HTTP application use-cases are the original and primary focus of http4k. Based on the Server as a Function concept, http4k provides a set of function types which can be used to write, test and deploy HTTP applications with simplicity and ease.

Make no mistake - this model is deceptively simple but exceptionally powerful. These core concepts are repeated and combined in many different ways over the various functionalities provided by the toolkit.


In http4k, an HttpMessage is an immutable entity representing either a Request or a Response.

This immutability is a powerful alternative to the mutable versions found in other web libraries, as it provides a exact record of the state of the messages as they travel through an HTTP application, so for debugging purposes you can time-travel through an application inspecting the exact state at any point in the stack.

Data class semantics for comparison also make HttpMessages incredibly simple to assert against in testing scenarios, and this ease of testing is one of the most important parts of the http4k ethos.


typealias HttpHandler = (Request) -> Response 

A simple function representing all incoming and outgoing HTTP calls.

HttpHandlers can be bound to a container (to create an Http4kServer) with 1 LOC. This decouples the server implementation from the business logic:

val jettyServer = app.asServer(Jetty(9000)).start()

An Http client is also a HttpHandler:

val client: HttpHandler = ApacheClient()

Because the client and server interfaces are the same, apps can simply be plugged together out-of-container by just injecting one into the other:

val app1: HttpHandler = MyApp1()
val app2: HttpHandler = MyApp2(app1)


fun interface Filter : (HttpHandler) -> HttpHandler

A function which decorates an HttpHandler to perform pre/post request processing. Filters can be composed together to make reusable "stacks" of behaviour which can be applied to a terminating HttpHandler - to yield another, decorated, HttpHandler.


interface Router {
    fun match(request: Request): RouterMatch

A selective request handler, which attempts to match an incoming call against a bound HttpHandler.