Ktor - A Kotlin Web Framework

Ktor is an asynchronous web framework written in and designed for Kotlin, leveraging coroutines and allowing you to write asynchronous code without having to manage any threads yourself.

Here is a bit more background information on Ktor. It is backed by Jetbrains, who are also the creators of Kotlin itself. Who better to make a Kotlin web framework than the people that work on the language?

Implementation

Dependencies

buildscript {
  ext.kotlin_version = '1.7.10'
  ext.ktor_version = '2.0.3'

  repositories {
    mavenCentral()
  }
  dependencies {
    classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
  }
}

apply plugin: 'java'
apply plugin: 'kotlin'

group 'ktor-and-kodein-di'
version '1.0.0'

repositories {
  mavenLocal()
  mavenCentral()
}

dependencies {
  implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"
  testImplementation "org.jetbrains.kotlin:kotlin-test:$kotlin_version"
  testImplementation "junit:junit:4.12"

  implementation "io.ktor:ktor-server-netty:$ktor_version"
  // each plugin is its own dependency which matches the name in code (default-headers = DefaultHeaders)
  implementation "io.ktor:ktor-serialization-jackson:$ktor_version"
  implementation "io.ktor:ktor-server-default-headers:$ktor_version"
  implementation "io.ktor:ktor-server-call-logging:$ktor_version"
  implementation "io.ktor:ktor-server-content-negotiation:$ktor_version"

  implementation group: 'org.kodein.di', name: 'kodein-di-generic-jvm', version: '6.5.1'
  implementation group: 'ch.qos.logback', name: 'logback-classic', version: '1.2.3'
  implementation group: 'com.datastax.oss', name: 'java-driver-core', version: '4.14.1'
}

• Ktor 2.0.3 uses a minimum version of Kotlin 1.7 from what I’ve determined from their documentation.
• Dependencies on ktor-server-netty and several ktor-server plugins are brought in. As ktor-server-netty suggests, Netty will be used for this post. Depending on which you choose to import, different underlying web servers can be used. The other available options are Netty, Jetty, Tomcat, and CIO. More information can be found in the supported engine documentation.
• Logback is brought in to handle logging. This is not included in the Ktor dependencies and is needed if you plan on doing any sort of logging.
• Kodein is a dependency injection framework written in Kotlin. I have used it loosely in this post, and due to the size of the code examples, I could probably remove it altogether.

Starting the Web Server

With the boring stuff out of the way, I can now run you through implementing a web server. The code below is all you need:

import io.ktor.server.engine.embeddedServer
import io.ktor.server.netty.Netty

fun main() {
  embeddedServer(Netty, port = 8080, module = Application::module).start()
}

fun Application.module() {
  // code that does stuff which is covered later
}

The code is pretty self-explanatory. The only piece worth highlighting is the Application.module function. The module parameter embeddedServer requires an Application.() -> Unit function to be provided that configures the server and will be the main entry point into the server code.

In the following sections, we will expand the contents Application.module so that your web server actually does something worthwhile.

Routing

All incoming requests will be rejected at the moment since there are no endpoints to handle them. By setting up the routing, you can specify valid paths that requests can travel along and the functions that will process the requests when they reach their destinations.

This is done inside of a Routing block (or multiple Routing blocks). Inside of a block, routes to different endpoints are set up:

import io.ktor.http.HttpStatusCode
import io.ktor.server.application.call
import io.ktor.server.request.receive
import io.ktor.server.response.respond
import io.ktor.server.response.respondText
import io.ktor.server.routing.Routing
import io.ktor.server.routing.get
import io.ktor.server.routing.post
import io.ktor.server.routing.route

routing {
  // All routes defined inside are prefixed with "/people"
  route("/people") {
    // Get a person
    get("/{id}") {
      val id = UUID.fromString(call.parameters["id"]!!)
      personRepository.find(id)?.let {
        call.respond(HttpStatusCode.OK, it)
      } ?: call.respondText(status = HttpStatusCode.NotFound) { "There is no record with id: $id" }
    }
    // Create a person
    post {
      val person = call.receive<Person>()
      val result = personRepository.save(person.copy(id = UUID.randomUUID()))
      call.respond(result)
    }
  }
}

The imports have been included due to the reliance on extension functions, making discovering functions without an IDE difficult.

routing is a convenience function to make the code flow smoothly. The context (a.k.a this) inside of routing is of type Routing. Furthermore, the functions route, get, and post are all extension functions of Routing.

