PM2 change cluster processes size at runtime

Question

does anyone know if it is possible to change in NodeJS PM2 the number of cluster processes for an application at runtime?

regards Philipp

To clarify, are you asking if it's possible to change the amount of cluster processes once you have started the app running? — Martyn Davies, Mar 28 '15 at 18:28

score 22 · Accepted Answer · edited Feb 25 '19 at 13:37

22

You can use pm2 scale to scale vertically the number of process at runtime, note that it only work with cluster mode. Example :

pm2 scale APPNAME 2 will scale the process to exactly 2 instances.
pm2 scale APPNAME +2 will add two process.
pm2 scale APPNAME -1 will remove one process.

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edited Feb 25 '19 at 13:37

dschu

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answered Sep 23 '16 at 18:51

vmarchaud

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Note that "pm2 scale APPNAME 2" will not add two processes, rather scale to a total of two processes. Use "+2" to add two processes. See my answer to scale using the Programmatic API too. – Andrew Apr 05 '18 at 20:59

score 2 · Answer 2 · answered Apr 20 '15 at 20:38

specify pm2 settings in json format:

{
 "apps": [{
    "name": "server",
    "script" : "index.js",
    "instances": 2,
    "exec_mode: "cluster",
    "cwd": "/path/to/script"
 }]
}

start the server:

pm2 start application.json

suppose you want to add 2 more instances, just run the same command again:

pm2 start application.json

check the processes list:

pm2 list

to test that all 4 instances are run in cluster mode:

pm2 restart server

it will restart each of the 4 processes.

score 0 · Answer 3 · answered Apr 05 '18 at 20:57

At runtime (after the application is started), there are 2 ways to "scale" the application:

1) With the command line (documented here under "Scaling your cluster in realtime"), like this:

pm2 scale <app name> <n>

Note that can be a consistent number which the cluster will scale up or down to. It can also be an addition such as pm2 scale app +3 in which case 3 more workers will be added to the cluster.

2) With the Programmatic API (docs are here, but scale is not documented). As it's not documented, here's how you do it:

pm2.scale(<APPNAME>, <SCALE_TO>, errback)

Note that is the number that will be scaled up or down to, not the number added or removed. Here's a full example of connecting to and scaling to 4 instances:

var pm2 = require('pm2');
pm2.connect(function (err) {
    pm2.scale('appname', 4, function(err, procs) {
        console.log('SCALE err: ', err);
        console.log('SCALE procs: ', procs);
    });
});

score 0 · Answer 4 · answered Mar 23 '23 at 21:23

We had the same problem and did not find any solution. To solve this problem i wrote a small plugin for PM2 which can help dynamically scale applications based on utilization demand. You can try and play with it https://www.npmjs.com/package/pm2-autoscale

Or just install it directly

pm2 install pm2-autoscale

When module detects that CPU utilisation is higher then expected it will start increasing instances to max CPUs-1 and if server has available free memory.

When module detects CPU utilization is decreasing it will stop useless instances.

PM2 change cluster processes size at runtime

4 Answers4