At Hexlabs we have been using AWS lambda functions for a number of use cases over the past few years. The basic idea is that you deploy code that reacts to a specified event in an AWS environment. AWS is then responsible for ensuring that the code to process the event is run in a timely manner. If no events take place, the code is not run and so there is no cost for whoever pays the bill.

On the other hand, if there are a large number of events happening, AWS can quickly spin up many instances of the lambda function to cope with the higher workload. From this we get services that are cost-effective and scalable with a minimal overhead to support.

A lambda function can be configured to react to an ever growing list of events in the AWS ecosystem but we are going to focus on one of the most common uses, reacting to HTTP requests sent to API gateway.

An API Gateway can be configured to invoke a lambda function when a specific endpoint and HTTP method is invoked. How to configure will be shown later but for now we will look at a basic function that could be invoked.

Basic API Gateway Lambda

As each AWS event will contain different information the payloads passed to a lambda function will look very different. AWS provides a Java SDK to aid handling these different events types. It is also possible to create your own handlers that read the events from InputStreams, but we will stick with the SDK for a first example. Below is a our basic Hello world lambda in Kotlin.

class HelloWorldHandler : RequestHandler<APIGatewayProxyRequestEvent, APIGatewayProxyResponseEvent> {
  override fun handleRequest(input: APIGatewayProxyRequestEvent?, context: Context?): APIGatewayProxyResponseEvent {
    return APIGatewayProxyResponseEvent().apply {
      statusCode = 200
      body = """{"msg": "hello world"}"""

This is saying that our lambda function will handle an APIGatewayProxyRequestEvent payload and return an APIGatewayProxyResponseEvent with status 200 and a “hello world” msg in a JSON body.

Once attached to an API gateway endpoint a request will return the HTTP response below.

> curl -i -X GET
HTTP/2 200
content-type: application/json
content-length: 22
date: Thu, 21 Nov 2019 11:31:36 GMT
x-amzn-requestid: 5ffe35e8-e8fe-4091-ad3b-803ebd060b6c
x-amz-apigw-id: DgdP2EgaDoEFfwg=
x-amzn-trace-id: Root=1-5dd67598-2e8a03500c5af4c99a467348;Sampled=0
x-cache: Miss from cloudfront
via: 1.1 (CloudFront)
x-amz-cf-pop: LHR62-C2
x-amz-cf-id: VNt8h2J0tBu_aJSKsVZlcx0mrBIg6C0BT3Jrd47u3TkJ8tF0_NNC-Q==

{"msg": "hello world"}%

If we examine the APIGatewayProxyRequestEvent sent to the lambda function we can see that all the information from the HTTP request is present alongside other AWS specific information.

    "resource": "/hello",
    "path": "/hello",
    "httpMethod": "GET",
    "headers": {
        "Accept": "*/*",
        "CloudFront-Forwarded-Proto": "https",
        "CloudFront-Is-Desktop-Viewer": "true",
        "CloudFront-Is-Mobile-Viewer": "false",
        "CloudFront-Is-SmartTV-Viewer": "false",
        "CloudFront-Is-Tablet-Viewer": "false",
        "CloudFront-Viewer-Country": "GB",
        "Host": "",
        "User-Agent": "curl/7.54.0",
        "Via": "2.0 (CloudFront)",
        "X-Amz-Cf-Id": "NQLoEfihGRaa9Mp22DS4J_mb507-xK44EYUB9FMRtE_RpE0BZWJzQ==",
        "X-Amzn-Trace-Id": "Root=1-5dd67549-1426c8f8f9891400c30a620",
        "X-Forwarded-For": ",",
        "X-Forwarded-Port": "443",
        "X-Forwarded-Proto": "https"
    "multiValueHeaders": {
        "Accept": [ "*/*" ],
        "CloudFront-Forwarded-Proto": [ "https" ],
        "CloudFront-Is-Desktop-Viewer": [ "true" ],
        "CloudFront-Is-Mobile-Viewer": [ "false" ],
        "CloudFront-Is-SmartTV-Viewer": [ "false" ],
        "CloudFront-Is-Tablet-Viewer": [ "false" ],
        "CloudFront-Viewer-Country": [ "GB" ],
        "Host": [ "" ],
        "User-Agent": [ "curl/7.54.0" ],
        "Via": [ "2.0 (CloudFront)" ],
        "X-Amz-Cf-Id": [ "NQLoEfihGRaa9Mp22DS4iJ_mb507-xK44EYUB9FMRtE_RpE0BZWJzQ==" ],
        "X-Amzn-Trace-Id": [ "Root=1-5dd67549-1426c8f82f9891400c30a620" ],
        "X-Forwarded-For": [ "," ],
        "X-Forwarded-Port": [ "443" ],
        "X-Forwarded-Proto": [ "https" ]
    "queryStringParameters": null,
    "multiValueQueryStringParameters": null,
    "pathParameters": null,
    "stageVariables": null,
    "requestContext": {
        "accountId": "11111111",
        "stage": "demo",
        "resourceId": "zt6sq",
        "requestId": "57073c9c-e46f-4856-be5f-a632f9d8c85f",
        "identity": {
            "cognitoIdentityPoolId": null,
            "accountId": null,
            "cognitoIdentityId": null,
            "caller": null,
            "apiKey": null,
            "sourceIp": "",
            "cognitoAuthenticationType": null,
            "cognitoAuthenticationProvider": null,
            "userArn": null,
            "userAgent": "curl/7.54.0",
            "user": null,
            "accessKey": null
        "resourcePath": "/hello",
        "httpMethod": "GET",
        "apiId": "tzpwxxbe",
        "path": "/demo/hello",
        "authorizer": null
    "body": null,
    "isBase64Encoded": false

