Kotlin for server side
Kotlin is a great fit for developing server-side applications, allowing you to write concise and expressive code while maintaining full compatibility with existing Java-based technology stacks and a smooth learning curve:
Scalability: Kotlin's support for coroutines helps build server-side applications that scale to massive numbers of clients with modest hardware requirements.
Interoperability: Kotlin is fully compatible with all Java-based frameworks, which lets you stay on your familiar technology stack while reaping the benefits of a more modern language.
Migration: Kotlin supports gradual, step by step migration of large codebases from Java to Kotlin. You can start writing new code in Kotlin while keeping older parts of your system in Java.
Tooling: In addition to great IDE support in general, Kotlin offers framework-specific tooling (for example, for Spring) in the plugin for IntelliJ IDEA Ultimate.
Learning Curve: For a Java developer, getting started with Kotlin is very easy. The automated Java to Kotlin converter included in the Kotlin plugin helps with the first steps. Kotlin Koans offer a guide through the key features of the language with a series of interactive exercises.
Frameworks for server-side development with Kotlin
Ktor is a framework built by JetBrains for creating Web applications in Kotlin, making use of coroutines for high scalability and offering an easy-to-use and idiomatic API.
kotlinx.html is a DSL that can be used to build HTML in a Web application. It serves as an alternative to traditional templating systems such as JSP and FreeMarker.
Micronaut is a modern, JVM-based, full-stack framework for building modular, easily testable microservice and serverless applications. It comes with a lot of built-in, handy features.
http4k is the functional toolkit with a tiny footprint for Kotlin HTTP applications, written in pure Kotlin. The library is based on the "Your Server as a Function" paper from Twitter and represents modeling both HTTP Servers and Clients as simple Kotlin functions that can be composed together.
Javalin is a very lightweight web framework for Kotlin and Java which supports WebSockets, HTTP2 and async requests.
The available options for persistence include direct JDBC access, JPA, as well as using NoSQL databases through their Java drivers. For JPA, the kotlin-jpa compiler plugin adapts Kotlin-compiled classes to the requirements of the framework.
Deploying Kotlin server-side applications
Kotlin applications can be deployed into any host that supports Java Web applications, including Amazon Web Services, Google Cloud Platform and more.
Google Cloud Platform offers a series of tutorials for deploying Kotlin applications to GCP, both for Ktor and App Engine and Spring and App engine. In addition there is an interactive code lab for deploying a Kotlin Spring application.
Users of Kotlin on the server side
Corda is an open-source distributed ledger platform, supported by major banks, and built entirely in Kotlin.
JetBrains Account, the system responsible for the entire license sales and validation process at JetBrains, is written in 100% Kotlin and has been running in production since 2015 with no major issues.
For a more in-depth introduction to the language, check out the Kotlin documentation on this site and Kotlin Koans.
Micronaut also has a lot of well-detailed guides, showing how you can build microservices in Kotlin.