Serverless Functions in Kotlin: AWS Lambda from A to Z

Introduction to Serverless Functions in Kotlin and AWS Lambda

Serverless architecture has become the standard for building scalable and cost-effective applications. AWS Lambda is one of the most popular services for running code without managing servers. However, most examples and documentation are focused on Java or Python. Kotlin, being a modern language for the JVM, offers concise syntax and full compatibility with the Java ecosystem, making it an excellent choice for Lambda functions.

In this article, we will cover the full cycle: from project setup to deployment and monitoring. You will learn how to write efficient serverless functions in Kotlin, handle events from API Gateway, S3, and DynamoDB, and optimize cold starts.

1. Project Setup and Dependencies

First, let's create a Gradle project with Kotlin support. The main dependency is com.amazonaws:aws-lambda-java-core, which provides interfaces for handlers.

Step 1.1: build.gradle.kts Structure

plugins {    kotlin("jvm") version "1.9.22"    id("io.github.microutils:kotlin-logging") version "3.0.5"}

dependencies {    implementation("com.amazonaws:aws-lambda-java-core:1.2.2")    implementation("com.amazonaws:aws-lambda-java-events:3.11.3")    implementation("com.fasterxml.jackson.module:jackson-module-kotlin:2.15.3")    implementation("org.slf4j:slf4j-simple:2.0.9")}

tasks.register<Zip>("buildLambda") {    from(sourceSets.main.get().output)    from(configurations.runtimeClasspath.get().map { if (it.isDirectory) it else zipTree(it) })    into("build/libs")    archiveFileName.set("lambda.zip")}

Note the buildLambda task — it creates a "fat" JAR that AWS Lambda can execute. Without this step, deployment will not work.

Step 1.2: Basic Handler Function

Create a file Handler.kt with a simple function that accepts Map<String, String> and returns a string:

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"        context.logger.log("Processing request for $name")        return "Hello, $name!"    }}

The RequestHandler interface is parameterized with two types: the input data type and the return value type. Kotlin allows using Map, String, as well as any data classes.

2. Working with AWS Events (API Gateway, S3, DynamoDB)

Serverless functions rarely work in isolation. More often, they react to events from other AWS services. Let's look at three popular scenarios.

2.1 Handling HTTP Requests via API Gateway

For integration with API Gateway, use the APIGatewayProxyRequestEvent class from aws-lambda-java-events:

import com.amazonaws.services.lambda.runtime.events.APIGatewayProxyRequestEventimport com.amazonaws.services.lambda.runtime.events.APIGatewayProxyResponseEventimport com.amazonaws.services.lambda.runtime.Contextimport com.amazonaws.services.lambda.runtime.RequestHandler

class ApiHandler : RequestHandler<APIGatewayProxyRequestEvent, APIGatewayProxyResponseEvent> {    override fun handleRequest(input: APIGatewayProxyRequestEvent, context: Context): APIGatewayProxyResponseEvent {        val body = "Received: ${input.body}"        return APIGatewayProxyResponseEvent()            .withStatusCode(200)            .withBody(body)            .withHeaders(mapOf("Content-Type" to "text/plain"))    }}

In this example, we extract the request body and return an HTTP response.