This is great for setting up simple one off endpoints but gets cumbersome very quickly if we want to set up a REST service with multiple endpoints. Functionality like checking HTTP methods, extracting from parameters encoding and decoding payloads that is part of most HTTP frameworks needs to be implemented by hand. For this reason we were very happy to discover HTTP4K.


HTTP4K is library written in Kotlin specifically to deal with HTTP programming. Rather than its own implement an HTTP server or client it provides a lightweight abstraction that can be easily adapted to existing HTTP servers and clients. Supported servers include:

  • Apache
  • Jetty
  • Ktor
  • Netty
  • Undertow
  • SunHttp

The library also has modules for Metrics, OpenAPI Contracts, OAuth, and more besides.

HTTP4K’s design is based on the paper Your Server as a Function that gave the idea that a server should be made from the 2 following types of functions:

A Service or HttpHandler that takes the incoming HTTP Request returning an HTTP Response:

typealias HttpHandler = (Request) -> Response

A Filter or Middleware that can apply operations pre or post processing by the HttpHandler e.g Tracing or Authentication `

interface Filter : (HttpHandler) -> HttpHandler

The other concept heavily used is a router that matches an incoming request to the appropriate HTTPHandler.

There are also functions that allow these basic components to be combined to make larger components of the same type. In concrete terms, if we want to create an API to fetch and create car information we could naively create some HTTPHandlers.

val carLens =<Car>().toLens()
val postCarHandler: HttpHandler = { request: Request ->
    val car = carLens(request)
    val id = carInventoryService.create(car)
    Response(Status.CREATED).with(CarInfo(id, car).jsonBody())
val getCarHandler: HttpHandler = {

These would be bound to specific HTTP request types using a router:

val postCarRoute: HttpHandler = routes(
    "/cars" bind POST to postCarHandler,
    "/cars" bind GET to getCarHandler

We can capture the time taken for each request to be handled using a filter

val latencyFilter: Filter = Filter { httpHandler: HttpHandler ->
    { request: Request ->
      val start = System.currentTimeMillis()
      val response = httpHandler(request)
      val latency = System.currentTimeMillis() - start"request took $latency ms")

The filter and routes can then be combined to create another HttpHandler

val service : HttpHandler = latencyFilter.then(postCarRoute)

As HttpHandler is just a function (Request) -> Response it can be invoked directly with a Request object for testing:

println(service(Request(Method.GET, "/cars")))

which gives

request took 469 ms
HTTP/1.1 200 OK
content-type: application/json; charset=utf-8


or we can run service as a Server and invoke remotely using a client, in this case a Ktor server and OkHttp client

val server  = service.asServer(KtorCIO(9000)).start()
val client = OkHttp()
println(client(Request(Method.GET, "http://localhost:9000/cars")))

which gives

Responding at
request took 0 ms
HTTP/1.1 200 OK
connection: keep-alive
content-type: application/json; charset=utf-8
transfer-encoding: chunked


A mystery line that may be of interest above is

val carLens =<Car>().toLens()

In HTTP4K a lens is something that can extract and decode to a specified format from a request while also being used to encode and update a response. The carLens above will decode JSON in a request to a car object

val ford: Car = carLens(Request(Method.POST, "/cars").body(
    """{"make":"Ford","model":"Fiesta","year":1985, "colour":"Blue"}"""))

and can also be used to modify a response while encoding a car object as JSON

val bmwResponse: Response =  Response(Status.OK)
    .with(carLens of Car(make = "BMW", model = "i8", year = 2019, colour = "White"))

HTTP4K also comes with predefined lenses to extract from Parameters and Headers in HTTP messages.

