I've created an application that connects to MongoDb atlas cluster and is hosted on heroku. My question is, when the api server application is idle for long time, and the next client request comes in, does the server application start from its defined main entry point (in my case it's `Application.module`())?

If a server is still running then yes.

Know that Heroku is know to shutdown idle inatances and reboot them when there is incoming request