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Kotlin server and client framework · by JetBrains

Ktor

JetBrains' lightweight Kotlin framework for asynchronous servers and HTTP clients, built on coroutines, with a plugin pipeline and no imposed structure.

Visit ktor.io ↗ Alternatives Open source Open source Self-hostable
Ktor homepage
§1

Overview

Ktor is a framework for creating asynchronous client and server applications in Kotlin, from microservices to multiplatform HTTP clients. It is written in Kotlin from the ground up and uses coroutines, so asynchronous code reads like ordinary imperative code and host implementations avoid blocking threads.

It is unopinionated: Ktor does not impose choices for logging, templating, persistence, serialization or dependency injection. Features are installed as plugins through a unified interception pipeline, and routes are declared with a Kotlin DSL. Applications run standalone on Netty or Jetty or in a servlet container such as Tomcat.

Ktor apps can run in a test environment that emulates a server without real networking, which makes integration tests fast. A Gradle plugin and a project generator at start.ktor.io help set up projects.

Ktor is an official JetBrains product, used internally at JetBrains, with IDE support in IntelliJ IDEA. The repository dates from 2015 and the project is Apache-2.0 licensed.

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Pricing and plans

Free and open source under the Apache-2.0 licence. No paid edition.

Source: ktor.io/ · prices as published, excluding tax.

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What it does

Coroutines
Asynchronous I/O with Kotlin coroutines throughout.
Routing DSL
Declarative routes written in Kotlin.
Plugins
Install features through an interception pipeline; write your own easily.
Multiple engines
Netty, Jetty or Servlet 3.0+ containers.
Test host
Test applications without real networking.
Multiplatform client
The same project provides a Kotlin Multiplatform HTTP client.
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Strengths and limitations

Strengths

  • Backed and used by JetBrains, with strong IDE tooling
  • Lightweight compared with Spring Boot
  • Idiomatic Kotlin with coroutines
  • Fast in-memory testing

Limitations

  • Kotlin only; Java-first teams get little benefit
  • Unopinionated: persistence, DI and project structure are your choice and your work
  • Smaller ecosystem than Spring Boot for enterprise integrations
  • No built-in ORM or DI container; you pick and wire those libraries yourself
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Who it suits

Good for

  • Kotlin microservices and APIs
  • Teams that share Kotlin code between server and clients
  • Developers wanting a lighter JVM framework

Look elsewhere if

  • Java-only teams
  • Enterprises needing Spring's breadth of integrations
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Alternatives to Ktor

Tool From Free option Stages
Phoenix Elixir web framework
— Open source
Encore Backend framework with infrastructure from code
$49/mo Free tier
AdonisJS Batteries-included TypeScript MVC framework
— Open source
FastAPI Python API framework
$20/mo Free tier
Hono Web-standards TypeScript web framework
— Open source
ASP.NET Core .NET web framework
— Open source

All backend frameworks →

§7

Questions

Who makes Ktor?

JetBrains, the company behind Kotlin and IntelliJ IDEA.

Ktor or Spring Boot?

Ktor is lighter and Kotlin-native; Spring Boot offers a much larger ecosystem and more built-in integrations.

Is Ktor free?

Yes, it is Apache-2.0 licensed.

Can Ktor run on Tomcat?

Yes, in any Servlet 3.0+ container, or standalone on Netty or Jetty.

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