Ktor — Async Web Framework for Kotlin by JetBrains

What it is

Ktor is a framework by JetBrains for building connected applications in Kotlin with minimal effort. It supports coroutines natively, runs on multiple engines (Netty, CIO, Jetty), and works for both server-side HTTP and client-side HTTP in Kotlin Multiplatform projects.

Ktor is designed for Kotlin developers who want a lightweight, non-opinionated framework that leverages Kotlin's language features rather than imposing its own abstractions.

How it saves time or tokens

Ktor's plugin-based architecture means you only include what you need. There is no framework boilerplate for features you do not use. Native coroutine support makes async I/O straightforward without callback hell. The same HTTP client works across JVM, Android, iOS, and JavaScript targets via Kotlin Multiplatform.

How to use

1. Create a project via IntelliJ IDEA or Gradle
2. Add Ktor dependencies to build.gradle.kts:

plugins {
    id("io.ktor.plugin") version "3.0.0"
    kotlin("plugin.serialization") version "2.0.0"
}

dependencies {
    implementation("io.ktor:ktor-server-core")
    implementation("io.ktor:ktor-server-netty")
    implementation("io.ktor:ktor-server-content-negotiation")
    implementation("io.ktor:ktor-serialization-kotlinx-json")
}

1. Write your server:

fun main() {
    embeddedServer(Netty, port = 8080) {
        install(ContentNegotiation) { json() }
        routing {
            get("/hello") {
                call.respond(mapOf("message" to "Hello, Ktor"))
            }
        }
    }.start(wait = true)
}

Example

// Full REST API with serialization
@Serializable
data class Task(val id: Int, val title: String, val done: Boolean)

fun Application.configureRouting() {
    val tasks = mutableListOf(
        Task(1, 'Build API', false),
        Task(2, 'Write tests', false)
    )
    routing {
        get("/tasks") { call.respond(tasks) }
        post("/tasks") {
            val task = call.receive<Task>()
            tasks.add(task)
            call.respond(HttpStatusCode.Created, task)
        }
    }
}

Related on TokRepo

• API tools — API development frameworks
• AI coding tools — development productivity resources

Common pitfalls

• Forgetting to install the ContentNegotiation plugin, which causes serialization to silently fail
• Mixing blocking I/O calls inside coroutine handlers, which blocks the event loop
• Not configuring CORS when building APIs consumed by browser frontends