route sets a base path to all its following endpoints. In this scenario, /people. get and post do not specify a path themselves since the base path is sufficient for their needs. If desired, a path could be added to each one, for example:

routing {
  // Get a person
  get("/people/{id}") {
    val id = UUID.fromString(call.parameters["id"]!!)
    personRepository.find(id)?.let {
      call.respond(HttpStatusCode.OK, it)
    } ?: call.respondText(status = HttpStatusCode.NotFound) { "There is no record with id: $id" }
  }
  // Create a person
  post("/people) {
    val person = call.receive<Person>()
    val result = personRepository.save(person.copy(id = UUID.randomUUID()))
    call.respond(result)
  }
}

fun Application.module() {
  val personRepository by kodein.instance<PersonRepository>()
  // Route requests to handler functions
  routing { people(personRepository) }
}

// Extracted to a separate extension function to tidy up the code
fun Routing.people(personRepository: PersonRepository) {
  route("/people") {
    // Get a person
    get("/{id}") {
      val id = UUID.fromString(call.parameters["id"]!!)
      personRepository.find(id)?.let {
        call.respond(HttpStatusCode.OK, it)
      } ?: call.respondText(status = HttpStatusCode.NotFound) { "There is no record with id: $id" }
    }
    // Create a person
    post {
      val person = call.receive<Person>()
      val result = personRepository.save(person.copy(id = UUID.randomUUID()))
      call.respond(result)
    }
  }
}

Contents of a request handler

The code that executes when a request is routed to a request handler is obviously pretty important. The function needs to do something after all…

Each handler function executes within the context of a coroutine. I did not really use this fact since each of the functions I have shown is fully synchronous.

For the remainder of this post, I will try not to mention coroutines too much since they are not particularly important for this simple REST API.

In this section, the get function will be examined a little closer:

get("/{id}") {
  val id = UUID.fromString(call.parameters["id"]!!)
  personRepository.find(id)?.let {
    call.respond(HttpStatusCode.OK, it)
  } ?: call.respondText(status = HttpStatusCode.NotFound) { "There is no record with id: $id" }
}

• call represents the context of the current request.
• parameters is a list of the request’s parameters.

The database searches for the corresponding record using  id from the path variables. In this scenario, if it exists, the record is returned along with the appropriate 200 OK. If it doesn’t, an error response is returned. Both respond and respondText alter the underlying response of the current call. You could do this manually, for example, by using:

call.response.status(HttpStatusCode.OK)
call.response.pipeline.execute(call, it)

Installing Plugins

In Ktor, plugins can be installed when needed. For example, Jackson JSON parsing can be added to handle and return JSON from your application.

Below are the plugins installed to the example application:

import io.ktor.http.HttpHeaders
import io.ktor.serialization.jackson.jackson
import io.ktor.server.application.install
import io.ktor.server.plugins.callloging.CallLogging
import io.ktor.server.plugins.contentnegotiation.ContentNegotiation
import io.ktor.server.plugins.defaultheaders.DefaultHeaders
import org.slf4j.event.Level

fun Application.module() {
  // Adds header to every response
  install(DefaultHeaders) { header(HttpHeaders.Server, "My ktor server") }
  // Controls what level the call logging is logged to
  install(CallLogging) { level = Level.INFO }
  // Setup jackson json serialisation
  install(ContentNegotiation) { jackson() }
}

• CallLogging logs information about outgoing responses and specifies what level to log them at. A logging library needs to be included for this to work. The output will look something like this:

  INFO  ktor.application.log - 200 OK: GET - /people/302a1a73-173b-491c-b306-4d95387a8e36

• ContentNegotiation tells the server to use Jackson for incoming and outbound requests. Remember this required including ktor-serialization-jackson as a dependency. You could also use GSON if you prefer.

For a list of the other plugins that Ktor includes, you can go to start.ktor.io, where you can view the existing plugins (by pretending to create a new application).

Installing plugins ties all the way back to the routing done earlier. routing delegates down to install inside its implementation. So you could write:

install(Routing) {
  route("/people") {
    get {
      // Implementation
    }
  }
}

Brief Mention for Kodein

I want to have a very brief look at Kodein since I used it in this post. Kodein is a dependency injection framework written in Kotlin, for Kotlin. Below is the super small amount of DI that I used for the example application:

val kodein = Kodein {
  bind<CqlSession>() with singleton { cassandraSession() }
  bind<PersonRepository>() with singleton { PersonRepository(instance()) }
}
val personRepository by kodein.instance<PersonRepository>()

Outside of the Kodein block, an instance PersonRespository is retrieved.

Yeah, I know; there isn’t much point to using Kodein here since I could have replaced it with a single line…

val personRepository = PersonRepository(cassandraSession())

Summary

In this post, we looked at initializing a web server using Ktor, routing requests to lambdas/handlers that generate responses, and installing plugins to the server. We mainly stayed at the surface level in this post and focused on the foundational knowledge to get you up and running with Ktor. For more information, it is worth going to ktor.io and viewing Ktor’s documentation and samples.