (eg: isMaster, fork methods etc.). This is great for zero-configuration process management because the process will automatically be forked in multiple instances. For example pm2 start -i 4 server.js will launch 4 instances of server.js and let the cluster module handle load balancing.