I just realized subscription won't working if graphql+prisma hosted on heroku server. Any one knows how to deal with it? { "error": "Could not connect to websocket endpoint wss://graphql-chatroom.herokuapp.com/. Please check if the endpoint url is correct." }

I think you should migrate to digitalocean for your setup. Heroku container deploys doesn't seem to support web socket connections.

Thanks for your recommendation. I am doing it now. Hope it works.