project-loom-c5m VS ktor

It is not. Blocking IO (with some exceptions mentioned in the JEP) will automatically be translated by the runtime into non-blocking IO when it occurs on virtual threads, and no OS threads will be blocked. You can have a million threads blocking on a million sockets (obviously without creating a million OS threads): https://github.com/ebarlas/project-loom-c5m

You can't do that with thread pools. You could achieve that scalability with async code, but then observability tools will not be able to track the IO operations and who initiated them, but with virtual threads you'll see exactly what business operation is doing what IO and why.

That might change in JDK 21 (with virtual threads). See this https://github.com/ebarlas/project-loom-c5m . It achieve 5 million persistent connections (again depends on the server capacity and kernal tuning) using normal simple blocking code (https://github.com/ebarlas/project-loom-c5m/blob/main/src/main/java/loomtest/EchoServer.java) . It's a far better better programming model compared to JS async/await.

In paralel Jetbrains has created a lot of KMP frameworks that are awesome. One of them is Ktor that helps "create asynchronous client and server applications."

Ktor, a powerful web framework built with Kotlin, offers a lightweight and flexible solution for building web applications. In this article, we will guide you through the process of creating a Ktor project manually using Gradle and SDKMAN!. By following the steps below, you'll have a basic Ktor project up and running in no time.

take a look at http4k and ktor for Kotlin specific frameworks Spring has first class support for Kotlin e.g. the following will give you reactive HTTP endpoints when using spring-webflux (this is what is use at work):

Ktor (Microservices framework by JetBrains)

For Android you should use a more mobile friendly framework like Retrofit or if you use Kotlin you can use the multi-platform Ktor library with it's client module

And last but not least, Ktor Client as our HTTP client. https://ktor.io/ It's a pretty amazing http client library and integrates well with Kotlinx serialization and Coroutines.

Kotlin is going to be great with almost every single JVM server. Spring works well with Kotlin, and is directly supported, but spring is also massive, very bloated. I recommend looking at https://github.com/ktorio/ktor which continues to serve all of my needs very well, integrates fantastically with Kotlin coroutines, and has very fast startup time and very good performance.

For the sake of the test, we use Ktor - the easiest way to do so is to use initializer.

I know you are looking for Spring Boot and/or Quarkus, but have you tried ktor. Just curios if you had a particular reason for choosing the other 2. Disclaimer: I haven't used ktor.