kotlin-ktor-patterns

Ktor Server Patterns

Umfassende Ktor-Muster für den Bau robuster, wartbarer HTTP-Server mit Kotlin-Koroutinen.

Wann aktivieren

• Erstellen von Ktor-HTTP-Servern
• Konfigurieren von Ktor-Plugins (Auth, CORS, ContentNegotiation, StatusPages)
• Implementieren von REST-APIs mit Ktor
• Einrichten der Abhängigkeitsinjektion mit Koin
• Schreiben von Ktor-Integrationstests mit testApplication
• Arbeiten mit WebSockets in Ktor

Anwendungsstruktur

Standard-Ktor-Projektlayout

src/main/kotlin/
├── com/example/
│   ├── Application.kt           # Einstiegspunkt, Modulkonfiguration
│   ├── plugins/
│   │   ├── Routing.kt           # Routendefinitionen
│   │   ├── Serialization.kt     # Content-Negotiation-Setup
│   │   ├── Authentication.kt    # Auth-Konfiguration
│   │   ├── StatusPages.kt       # Fehlerbehandlung
│   │   └── CORS.kt              # CORS-Konfiguration
│   ├── routes/
│   │   ├── UserRoutes.kt        # /users-Endpunkte
│   │   ├── AuthRoutes.kt        # /auth-Endpunkte
│   │   └── HealthRoutes.kt      # /health-Endpunkte
│   ├── models/
│   │   ├── User.kt              # Domainmodelle
│   │   └── ApiResponse.kt       # Antwort-Hüllen
│   ├── services/
│   │   ├── UserService.kt       # Geschäftslogik
│   │   └── AuthService.kt       # Auth-Logik
│   ├── repositories/
│   │   ├── UserRepository.kt    # Datenzugriffsschnittstelle
│   │   └── ExposedUserRepository.kt
│   └── di/
│       └── AppModule.kt         # Koin-Module
src/test/kotlin/
├── com/example/
│   ├── routes/
│   │   └── UserRoutesTest.kt
│   └── services/
│       └── UserServiceTest.kt

Anwendungs-Einstiegspunkt

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

fun Application.module() {
    configureSerialization()
    configureAuthentication()
    configureStatusPages()
    configureCORS()
    configureDI()
    configureRouting()
}

Routing-DSL

Basis-Routen

// plugins/Routing.kt
fun Application.configureRouting() {
    routing {
        userRoutes()
        authRoutes()
        healthRoutes()
    }
}

// routes/UserRoutes.kt
fun Route.userRoutes() {
    val userService by inject<UserService>()

    route("/users") {
        get {
            val users = userService.getAll()
            call.respond(users)
        }

        get("/{id}") {
            val id = call.parameters["id"]
                ?: return@get call.respond(HttpStatusCode.BadRequest, "Fehlende ID")
            val user = userService.getById(id)
                ?: return@get call.respond(HttpStatusCode.NotFound)
            call.respond(user)
        }

        post {
            val request = call.receive<CreateUserRequest>()
            val user = userService.create(request)
            call.respond(HttpStatusCode.Created, user)
        }

        put("/{id}") {
            val id = call.parameters["id"]
                ?: return@put call.respond(HttpStatusCode.BadRequest, "Fehlende ID")
            val request = call.receive<UpdateUserRequest>()
            val user = userService.update(id, request)
                ?: return@put call.respond(HttpStatusCode.NotFound)
            call.respond(user)
        }

        delete("/{id}") {
            val id = call.parameters["id"]
                ?: return@delete call.respond(HttpStatusCode.BadRequest, "Fehlende ID")
            val deleted = userService.delete(id)
            if (deleted) call.respond(HttpStatusCode.NoContent)
            else call.respond(HttpStatusCode.NotFound)
        }
    }
}

Routenorganisation mit authentifizierten Routen

fun Route.userRoutes() {
    route("/users") {
        // Öffentliche Routen
        get { /* Benutzer auflisten */ }
        get("/{id}") { /* Benutzer abrufen */ }

        // Geschützte Routen
        authenticate("jwt") {
            post { /* Benutzer erstellen – erfordert Authentifizierung */ }
            put("/{id}") { /* Benutzer aktualisieren – erfordert Authentifizierung */ }
            delete("/{id}") { /* Benutzer löschen – erfordert Authentifizierung */ }
        }
    }
}

Content-Negotiation & Serialisierung

kotlinx.serialization-Setup

// plugins/Serialization.kt
fun Application.configureSerialization() {
    install(ContentNegotiation) {
        json(Json {
            prettyPrint = true
            isLenient = false
            ignoreUnknownKeys = true
            encodeDefaults = true
            explicitNulls = false
        })
    }
}

