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Version: 4.2

Ktor DI Bridge

Ktor 3.4 ships its own dependency injection plugin (dependencies { provide<T> { } } and by dependencies). This page shows how to connect it to Koin with the bridge { } option of the Koin plugin, in one or both directions. You need the Koin plugin installed as in Koin for Ktor.

The bridge is available since Koin 4.2. Its functions are marked @KoinExperimentalAPI.

Configuring the bridge​

fun Application.module() {
    install(Koin) {
        slf4jLogger()
        bridge {
            ktorToKoin()
            koinToKtor()
        }
        modules(appModule)
    }

    dependencies {
        provide<KtorSpecificService> { KtorSpecificServiceImpl() }
    }
}
OptionEffect
ktorToKoin()Ktor's by dependencies can resolve Koin definitions
koinToKtor()Koin's get(), inject() and constructor injection can resolve Ktor DI dependencies

Enable only the direction you use.

Ktor resolving from Koin​

With ktorToKoin(), the plugin installs Ktor's DI plugin with a dependency map backed by Koin:

fun Application.module() {
    install(Koin) {
        bridge { ktorToKoin() }
        modules(module {
            single<HelloService> { HelloServiceImpl() }
        })
    }

    routing {
        get("/hello") {
            val helloService: HelloService by dependencies // declared in Koin
            call.respondText(helloService.sayHello())
        }
    }
}

A Ktor dependency key with a name maps to a Koin named() qualifier with the same value.

Koin resolving from Ktor DI​

With koinToKtor(), Koin adds a resolution extension: when a type has no Koin definition, Koin asks Ktor DI for it.

fun Application.module() {
    install(Koin) {
        bridge { koinToKtor() }
        modules(appModule)
    }

    dependencies {
        provide<DatabaseConnection> { DatabaseConnectionImpl() }
    }

    routing {
        get("/data") {
            val database: DatabaseConnection by inject() // declared in Ktor DI
            call.respondText(database.query())
        }
    }
}

Koin definitions can also take Ktor DI types as constructor parameters. A Koin qualifier is passed to Ktor as the dependency name. If Ktor DI has no such dependency either, Koin logs a warning and the resolution fails as for any missing definition.

Splitting infrastructure and application code​

A common split keeps framework-level objects in Ktor DI and application classes in Koin, with koinToKtor() only:

fun Application.module() {
    // Ktor DI: infrastructure
    val database = Database(environment.config)
    dependencies {
        provide<Database> { database }
        provide<ApplicationConfig> { environment.config }
    }

    // Koin: application classes
    install(Koin) {
        slf4jLogger()
        bridge { koinToKtor() }
        modules(appModule)
    }
}

val appModule = module {
    singleOf(::CustomerRepository) // receives Database from Ktor DI
    singleOf(::CustomerService)
}

Limits​

  • Compile-time checks: the Koin Compiler Plugin does not see Ktor DI declarations. If compile-time safety reports a Ktor DI type as missing, mark it @Provided, as described in Compile-Time Safety.
  • KoinIsolated: in Koin 4.2, the KoinIsolated plugin accepts the bridge { } block but does not apply it. Use install(Koin) when you need the bridge.
  • Experimental: the bridge API may change in a future release.