Server as a Function. In Kotlin. Typesafe. Without the Server.

november 2017 / @daviddenton

Meet http4k

http4k is an HTTP toolkit written in Kotlin that enables the serving and consuming of HTTP services in a functional and consistent way.

Whenever (yet another) new JVM HTTP framework is released, the inevitable question that rightly get asked is "How it this different to X?". In this post, I'm going to briefly cover what http4k is, how we think it's different, and address some of those bold claims from the title of this post.

Here's a quick rundown of what we think those differences are:

  • http4k is small. Written in pure, functional Kotlin, with zero dependencies.
  • http4k is simple. Like, really simple. No static API magic, no annotations, no reflection.
  • http4k is immutable. It relies on an immutable HTTP model, which makes it a snap to test and debug.
  • http4k is symmetric. It supports remote calls as a first-class concern, and the remote HTTP model is identical to the incoming HTTP model.
  • http4k is typesafe. Say goodbye to all your validation and marshalling boilerplate and hello to automatic request validation and data class-based contracts for HTTP bodies using the Lens API.
  • http4k is serverless. Or rather - server independent. Test an app out of container and then deploy it into any supported local container with 1 LOC - or as a function into AWS Lambda.

Oh god, not another framework! Why does this even exist?!?

Firstly - we don't consider http4k to be a framework - it's a set of libraries providing a functional toolkit to serve and consume HTTP services, focusing on simple, consistent, and testable APIs. Hence, whilst it does provide support for various APIs relevant to serving and consuming HTTP, it does not provide every integration under the sun - merely simple points to allow those integrations to be hooked in.

Another thing to say is that (not very much) of http4k is new - it's rather the distillation of 15 years worth of experience of using various server-side libraries and hence most of the good ideas are stolen. For instance - the routing module is inspired by UtterlyIdle, the basic "Server as a function" model is stolen from Finagle, and the contract module OpenApi/Swagger generator is ported from Fintrospect.

With the growing adoption of Kotlin, we wanted something that would fully leverage the functional features of the language and it felt like a good time to start something from scratch, whilst avoiding the magic that plagues other frameworks. Hence, http4k is primarily designed to be a Kotlin-first library.

Claim A: Small, simple, immutable.

Based on the awesome "Your Server as a Function" paper from Twitter, http4k apps are modelled by composing 2 types of simple, independent function.

Function 1: HttpHandler

An HttpHandler represents an HTTP endpoint. It's not even an Interface, modelled merely as a Typealias:

typealias HttpHandler = (Request) -> Response

Below is a entire http4k application that echoes the request body back in the response. It only relies on the http4k-core module, which itself has zero dependencies:

val app = { request: Request -> Response(OK).body(request.body) }
val server = app.asServer(SunHttp(8000)).start()

The Request and Response objects in there are immutable data classes/POKOs, so testing the app requires absolutely no extra infrastructure - just call the function, it's as easy as:

class AppTest {
    fun `echoes request body`() {
        assertThat(app(Request(POST, "/").body("hello")), equalTo(Response(OK).body("hello")))

To plug it into a different Server-backend, just depend on the relevant module (Jetty, Undertow, Netty, Apache (httpcore), Ktor CIO, Ktor Netty, and SunHttp are available) and change the call to asServer().

Function 2: Filter

Filters provides pre and post Request processing and are simply:

interface Filter : (HttpHandler) -> HttpHandler

For API conciseness and discoverability reasons this is modelled as an Interface and not a Typealias - it also has a couple of Kotlin extension methods to allow you to compose Filters with HttpHandlers and other Filters:

val setContentType = Filter { next ->
        { request -> next(request).header("Content-Type", "text/plain") }
val repeatBody = Filter { next ->
        { request -> next(request.body(request.bodyString() + request.bodyString())) }
val composedFilter: Filter = repeatBody.then(setContentType)
val decoratedApp: HttpHandler = composedFilter.then(app)

Filters are also trivial to test independently, because they are generally just stateless functions.


http4k's nestable routing looks a lot like every other Sinatra-style framework these days, and allows for infinitely nesting HttpHandlers - this just exposes another HttpHandler so you can easily extract, test and reuse sets of routes as easily as you could with one:

val app: HttpHandler = routes(
    "/app" bind GET to decoratedApp,
    "/other" bind routes(
        "/delete" bind DELETE to { _: Request -> Response(OK) },
        "/post/{name}" bind POST to { request: Request -> 
            Response(OK).body("you POSTed to ${request.path("name")}") 

And that it - those functions are everything you need to know to write a simple http4k application. The http4k-core module rocks in at about 700kb, and has zero dependencies (other than the Kotlin language itself). Additionally, everything in the core is functional and predictable - there is no static API magic going on under the covers (making it difficult to have multiple apps in the same JVM), no annotations, no compiler-plugins, and no reflection.

