http4k is an HTTP toolkit written in pure Kotlin for serving and consuming HTTP services. One of the key ideas behind http4k is that HTTP apps are modelled by composing two simple functions — HttpHandler and Filter. They derive inspiration from Twitter's "Your Server as a Function" paper. It's very lightweight with the core module having no dependencies apart from Kotlin StdLib. Apart from its elegance and simplicity, we also like its emphasis on testability — given that the entities in the libraries are immutable and the routes in the app, as well as the app itself, are just functions, they're super easy to test. One of the things to be aware of, though, is that we don't have nonblocking or coroutines support in http4k yet.