Microservices in Kotlin: A Roadmap for Beginners

Why Kotlin is the Best Choice for Microservices in 2024?

Have you ever written a microservice in Python and then thought, "What if it ran faster, consumed less memory, and the code remained just as readable?" Sound familiar? Then welcome to the world of Kotlin — a language that Google chose for Android, and the backend developer community chose for creating lightweight and performant microservices.

Kotlin fits perfectly into a microservice architecture: it is statically typed, compiles to JVM bytecode (or native code via Kotlin/Native), but doesn't require tons of boilerplate like Java. And most importantly, its coroutines allow you to handle thousands of requests on a single thread without the pain of callback hell.

In this article, we won't lecture you on theory. We'll just go ahead and write two microservices: User Service (REST API) and Notification Service (simulating sending emails). All in pure Kotlin + Ktor. Let's go!

1. Toolkit: What to Install Before Starting

Before writing code, let's make sure your computer is ready for action.

What you need to install:

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• JDK 17+ — Kotlin runs on the JVM, so you can't do without Java. Download from Adoptium.
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• IntelliJ IDEA Community Edition — the best IDE for Kotlin (yes, it's from JetBrains, and yes, it's free).
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• Gradle (or Maven) — build system. It's already built into IntelliJ, but if you want to install it separately — gradle.org.
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• Docker (optional, but will greatly simplify deployment).
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If you have macOS or Linux — you're already halfway to success. On Windows, I recommend WSL2.

2. Creating Your First Microservice: User Service on Ktor

Ktor is a framework from the creators of Kotlin. It is extremely lightweight: you decide which modules to include (routing, serialization, logging). No magic — just pure code.

2.1. Project Generation

The fastest way is to use the Ktor Project Generator. Select:

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• Build system: Gradle Kotlin DSL
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• Engine: Netty (the most popular)
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• Plugins: Routing, Content Negotiation (JSON), Status Pages
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Download the archive, open it in IntelliJ — and in 2 minutes you have a ready-made skeleton.

2.2. Writing the REST API: Getting Users

Let's create a simple model and endpoint. The code will be as readable as possible:

// Model.kt\ndata class User(\n    val id: Int,\n    val name: String,\n    val email: String\n)\n\n// UserRepository.kt\nobject UserRepository {\n    private val users = mutableListOf(\n        User(1, "Alice", "alice@example.com"),\n        User(2, "Bob", "bob@example.com")\n    )\n\n    fun findAll(): List<User> = users\n    fun findById(id: Int): User? = users.find { it.id == id }\n}\n\n// Application.kt (entry point)\nfun main() {\n    embeddedServer(Netty, port = 8080) {\n        configureSerialization()\n        configureRouting()\n    }.start(wait = true)\n}\n\n// Routing.kt\nfun Application.configureRouting() {\n    routing {\n        route("/api/v1/users") {\n            get {\n                call.respond(UserRepository.findAll())\n            }\n            get("/{id}") {\n                val id = call.parameters["id"]?.toIntOrNull()\n                if (id == null) {\n                    call.respond(HttpStatusCode.BadRequest, "Invalid ID")\n                    return@get\n                }\n                val user = UserRepository.findById(id)\n                if (user != null) {\n                    call.respond(user)\n                } else {\n                    call.respond(HttpStatusCode.NotFound, "User not found")\n