When deploying an app created with Sinatra to Heroku, an error occurs that it is useless without config.ru. So I created config.ru and described various things, and it worked without problems. I wondered what config.ru is doing in the first place and decided to leave it as a memorandum.
It seems to be an abbreviation for rack up.
A command provided by a library (gem) called Rack.
Interface between web server and Ruby or Ruby frameworks (such as Rails and Sinatra). Without this, requests from the web server will not reach the app. Since the web server and frameworks such as Rails are talking in different languages, the framework side does not understand what is being said even if the web server asks. An image that allows Rack to translate both languages and exchange requests and responses by sandwiching them.
config.ru Rack works by passing the app's object to run.
Rack is a bridge between web servers and application servers. I got an error when deploying because the application server couldn't answer the web server request.