Create a custom ApplicationEngine

Estimated reading time: 3 minutes

Ktor’s HTTP client and server provide a common interface while allowing to use several different engines to perform or to handle HTTP requests.

Ktor includes several artifacts with several engines:

  • For the server: Netty, Jetty, Tomcat, CIO, TestEngine.
  • For the client: ApacheEngine, JettyHttp2Engine, CIOEngine, TestHttpClientEngine.

Server’s ApplicationEngine


A simplified version of the ApplicationEngine looks like this:

interface ApplicationEngineFactory<
    out TEngine : ApplicationEngine,
    TConfiguration : ApplicationEngine.Configuration
> {
    fun create(environment: ApplicationEngineEnvironment, configure: TConfiguration.() -> Unit): TEngine

interface ApplicationEngine {
    val environment: ApplicationEngineEnvironment
    fun start(wait: Boolean = false): ApplicationEngine
    fun stop(gracePeriod: Long, timeout: Long, timeUnit: TimeUnit)

    open class Configuration {
        val parallelism: Int
        var connectionGroupSize: Int
        var workerGroupSize: Int
        var callGroupSize: Int

interface ApplicationEngineEnvironment {
    val connectors: List<EngineConnectorConfig>
    val application: Application
    fun start()
    fun stop()

    val classLoader: ClassLoader
    val log: Logger
    val config: ApplicationConfig
    val monitor: ApplicationEvents

interface EngineConnectorConfig {
    val type: ConnectorType
    val host: String
    val port: Int

data class ConnectorType(val name: String) {
    companion object {
        val HTTP = ConnectorType("HTTP")
        val HTTPS = ConnectorType("HTTPS")

abstract class BaseApplicationEngine(
    final override val environment: ApplicationEngineEnvironment,
    val pipeline: EnginePipeline = defaultEnginePipeline(environment)
) : ApplicationEngine {
    val application: Application

The ApplicationEngineFactory

Each implementation of the ApplicationEngineFactory along the a subtyped ApplicationEngine.Configuration are the public exposed APIs for each engine.

fun ApplicationEngineFactory.create(environment: ApplicationEngineEnvironment, configure: TConfiguration.() -> Unit): TEngine

The ApplicationEngineFactory.create is in charge of instantiating the right subtyped ApplicationEngine.Configuration, and call the provided configure: TConfiguration.() -> Unit lambda that should mutate the configuration object, and constructs an implementation of the ApplicationEngine probably subtype of BaseApplicationEngine.

For example:

class MyApplicationEngineFactory
    <MyApplicationEngine, MyApplicationEngineConfiguration>

    fun create(
        environment: ApplicationEngineEnvironment,
        configure: MyApplicationEngineConfiguration.() -> Unit
    ): MyApplicationEngine {
        val configuration = MyApplicationEngineConfiguration()
        return MyApplicationEngine(environment, configuration)

The ApplicationEngine and BaseApplicationEngine

The interface ApplicationEngine, with an abstract implementation BaseApplicationEngine is the one in charge of starting and stopping the application. It holds the ApplicationEngineEnvironment and the constructed configuration of the application.

This class has two methods:

  • The start method: it should connect to the ApplicationEngineEnvironment.connectors that come from the environment, start the environment, and to start and configure the engine to trigger an execute of the application pipeline for each HTTP request with an ApplicationCall.
  • The stop method: should stop the engine, the environment and to unregister all the things registered by the start method.

The BaseApplicationEngine exposes an ApplicationEngineEnvironment passed to the constructor, while creates an EnginePipeline used as intermediary to pre-intercept the application pipeline. It also installs default transformations the the receive and end pipelines and logs the defined connection endpoints.

For example:

class MyApplicationEngine(
    environment: ApplicationEngineEnvironment,
    configuration: MyApplicationEngineConfiguration
) : BaseApplicationEngine(environment) {
    val myEngine = MyEngine()

    override fun start(wait: Boolean): MyApplicationEngine {
        return this
    override fun stop(gracePeriod: Long, timeout: Long, timeUnit: TimeUnit) {
    private fun handleRequest(request: MyEngineCall) {
        val call: ApplicationCall = request.toApplicationCall()