Migrating from 2.2.x to 3.0.x

This guide provides instructions on how to migrate your Ktor application from the 2.2.x version to 3.0.x.

Ktor Server

ApplicationEngine, ApplicationEnvironment, and Application

Several design changes have been introduced to improve configurability and provide a more defined separation between the ApplicationEngine, ApplicationEnvironment and Application instances.

Before v3.0.0, ApplicationEngine managed ApplicationEnvironment, which in turn managed Application.

In the current design, however, Application is responsible for creating, owning, and initiating both ApplicationEngine and ApplicationEnvironment.

This restructuring comes with the following set of breaking changes:

ApplicationEngineEnvironmentBuilder and applicationEngineEnvironment classes are renamed..
start() and stop() methods are removed from ApplicationEngineEnvironment.
Introduction of ApplicationPropertiesBuilder.
embeddedServer() returnsEmbeddedServer instead of ApplicationEngine.

These changes will impact existing code that relies on the previous model.

Renamed classes

Package	2.x.x	3.0.x
`io.ktor:ktor-server-core`	`ApplicationEngineEnvironmentBuilder`	`ApplicationEnvironmentBuilder`
`io.ktor:ktor-server-core`	`applicationEngineEnvironment`	`applicationEnvironment`

start() and stop() methods are removed from ApplicationEngineEnvironment

With the merge of AplicationEngineEnvironment to ApplicationEnvironment, the start() and stop() methods are now only accessible through ApplicationEngine.

2.x.x	3.0.x
`ApplicationEngineEnvironment.start()`	`ApplicationEngine.start()`
`ApplicationEngineEnvironment.stop()`	`ApplicationEngine.stop()`

Additionally, in the following table you can see the list of removed properties and their current corresponding ownership:

2.x.x	3.0.x
`ApplicationEngineEnvironment.connectors`	`ApplciationEngine.Configuration.connectors`
`ApplicationEnvironment.developmentMode`	`Application.developmentMode`
`ApplicationEnvironment.monitor`	`Application.monitor`
`ApplicationEnvironment.parentCoroutineContext`	`Application.parentCoroutineContext`
`ApplicationEnvironment.rootPath`	`Application.rootPath`

The ownership changes can be illustrated through the following example:

import io.ktor.server.application.*
import io.ktor.server.cio.*
import io.ktor.server.engine.*
import org.slf4j.helpers.NOPLogger

fun defaultServer(module: Application.() -> Unit) =
  embeddedServer(CIO,
    environment = applicationEngineEnvironment {
      log = NOPLogger.NOP_LOGGER
      connector { 
          port = 8080
      }
      module(module)
    }
  )

import io.ktor.server.application.*
import io.ktor.server.cio.*
import io.ktor.server.engine.*
import org.slf4j.helpers.NOPLogger

fun defaultServer(module: Application.() -> Unit) =
  embeddedServer(CIO,
    environment = applicationEnvironment { log = NOPLogger.NOP_LOGGER },
    configure = {
      connector {
          port = 8080
      }
    },
    module
  )

Introduction of ApplicationPropertiesBuilder

A new entity, ApplicationPropertiesBuilder, is introduced for configuring Application properties represented by the ApplicationProperties class. The class contains properties, previously available in ApplicationEnvironment.

Introduction of EmbeddedServer

The class EmbeddedServer is introduced and used to replace ApplicationEngine as a return type of the embeddedServer() function.

For more details about the model change, see issue KTOR-3857 on YouTrack.

ktor-server-host-common module has been removed

Due to Application requiring knowledge of ApplicationEngine, the contents of ktor-server-host-common module have been merged into ktor-server-core, namely the io.ktor.server.engine package.

Ensure that your dependencies are updated accordingly. In most cases, you can simply remove the ktor-server-host-common dependency.

Locations plugin has been removed

The Locations plugin for the Ktor server has been removed. To create type-safe routing, use the Resources plugin instead. This requires the following changes:

Replace the io.ktor:ktor-server-locations artifact with io.ktor:ktor-server-resources.
The Resources plugin depends on the Kotlin serialization plugin. To add the serialization plugin, see the kotlinx.serialization setup.
Update the plugin import from io.ktor.server.locations.* to io.ktor.server.resources.*.
Additionally, import the Resource module from io.ktor.resources.

The following example shows how to implement these changes:

import io.ktor.server.locations.*

@Location("/articles")
class article(val value: Int)

fun Application.module() {
    install(Locations)
    routing {
        get<article> {
            // Get all articles ...
            call.respondText("List of articles")
        }
    }
}

import io.ktor.resources.Resource
import io.ktor.server.resources.*

@Resource("/articles")
class Articles(val value: Int)

fun Application.module() {
    install(Resources)
    routing {
        get<Articles> {
            // Get all articles ...
            call.respondText("List of articles")
        }
    }
}

For more information on working with Resources, refer to Type-safe routing.

Session encryption method update

The encryption method offered by the Sessions plugin has been updated to enhance security.

Specifically, the SessionTransportTransformerEncrypt method, which previously derived the MAC from the decrypted session value, now computes it from the encrypted value.

To ensure compatibility with existing sessions, Ktor has introduced the backwardCompatibleRead property. For current configurations, include the property in the constructor of SessionTransportTransformerEncrypt:

install(Sessions) {
  cookie<UserSession>("user_session") {
    // ...
    transform(
      SessionTransportTransformerEncrypt(
        secretEncryptKey, // your encrypt key here
        secretSignKey, // your sign key here
        backwardCompatibleRead = true
      )
    )
  }
}

For more information on session encryption in Ktor, see Sign and encrypt session data.

Last modified: 30 April 2024