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Ryan Harrison My blog, portfolio and technology related ramblings

RESTful Kotlin with Ktor and Exposed

Updated for Ktor 2.1 and Kotlin 1.7.20+

I’ve been writing a lot more Kotlin recently and have been really liking the language so far. I’ll probably write another post pointing out some of my favourite features, but in short it’s basically Java, but without all the annoying stuff. I think in terms of adoption it’s still very early days for Kotlin, but due to the great interop with Java and being an official language for Android development, I wouldn’t be surprised if it doesn’t start to become extremely popular over the next few years.

Kotlin is pretty versatile, even though most people no doubt focus on the Android side of things. That doesn’t mean however that server side development isn’t also supported - in fact, quite the opposite. The Spring framework already has built-in support for Kotlin and many other libraries are also focusing attention on it. You could use such Java focused libraries, but instead you could use the dedicated Kotlin libraries - some of which are supported by JetBrains themselves.

All the code for the following example is available in the GitHub project kotlin-ktor-exposed-starter.

Create a Kotlin Project

First things first on our way to creating a barebones REST server in Kotlin. Open up IntelliJ and create a new Kotlin project. This will create the basic file structure and a gradle build file (in this example we will be using the Kotlin DSL). To make sure everything is working, you can run the basic Hello World:

fun main(args: Array<String>) {
    println("Hello World!")

Setting up Ktor Async Web Framework

Ktor is great library for creating simplistic and lightweight web services in Kotlin. It’s completely asynchronous through the use of coroutines and as such should scale very well with load. It’s still in active development so might have some rough edges, but on the whole it seems solid. The documentation is somewhat lacking, but has improved significantly since the 0.x days.

Add the following to build.gradle.kts to add ktor as a dependency and allow the use of kotlin coroutines (an experimental feature as of writing). We are also using the new kotlinx.serialization library instead of traditional libraries such as Jackson for slightly better performance

plugins {
    kotlin("jvm") version "1.7.20"
    kotlin("plugin.serialization") version "1.7.20"

repositories {
val ktorVersion = "2.1.2"
dependencies {
    // ommitted test dependencies

The full example Gradle file is available here. In this case, we’re making use of the Netty application server, although servlet based options are also available (although not sure why you would want to sacrifice async).

Create a Ktor application

The main configuration for a Ktor app (module) is very straightforward (MainKt file):

fun Application.module() {
    install(WebSockets) {
        contentConverter = KotlinxWebsocketSerializationConverter(JsonMapper.defaultMapper)

    install(ContentNegotiation) {

    val widgetService = WidgetService()

    install(Routing) {

fun main(args: Array<String>) {
    embeddedServer(Netty, commandLineEnvironment(args)).start(wait = true)

Here a new Ktor module is created. Ktor is configured around the concept of plugins which can be installed into the main request pipeline. In this example we add pluginsto add default headers to all our responses, log our calls for debugging and also configure processing of JSON requests and conversion to responses. Finally, the main Routing feature is used to designate which paths to handle in our app. We defer to another extension method defined elsewhere to define the routes for a widget RESTful service. To run the application, the main method starts a Netty server pointing to the module we just created.

Ktor is configuring using an application.conf similar to how Spring would use

ktor {
    deployment {
        port = 8080
        watch = [ build ]

    application {
        modules = [ MainKt.module ]

Here we provide the port Ktor should bind to and the path to the main module path which will be used for our app.

Of course for anything to actually happen, we need to define the widget routes and service themselves.

Defining Routes

Here is the definition of the widget extension method which defines the interface for our service:

fun Route.widget(widgetService: WidgetService) {

    route("/widgets") {

        get {

        get("/{id}") {
            val id = call.parameters["id"]?.toInt() ?: throw IllegalStateException("Must provide id")
            val widget = widgetService.getWidget(id)
            if (widget == null) call.respond(HttpStatusCode.NotFound)
            else call.respond(widget)

        post {
            val widget = call.receive<NewWidget>()
            call.respond(HttpStatusCode.Created, widgetService.addWidget(widget))

        put {
            val widget = call.receive<NewWidget>()
            val updated = widgetService.updateWidget(widget)
            if (updated == null) call.respond(HttpStatusCode.NotFound)
            else call.respond(HttpStatusCode.OK, updated)

        delete("/{id}") {
            val id = call.parameters["id"]?.toInt() ?: throw IllegalStateException("Must provide id")
            val removed = widgetService.deleteWidget(id)
            if (removed) call.respond(HttpStatusCode.OK)
            else call.respond(HttpStatusCode.NotFound)

As you can see the Ktor DSL is very intuitive thanks mainly to extension methods and lambda parameter syntax in Kotlin. The basic HTTP method are defined for dealing with widgets - each of which defer to our service which can do all the database access etc.

Note that the post and put methods expect an instance of the NewWidget class (as converted via JSON). This is defined as a Kotlin data class for a widget instance with an optional id:

data class NewWidget(
        val id: Int? = null,
        val name: String,
        val quantity: Int
data class Widget(
        val id: Int,
        val name: String,
        val quantity: Int,
        val dateUpdated: Long

Just like if we were using Jackson, simple data objects work well with Ktor. As we are using kotlinx.serialization we just need to add the @Serializable attribute to the compiler plugin knows to visit these classes and generate the full serializable classes.

