Serverless Functions in Kotlin: AWS Lambda — Complete Guide

Introduction to Serverless Computing with Kotlin

Serverless architecture has become a standard for building scalable and cost-effective applications. AWS Lambda is one of the most popular services that allows you to run code without managing servers. Although Lambda is most often associated with Node.js or Python, the Kotlin language, thanks to its conciseness, type safety, and full compatibility with the Java ecosystem, is becoming an excellent choice for writing serverless functions.

In this article, we will thoroughly examine how to create, test, and deploy serverless functions on Kotlin for AWS Lambda. You will learn how to set up a project, handle events from API Gateway, S3, and DynamoDB, as well as optimize performance.

Setting up a Kotlin Project for AWS Lambda

To get started, we need a Gradle project with the correct dependencies. The main advantage of Kotlin is the ability to use all Java libraries, including the official AWS SDK.

Creating a Gradle Project

Create a build.gradle.kts file with the following configuration:

plugins {    kotlin("jvm") version "1.9.22"    id("io.github.deweyjose.lambda-packager") version "0.1.2"}

dependencies {    implementation("com.amazonaws:aws-lambda-java-core:1.2.3")    implementation("com.amazonaws:aws-lambda-java-events:3.11.4")    implementation("com.google.code.gson:gson:2.10.1")

    testImplementation("org.junit.jupiter:junit-jupiter:5.10.2")    testImplementation("org.mockito:mockito-core:5.11.0")}

tasks.withType<Jar> {    duplicatesStrategy = DuplicatesStrategy.EXCLUDE    from(configurations.runtimeClasspath.get().map { if (it.isDirectory) it else zipTree(it) })}

Handler Function Structure

Any Lambda function in Kotlin must implement the RequestHandler or RequestStreamHandler interface. Let's look at a simple HelloHandler example:

import com.amazonaws.services.lambda.runtime.Contextimport com.amazonaws.services.lambda.runtime.RequestHandler

class HelloHandler : RequestHandler<Map<String, String>, String> {    override fun handleRequest(input: Map<String, String>, context: Context): String {        val name = input["name"] ?: "World"        return "Hello, $name! Greetings from Kotlin Lambda!"    }}

Note: Kotlin allows using Map<String, String> for working with JSON events, but for complex structures it is better to create a Data Class.

Working with Various AWS Triggers

Serverless functions rarely work in isolation. Let's look at three popular scenarios: API Gateway, S3, and DynamoDB.

API Gateway + Lambda on Kotlin

To handle HTTP requests, use the APIGatewayProxyRequestEvent class:

import com.amazonaws.services.lambda.runtime.events.APIGatewayProxyRequestEventimport com.amazonaws.services.lambda.runtime.events.APIGatewayProxyResponseEvent

class ApiHandler : RequestHandler<APIGatewayProxyRequestEvent, APIGatewayProxyResponseEvent> {    override fun handleRequest(event: APIGatewayProxyRequestEvent, context: Context): APIGatewayProxyResponseEvent {        val body = """            {"message": "Received ${event.httpMethod} request to ${event.path}",             "queryParams": "${event.queryStringParameters}"}        """.trimIndent()

        return APIGatewayProxyResponseEvent()            .withStatusCode(200)            .withHeaders(mapOf("Content-Type" to "application/json"))            .withBody(body)    }}

Processing S3 Events

When a new object is uploaded to an S3 bucket, Lambda can automatically trigger:

import com.amazonaws.services.lambda.runtime.events.S3Eventimport com.amazonaws.services.s3.AmazonS3ClientBuilder

class S3ProcessorHandler : RequestHandler<S3Event, String>