Aadd watch to the config.json file:

{

  "apps": [{

    "name": "App1",

    "script": "app1/server.js",

    "log_file": "log/app1.log",

    "error_file": "log/app1-err.log",

    "watch": true,

    "ignore_watch": ["file"]  //inside app1 folder

  },{

    "name": "App2",

    "script": "app2/server.js",

    "log_file": "log/app2.log",

    "error_file": "log/app2-err.log"

  }]

}

After change app1/server.js, run

pm2 list

You can see App1's pid is changed, but App2 is not. Because we enable the watch on App1 not on App2.