Hello 🙂 I am new to Kotlin for backend dev and I was wondering what is the advantage of ktor compared to Spring?

Comparing the two is kind of difficult, Spring is an entire framework encompassing different concerns of a server, while ktor mostly is a wrapper of a web server that allows you to easily setup your endpoints and filters.

I would add that ktor is much less opinionated than spring.

Okay, so ktor is what Sinatra is to Rails or Flask is to Django, right ?

Yes, the Flask comparison is good. I don´t know about Sinatra.