Serialisierbare Modelle

@Serializable
data class UserResponse(
    val id: String,
    val name: String,
    val email: String,
    val role: Role,
    @Serializable(with = InstantSerializer::class)
    val createdAt: Instant,
)

@Serializable
data class CreateUserRequest(
    val name: String,
    val email: String,
    val role: Role = Role.USER,
)

@Serializable
data class ApiResponse<T>(
    val success: Boolean,
    val data: T? = null,
    val error: String? = null,
) {
    companion object {
        fun <T> ok(data: T): ApiResponse<T> = ApiResponse(success = true, data = data)
        fun <T> error(message: String): ApiResponse<T> = ApiResponse(success = false, error = message)
    }
}

@Serializable
data class PaginatedResponse<T>(
    val data: List<T>,
    val total: Long,
    val page: Int,
    val limit: Int,
)

Benutzerdefinierte Serialisierer

object InstantSerializer : KSerializer<Instant> {
    override val descriptor = PrimitiveSerialDescriptor("Instant", PrimitiveKind.STRING)
    override fun serialize(encoder: Encoder, value: Instant) =
        encoder.encodeString(value.toString())
    override fun deserialize(decoder: Decoder): Instant =
        Instant.parse(decoder.decodeString())
}

Authentifizierung

JWT-Authentifizierung

// plugins/Authentication.kt
fun Application.configureAuthentication() {
    val jwtSecret = environment.config.property("jwt.secret").getString()
    val jwtIssuer = environment.config.property("jwt.issuer").getString()
    val jwtAudience = environment.config.property("jwt.audience").getString()
    val jwtRealm = environment.config.property("jwt.realm").getString()

    install(Authentication) {
        jwt("jwt") {
            realm = jwtRealm
            verifier(
                JWT.require(Algorithm.HMAC256(jwtSecret))
                    .withAudience(jwtAudience)
                    .withIssuer(jwtIssuer)
                    .build()
            )
            validate { credential ->
                if (credential.payload.audience.contains(jwtAudience)) {
                    JWTPrincipal(credential.payload)
                } else {
                    null
                }
            }
            challenge { _, _ ->
                call.respond(HttpStatusCode.Unauthorized, ApiResponse.error<Unit>("Ungültiges oder abgelaufenes Token"))
            }
        }
    }
}

// Benutzer aus JWT extrahieren
fun ApplicationCall.userId(): String =
    principal<JWTPrincipal>()
        ?.payload
        ?.getClaim("userId")
        ?.asString()
        ?: throw AuthenticationException("Keine userId im Token")

Auth-Routen

fun Route.authRoutes() {
    val authService by inject<AuthService>()

    route("/auth") {
        post("/login") {
            val request = call.receive<LoginRequest>()
            val token = authService.login(request.email, request.password)
                ?: return@post call.respond(
                    HttpStatusCode.Unauthorized,
                    ApiResponse.error<Unit>("Ungültige Anmeldedaten"),
                )
            call.respond(ApiResponse.ok(TokenResponse(token)))
        }

        post("/register") {
            val request = call.receive<RegisterRequest>()
            val user = authService.register(request)
            call.respond(HttpStatusCode.Created, ApiResponse.ok(user))
        }

        authenticate("jwt") {
            get("/me") {
                val userId = call.userId()
                val user = authService.getProfile(userId)
                call.respond(ApiResponse.ok(user))
            }
        }
    }
}

Status-Seiten (Fehlerbehandlung)

// plugins/StatusPages.kt
fun Application.configureStatusPages() {
    install(StatusPages) {
        exception<ContentTransformationException> { call, cause ->
            call.respond(
                HttpStatusCode.BadRequest,
                ApiResponse.error<Unit>("Ungültiger Anforderungstext: ${cause.message}"),
            )
        }

        exception<IllegalArgumentException> { call, cause ->
            call.respond(
                HttpStatusCode.BadRequest,
                ApiResponse.error<Unit>(cause.message ?: "Ungültige Anfrage"),
            )
        }

        exception<AuthenticationException> { call, _ ->
            call.respond(
                HttpStatusCode.Unauthorized,
                ApiResponse.error<Unit>("Authentifizierung erforderlich"),
            )
        }

        exception<AuthorizationException> { call, _ ->
            call.respond(
                HttpStatusCode.Forbidden,
                ApiResponse.error<Unit>("Zugriff verweigert"),
            )
        }

        exception<NotFoundException> { call, cause ->
            call.respond(
                HttpStatusCode.NotFound,
                ApiResponse.error<Unit>(cause.message ?: "Ressource nicht gefunden"),
            )
        }

        exception<Throwable> { call, cause ->
            call.application.log.error("Unbehandelte Ausnahme", cause)
            call.respond(
                HttpStatusCode.InternalServerError,
                ApiResponse.error<Unit>("Interner Serverfehler"),
            )
        }

        status(HttpStatusCode.NotFound) { call, status ->
            call.respond(status, ApiResponse.error<Unit>("Route nicht gefunden"))
        }
    }
}