HTTP4K Serverless module

While an AWS lambda functions doesn’t equate to a server the APIGateway Request and Response events have been adapted to the HTTP4k model in the HTTP4K Serverless module. In this case we implement a supplied AppLoader interface.

object InventoryHandlerLambdaFunction : AppLoader {
override fun invoke(env: Map<String, String>): HttpHandler =
      "/cars" bind Method.POST to postCarHandler,
      "/cars" bind Method.GET to getCarHandler

We now have access to HTTP4Ks functionality and are able to create powerful and succinct REST services inside our lambda function.


Setting up a Lambda function in AWS involves a number of operations that can be off putting. These include setup and configuration for:

  • IAM roles
  • Policies
  • Log groups
  • AWS Gateway

There are a number of libraries that help with this configuration but seeing as this is a Kotlin article, we are going to use Kloudformation an open source library developed by Hexlabs.

Kloudformation allows users to generate AWS infrastructure templates (AKA Cloudformation, see what we did there) using Kotlin for over 300 AWS resources, it also has a serverless module that greatly reduces boilerplate when creating templates for lambda functions.

The code below is enough to generate a Cloudformation template for our lambda setup against API gateway. The full generated template can be viewed here.

class Stack : StackBuilder {
  override fun KloudFormation.create(args: List<String>) {
    val (code) = args
    serverless(serviceName = "htt4k-aws-demo", stage = "demo", bucketName = +"lambda-cf-bucket") {
      serverlessFunction(functionId = "demo-http4k", codeLocationKey = +code,
        handler = +"org.http4k.serverless.lambda.LambdaFunction::handle",
        runtime = +"java8") {
        lambdaFunction {
          environment {
            variables(json(mapOf("HTTP4K_BOOTSTRAP_CLASS" to "io.hexlabs.vehicle.inventory.InventoryHandler")))
        http {
          path("/inventory/cars") {
            path("{id}") {

A few parts to note.

  • The http {..} is optional and specifies endpoints bound from API gateway to our lambda function.
  • When using HTTP4K the handler field should always be set to org.http4k.serverless.lambda.LambdaFunction::handle
  • The object that Implements the HTTP4K AppLoader we mentioned earlier is set as an environment variable HTTP4K_BOOTSTRAP_CLASS

At this point the Stack can be translated to a Cloudformation template and deployed using the AWS CLI. Alternatively we can use Kloudformations own CLI against the Stack.kt file. The Kloudformation CLI can be installed using:

curl -s | sh

Once installed ensure AWS credentials are stored in ~/.aws/credentials and simply run:

kloudformation -v 1.1.76 -m serverless@1.1.2 deploy -stack-name htt4k-aws-demo -bucket lambda-cf-bucket -location ./build/libs/http4k-aws-demo-uber.jar

This will deploy our lambda giving the ServiceEndpoint as an output:

ApiDeployment23bce43cb19b486bbd0cf1ec0bbdf956 AWS::ApiGateway::Deployment Status became CREATE_COMPLETE (rk5a9v)
ApiDeployment654780393ab0422185c845adb03ac95e AWS::ApiGateway::Deployment Status became CREATE_COMPLETE (lfz1n4)
htt4k-aws-demo AWS::CloudFormation::Stack Status became CREATE_COMPLETE (arn:aws:cloudformation:eu-west-1:1111111:stack/htt4k-aws-demo/4aead360-0bae-11ea-89d2-060f8efab01c)

Stack Create Complete


We are now able to code build and deploy a lambda function all using Kotlin. If you interested the code can be found here. Some may be wary of using lambda functions with the JVM due to cold starts. while this isn’t as big a problem as in the past, HTTP4K can also be compiled to a native image using Graalvm reducing this time. This will be a topic for future articles but in the meantime if you interested we have a basic implementation available here.