Setting up Exposed

Exposed is another JetBrains sponsored library (though not officially) for database interactions in Kotlin. It is a kind of ORM, but unlike Hibernate it’s very simple and lightweight. In this post we’re going to use H2 as a simple in-memory database and HikariCP for connection pooling. Add the following dependencies:


Exposed has two ways of interacting with databases - their DSL and DAO. In this post I focus only on the DSL (sql builder) as I think that’s where the library excels. The DAO syntax is nice, but introduces complexity when dealing with web frameworks as you have to convert to your own model class manually. The following defines a Table for widgets:

object Widgets : Table() {
    val id = integer("id").autoIncrement()
    val name = varchar("name", 255)
    val quantity = integer("quantity")
    val dateUpdated = long("dateUpdated")
    override val primaryKey = PrimaryKey(id)

It’s quite straightforward, we just define our columns as fields and use the fluent column builder to define attributes. We can then make use of the Widgets object application wide to query the table.

Connection Pooling and Database Threads

Now we can setup a connection pool for database interaction. This example uses HikariCP as it’s the most widely used library for this at the moment:

private fun hikari(): HikariDataSource {
    val config = HikariConfig().apply {
        driverClassName = "org.h2.Driver"
        jdbcUrl = "jdbc:h2:mem:test"
        maximumPoolSize = 3
        isAutoCommit = false
        transactionIsolation = "TRANSACTION_REPEATABLE_READ"
    return HikariDataSource(config)

Now we can tell Exposed to connect to our H2 db and create the widgets table. Note: in the example project I use Flyway to perform proper database migrations for table creation. Check out the repo for details as this is beyond the scope of this post.

transaction {

A key thing to note when dealing with the async world is that you really don’t want to block any of the threads that are handling web requests. Unlike the standard servlet model where each request is tied to a thread, when you block in an async app you are essentially also blocking any other work from being done. If you do this a lot or have a spike in load, your app will grind to a halt.

This presents a problem when using standard JDBC to query our database because the framework is inherently blocking and so our threads will cease when waiting for results sets. To get around this, we must do our database queries on a dedicated thread pool. This is only really possible through coroutines which can suspend and resume as needed. The flow will be:

  1. Coroutine A starts to handle main web request from user
  2. Database query needed so another coroutine B is starting on another thread pool to perform this blocking operation
  3. A suspends execution until B is finished. Due to the nature of coroutines, the underlying thread is then free to perform other work (handling other requests)
  4. Background coroutine B finishes after database query. Thread is returned to the thread pool for other queries etc
  5. A resumes execution by restoring the previous state it had before suspension. It now has access to the query results which can be passed back as the response. Note that the coroutine A may now be executing on a different thread than in step 1 (pretty cool right?)

This might sound like a lot of work (and it is), but thanks to the coroutines library in Kotlin, this is thankfully very easy to accomplish. The following helper method, which is used across all database interaction in our service class, runs a block of code inside a transaction in this new coroutine. Dispatchers.IO references a thread pool managed by Kotlin coroutines that is meant for blocking IO operations like these. Once called, this function will suspend the current coroutine and launch a new one on the special IO thread pool - which will then block whilst the database transaction is performed. When the result is ready, the coroutine is resumed and returned to the initial caller.

suspend fun <T> dbExec(
    block: () -> T
): T = withContext(Dispatchers.IO) {
    transaction { block() }

The method is marked as suspend which will allow the suspension of A described in step 3.

Database Queries with Exposed

Finally, we need to define the WidgetService which will be making use of the database we just set up. The whole code is available in the GitHub project, but here is the method to retrieve a specific widget:

suspend fun getWidget(id: Int): Widget? = dbExec { {
        ( eq id)
    }.map { toWidget(it) }

As you can see we make use of the dbExec helper to perform our query. The Exposed DSL for queries is nice and easy to read. The result of the select is a ResultRow, so I define a helper to perform the mapping to our model class:

private fun toWidget(row: ResultRow): Widget =
        id = row[],
        name = row[],
        quantity = row[Widgets.quantity],
        dateUpdated = row[Widgets.dateUpdated]

Something like Hibernate (or the DAO in Exposed) would do this automatically, but Exposed is just a lightweight wrapper around the sql so we have full control of what’s happening. Here is the method to add and delete a widget - again fairly intuitive to read:

suspend fun addWidget(widget: NewWidget): Widget {
    var key = 0
    dbExec {
        key = (Widgets.insert {
            it[name] =
            it[quantity] = widget.quantity
            it[dateUpdated] = System.currentTimeMillis()
        } get
    return getWidget(key)!!.also {
        onChange(ChangeType.CREATE, key, it)

suspend fun deleteWidget(id: Int): Boolean {
    return dbExec {
        Widgets.deleteWhere { eq id } > 0
    }.also {
        if (it) onChange(ChangeType.DELETE, id)

And that’s it! Pretty straightforward in terms of lines of code to create a REST server with database interaction. Start the app as you would any other program (no need to deploy to any app server) and test out the widget routes.

The full example is available in the GitHub project kotlin-ktor-exposed-starter. This also contains some extra’s:

  • Database migrations using Flyway
  • Notifications with Ktor websockets
  • Unit and integration testing of our services using a fully running server and Rest Assured
  • Code coverage and reporting using Kover