CORS-Konfiguration

// plugins/CORS.kt
fun Application.configureCORS() {
    install(CORS) {
        allowHost("localhost:3000")
        allowHost("example.com", schemes = listOf("https"))
        allowHeader(HttpHeaders.ContentType)
        allowHeader(HttpHeaders.Authorization)
        allowMethod(HttpMethod.Put)
        allowMethod(HttpMethod.Delete)
        allowMethod(HttpMethod.Patch)
        allowCredentials = true
        maxAgeInSeconds = 3600
    }
}

Koin-Abhängigkeitsinjektion

Moduldefinition

// di/AppModule.kt
val appModule = module {
    // Datenbank
    single<Database> { DatabaseFactory.create(get()) }

    // Repositories
    single<UserRepository> { ExposedUserRepository(get()) }
    single<OrderRepository> { ExposedOrderRepository(get()) }

    // Services
    single { UserService(get()) }
    single { OrderService(get(), get()) }
    single { AuthService(get(), get()) }
}

// Anwendungssetup
fun Application.configureDI() {
    install(Koin) {
        modules(appModule)
    }
}

Koin in Routen verwenden

fun Route.userRoutes() {
    val userService by inject<UserService>()

    route("/users") {
        get {
            val users = userService.getAll()
            call.respond(ApiResponse.ok(users))
        }
    }
}

Koin für Tests

class UserServiceTest : FunSpec(), KoinTest {
    override fun extensions() = listOf(KoinExtension(testModule))

    private val testModule = module {
        single<UserRepository> { mockk() }
        single { UserService(get()) }
    }

    private val repository by inject<UserRepository>()
    private val service by inject<UserService>()

    init {
        test("getUser gibt Benutzer zurück") {
            coEvery { repository.findById("1") } returns testUser
            service.getById("1") shouldBe testUser
        }
    }
}

Anforderungsvalidierung

// Anforderungsdaten in Routen validieren
fun Route.userRoutes() {
    val userService by inject<UserService>()

    post("/users") {
        val request = call.receive<CreateUserRequest>()

        // Validieren
        require(request.name.isNotBlank()) { "Name ist erforderlich" }
        require(request.name.length <= 100) { "Name darf maximal 100 Zeichen lang sein" }
        require(request.email.matches(Regex(".+@.+\\..+"))) { "Ungültiges E-Mail-Format" }

        val user = userService.create(request)
        call.respond(HttpStatusCode.Created, ApiResponse.ok(user))
    }
}

// Oder eine Validierungserweiterung verwenden
fun CreateUserRequest.validate() {
    require(name.isNotBlank()) { "Name ist erforderlich" }
    require(name.length <= 100) { "Name darf maximal 100 Zeichen lang sein" }
    require(email.matches(Regex(".+@.+\\..+"))) { "Ungültiges E-Mail-Format" }
}

WebSockets

fun Application.configureWebSockets() {
    install(WebSockets) {
        pingPeriod = 15.seconds
        timeout = 15.seconds
        maxFrameSize = 64 * 1024 // 64 KiB – nur erhöhen, wenn Ihr Protokoll größere Frames erfordert
        masking = false // Server-zu-Client-Frames sind gemäß RFC 6455 unverdeckt; Client-zu-Server werden von Ktor immer verdeckt
    }
}

fun Route.chatRoutes() {
    val connections = Collections.synchronizedSet<Connection>(LinkedHashSet())

    webSocket("/chat") {
        val thisConnection = Connection(this)
        connections += thisConnection

        try {
            send("Verbunden! Benutzer online: ${connections.size}")

            for (frame in incoming) {
                frame as? Frame.Text ?: continue
                val text = frame.readText()
                val message = ChatMessage(thisConnection.name, text)

                // Snapshot unter Sperre, um ConcurrentModificationException zu vermeiden
                val snapshot = synchronized(connections) { connections.toList() }
                snapshot.forEach { conn ->
                    conn.session.send(Json.encodeToString(message))
                }
            }
        } catch (e: Exception) {
            logger.error("WebSocket-Fehler", e)
        } finally {
            connections -= thisConnection
        }
    }
}

data class Connection(val session: DefaultWebSocketSession) {
    val name: String = "Benutzer-${counter.getAndIncrement()}"



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