How to write async/reactive code with Quarkus?

[danechitoaie]

Hi,
I'm new to Quarkus and it seems to "click" with me. I very much prefer it vs the alternatives but I'm a bit confused about reactive and non reactive modes.

I was reading this article https://medium.com/abbeal/quarkus-why-you-should-explore-f5c14dc3c80a and in there it's mentioned that:

So no matter what your programming paradigm is, it’s possible to write reactive or imperative code, both will respond in a non-blocking way.

So when is the code in Quarkus blocking (sync?) and when it's non blocking (async?)?
Is the code non blocking only when using reactive routes and doing everything with Uni/Multi? And blocking otherwise?

I'm coming from a Node.js/Python background so I'm trying to understand how things work with Quarkus and what is the equivalent of sync/async.