Load a variable from dotenv file when starting PM2

Question

I am starting instances of my app as a package.json script with PM2 this way:

"start:pm2": "pm2 start -i max node myapp.js"

I found out that not all members in the team always want to use max as a value for instances number while developing, but prefer to use some lower value.

To not change package.json I would better let them change the value inside .env file because we already use it so that the value from it would be used as the parameter to pm2.

I know I can create a wrapper js or bash script to load the variable from .env file and pass it to pm2 but it would be better to have a solution without it.

How can I achieve this?

Answer 1

You can create an ecosystem.config.js file and declare your environment variables under the “env:” attribute, in your case the NODE_APP_INSTANCE can be used to set the number of instances:

module.exports = {
  apps : [{
    name: "MyApp",
    script: "./myapp.js",
    env: {
      NODE_ENV: "development",
      NODE_APP_INSTANCE: "max"
    },
    env_production: {
      NODE_ENV: "production",
    }
  }]
}

Then call pm2 start or pm2 start /path/to/ecosystem.config.js to load an ecosystem from an other folder.

Answer 2

A better pattern here is to remove dotenv from your code and "require" it on the command line. This makes your code nicely transportable between any environment (including cloud-based) - which is one of the main features of environment variables.

a) code up your.env file alongside your script (eg app.js)

b) to run your script without pm2:

node -r dotenv/config app.js

c) in pm2.config.js:

module.exports = {
  apps : [{
    name      : 'My Application',
    script    : 'app.js',
    node_args : '-r dotenv/config',
    ...
  }],
}

and then pm2 start pm2.config.js

Note: the use of dotenv/config on the command line is one of the best practices recommended by dotenv themselves