Claim B. Symmetric HTTP

Out of the multitude of JVM http frameworks out there, not many actually consider how you app talks to other services, yet in this Microservice™ world that's an absolutely massive part of what many apps do!

As per a core principle behind "Server as a Function", http4k provides a symmetric API for HTTP clients - ie. it's exactly the same API as is exposed in http4k server applications - the HttpHandler. Here's that entire API again, just in case you've forgotten:

typealias HttpHandler = (Request) -> Response

What does that mean in practice? Well - for one thing, it's less for your brain to learn because you already know the API:

val client: HttpHandler = ApacheClient()
val response: Response = client(Request(GET, "http://server/path"))

For another, it means that since clients are just functions you can easily stub them for testing, and since applications and clients are interchangeable, they can be plugged together in memory without putting them on the network - which makes testing insanely fast:

fun MyApp1(): HttpHandler = { Response(OK) }
fun MyApp2(app1: HttpHandler): HttpHandler = { app1(it) }

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

http4k provides a HTTP client adapters for both Apache and OkHttp, all with streaming support.

Claim C. Typesafe HTTP with Lenses

The immutable http4k model for HTTP objects contains all the usual suspect methods for getting values from the messages. For instance, if we are expecting a search parameter with a query containing a page number:

val request = Request(GET, "http://server/search?page=123")
val page: Int = request.query("page")!!.toInt

...but we also want to ensure that the expected values are both present and valid, since the above example will fail if either of those things is not true. For this purpose, we can use a Lens to enforce the expected HTTP contract.

The use of Lenses in http4k applications can remove the need for writing any parsing or validation code for all incoming data (including Forms), as validations are taken care of by the library.

Lens basics

A Lens is a bi-directional entity which can be used to either get or set a particular value from/onto an HTTP message. http4k provides a DSL to configure these lenses to target particular parts of the message, whilst at the same time specifying the requirement for those parts (i.e. mandatory or optional) and the type. For the above example, we could use the Query Lens builder and then invoke() the Lens on the message to extract the target value:

val pageLens ="page")
val page: Int = pageLens(Request(GET, "http://server/search?page=123"))

If the query parameter is missing or not an Int, the lens extraction operation will fail. There are similar Lens builder functions for all parts of the HTTP message (Header, Path, Body, FormField etc..), and functions for all common JVM primitive types. They are all completely typesafe - there is no reflection or magic going on - just marshalling of the various entities (in this case String to Int conversion).

In the case of failure, we need to apply a Filter to detect the errors and convert them to a BAD_REQUEST response:

val queryName = Query.string().required("name")
val app: HttpHandler = routes(
      "/post" bind POST to { request: Request -> Response(OK).body(queryName(request)) }

val app = ServerFilters.CatchLensFailure.then(handler)(Request(GET, "/hello/2000-01-01?myCustomType=someValue"))

Lenses can also be applied with a correctly typed value (via invoke()) to set it onto a target object - and as HTTP messages in http4k are immutable, this results in a copy of the modified message:

val pageSizeLens ="page")
val page: Response = pageLens(Response(OK), 123)
// or apply multiple lenses using with()
val updated: Request = Request(GET, "/foo").with(pageLens of 123, pageSizeLens of 10)

Securely extracting JDK primitives from HTTP messages is great, but we really want to avoid primitives entirely and go straight to domain types. During construction of Lens, the builders allow mapping to occur so we can leverage Kotlin data classes. This works for both get and set operations:

data class MyDate(val value: LocalDate)
val dateQuery = Query.localDate().map(::MyDate, MyDate::value).required("date")
val myDate: MyDate = dateQuery(Request(GET, "http://server/search?date=2000-01-01"))

