Eclipse Vert.x is a tool-kit for building reactive applications on the JVM: it is event-driven and nonblocking, this means your app can handle a lot of concurrency using a small number of kernel threads. Most of us are used to writing interactive code and going the reactive way is not a trivial paradigm shift for everyone: programming using asynchronous APIs can be more challenging than using a direct synchronous style, in particular, if you have several operations that you want to do in sequence. Also error propagation is often more complex when using asynchronous APIs. With Kotlin you can write inter-reactive Vert.x applications: perform asynchronous operations and receive events in a synchronous way. Inter-reactive allows you to work with asynchronous APIs, but using a direct synchronous style that you’re already familiar with! Julien Viet is the Vert.x project lead and works at Red Hat. Julien has been developing open source software since 2002. Julien holds an engineering degree from Institut National des Télécommunications.