Lensing HTTP bodies with Data classes

Some of the supported message libraries (eg. GSON, Jackson, Moshi, XML) provide the mechanism to automatically marshall data objects to/from JSON and XML using reflection (oops - looks like we broke our reflection promise - but technically we're not doing it ;) !). This behaviour is supported in http4k Lenses through the use of the auto() method, which will marshall objects to/from HTTP messages:

data class Email(val value: String)
data class Message(val subject: String, val from: Email, val to: Email)

val messageLens =<Message>().toLens()
val body = """{"subject":"hello","from":{"value":"[email protected]"},"to":{"value":"[email protected]"}}"""
val message: Message = messageLens(Request(GET, "/").body(body))

This mechanism works for all incoming and outgoing JSON and XML Requests and Responses. To assist with developing whilst using this type of auto-marshalling, we have created a tool to automatically generate a set of data classes for a given messages.

Claim D. Serverless

Ah yes - Serverless - the latest in the Cool Kids Club and killer fodder for the resume. Well, since http4k is server independent, it turns out to be fairly trivial to deploy full applications to AWS Lambda, and then call them by setting up the API Gateway to proxy requests to the function. Effectively, the combination of these two services become just another Server back-end supported by the library.

In order to achieve this, only a single interface AppLoader needs to be implemented - this is responsible for creating the HttpHandler which is adapted to the API of the ApiGatewayProxyRequest/ApiGatewayProxyResponse used by AWS, and then a single class ApiGatewayV2LambdaFunction extended. As this is AWS, there is a fair amount of configuration required to make this possible, but the only http4k specific config is to set the function execution to call org.http4k.MyLambdaFunction

Here's a simple example:

object TweetEcho : AppLoader {
    override fun invoke(env: Map<String, String>): HttpHandler = {

class MyLambdaFunction : ApiGatewayV2LambdaFunction(TweetEcho)

Since http4k is very dependency-light, full binary uploads of these AWS Lambdas tend to be very small - and by utilising Proguard we've seen the size of a Lambda UberJar go as small as 150kb.

Introduced in v3.0.0, this support is available in the http4k-serverless-lambda module.

The final word(s)!

As pointed out above, http4k-core module has zero dependencies. It is also small, even though it also provides:

  • Support for static file-serving with HotReload.
  • A bunch of useful Filters for stuff like Zipkin Request Tracing.
  • Support for Request Contexts.
  • Facilities to record and replay HTTP traffic.

There are also a bunch of other modules available, all presented with the same concentration on Testability, API simplicity and consistency:

  • ViewModel driven templating engine support (Handlerbars etc) with HotReload.
  • Popular JSON/XML (Gson, Jackson, Moshi, etc) library support for HTTP bodies.
  • Typesafe HTML Form and Multipart Forms processing, with support for Streaming uploads to a storage service. Forms can also be configured to collect errors instead of just rejecting outright.
  • Typesafe contract module, providing live [OpenApi v2 & v3] documentation.
  • AWS request signing.
  • Resilience4j integration, including Circuit Breakers & Rate Limiting.
  • Testing support via Hamkrest matchers and an in-memory WebDriver implementation.

Finally, http4k is proven in production, it has been adopted in at least 2 global investment banks and is serving the vast majority of traffic for a major publishing website (in the top 1000 sites globally according to - ie. easily serving 10s of million hits per day on a few nodes) since March 2017.

You can see a few example applications here, including a bootstrap project for creating a Github -> Travis -> Heroku CD pipeline in a single command.

Well, that's it for this whirlwind tour - we hope you found it worth reading this far! We'd love you to try out http4k and feedback why you love/hate/are indifferent to it :) . And if you want to get involved or chat to the authors, we hang out in the friendly #http4k channel @ slack.kotlinlang,org.

  • "But... but... but... asynchronous! And Webscale!", I heard them froth. Yes, you are correct - "Server as a Function" is based on asynchronous functions and http4k exposes a synchronous API. However, our experience suggests that for the vast majority of apps, this actually makes API integration harder unless you've got async all the way down - and that is assuming that async clients are actually available for all your various remote dependencies. We found that this plainly didn't matter for our use-cases so went for Simple API™ instead... it's possible however that Kotlin co-routines will allow us to revisit this decision.
  • (UPDATE) Websockets? Yep - simple, testable, and now available in v3.2.1! See the introductory blog post for details!