Ktor Server Patterns
Comprehensive Ktor patterns for building robust, maintainable HTTP servers with Kotlin coroutines.
When to Activate
- Building Ktor HTTP servers
- Configuring Ktor plugins (Auth, CORS, ContentNegotiation, StatusPages)
- Implementing REST APIs with Ktor
- Setting up dependency injection with Koin
- Writing Ktor integration tests with testApplication
- Working with WebSockets in Ktor
Application Structure
Standard Ktor Project Layout
text
src/main/kotlin/├── com/example/│ ├── Application.kt # Entry point, module configuration│ ├── plugins/│ │ ├── Routing.kt # Route definitions│ │ ├── Serialization.kt # Content negotiation setup│ │ ├── Authentication.kt # Auth configuration│ │ ├── StatusPages.kt # Error handling│ │ └── CORS.kt # CORS configuration│ ├── routes/│ │ ├── UserRoutes.kt # /users endpoints│ │ ├── AuthRoutes.kt # /auth endpoints│ │ └── HealthRoutes.kt # /health endpoints│ ├── models/│ │ ├── User.kt # Domain models│ │ └── ApiResponse.kt # Response envelopes│ ├── services/│ │ ├── UserService.kt # Business logic│ │ └── AuthService.kt # Auth logic│ ├── repositories/│ │ ├── UserRepository.kt # Data access interface│ │ └── ExposedUserRepository.kt│ └── di/│ └── AppModule.kt # Koin modulessrc/test/kotlin/├── com/example/│ ├── routes/│ │ └── UserRoutesTest.kt│ └── services/│ └── UserServiceTest.ktApplication Entry Point
kotlin
// Application.ktfun main() { embeddedServer(Netty, port = 8080, module = Application::module).start(wait = true)}
fun Application.module() { configureSerialization() configureAuthentication() configureStatusPages() configureCORS() configureDI() configureRouting()}Routing DSL
Basic Routes
kotlin
// plugins/Routing.ktfun Application.configureRouting() { routing { userRoutes() authRoutes() healthRoutes() }}
// routes/UserRoutes.ktfun 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, "Missing 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, "Missing 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, "Missing id") val deleted = userService.delete(id) if (deleted) call.respond(HttpStatusCode.NoContent) else call.respond(HttpStatusCode.NotFound) } }}Route Organization with Authenticated Routes
kotlin
fun Route.userRoutes() { route("/users") { // Public routes get { /* list users */ } get("/{id}") { /* get user */ }
// Protected routes authenticate("jwt") { post { /* create user - requires auth */ } put("/{id}") { /* update user - requires auth */ } delete("/{id}") { /* delete user - requires auth */ } } }}Content Negotiation & Serialization
kotlinx.serialization Setup
kotlin
// plugins/Serialization.ktfun Application.configureSerialization() { install(ContentNegotiation) { json(Json { prettyPrint = true isLenient = false ignoreUnknownKeys = true encodeDefaults = true explicitNulls = false }) }}Serializable Models
kotlin
@Serializabledata class UserResponse( val id: String, val name: String, val email: String, val role: Role, @Serializable(with = InstantSerializer::class) val createdAt: Instant,)
@Serializabledata class CreateUserRequest( val name: String, val email: String, val role: Role = Role.USER,)
@Serializabledata 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) }}
@Serializabledata class PaginatedResponse<T>( val data: List<T>, val total: Long, val page: Int, val limit: Int,)Custom Serializers
kotlin
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())}Authentication
JWT Authentication
kotlin
// plugins/Authentication.ktfun 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>("Invalid or expired token")) } } }}
// Extracting user from JWTfun ApplicationCall.userId(): String = principal<JWTPrincipal>() ?.payload ?.getClaim("userId") ?.asString() ?: throw AuthenticationException("No userId in token")Auth Routes
kotlin
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>("Invalid credentials"), ) 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 Pages (Error Handling)
kotlin
// plugins/StatusPages.ktfun Application.configureStatusPages() { install(StatusPages) { exception<ContentTransformationException> { call, cause -> call.respond( HttpStatusCode.BadRequest, ApiResponse.error<Unit>("Invalid request body: ${cause.message}"), ) }
exception<IllegalArgumentException> { call, cause -> call.respond( HttpStatusCode.BadRequest, ApiResponse.error<Unit>(cause.message ?: "Bad request"), ) }
exception<AuthenticationException> { call, _ -> call.respond( HttpStatusCode.Unauthorized, ApiResponse.error<Unit>("Authentication required"), ) }
exception<AuthorizationException> { call, _ -> call.respond( HttpStatusCode.Forbidden, ApiResponse.error<Unit>("Access denied"), ) }
exception<NotFoundException> { call, cause -> call.respond( HttpStatusCode.NotFound, ApiResponse.error<Unit>(cause.message ?: "Resource not found"), ) }
exception<Throwable> { call, cause -> call.application.log.error("Unhandled exception", cause) call.respond( HttpStatusCode.InternalServerError, ApiResponse.error<Unit>("Internal server error"), ) }
status(HttpStatusCode.NotFound) { call, status -> call.respond(status, ApiResponse.error<Unit>("Route not found")) } }}CORS Configuration
kotlin
// plugins/CORS.ktfun 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 Dependency Injection
Module Definition
kotlin
// di/AppModule.ktval appModule = module { // Database 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()) }}
// Application setupfun Application.configureDI() { install(Koin) { modules(appModule) }}Using Koin in Routes
kotlin
fun Route.userRoutes() { val userService by inject<UserService>()
route("/users") { get { val users = userService.getAll() call.respond(ApiResponse.ok(users)) } }}Koin for Testing
kotlin
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 returns user") { coEvery { repository.findById("1") } returns testUser service.getById("1") shouldBe testUser } }}Request Validation
kotlin
// Validate request data in routesfun Route.userRoutes() { val userService by inject<UserService>()
post("/users") { val request = call.receive<CreateUserRequest>()
// Validate require(request.name.isNotBlank()) { "Name is required" } require(request.name.length <= 100) { "Name must be 100 characters or less" } require(request.email.matches(Regex(".+@.+\\..+"))) { "Invalid email format" }
val user = userService.create(request) call.respond(HttpStatusCode.Created, ApiResponse.ok(user)) }}
// Or use a validation extensionfun CreateUserRequest.validate() { require(name.isNotBlank()) { "Name is required" } require(name.length <= 100) { "Name must be 100 characters or less" } require(email.matches(Regex(".+@.+\\..+"))) { "Invalid email format" }}WebSockets
kotlin
fun Application.configureWebSockets() { install(WebSockets) { pingPeriod = 15.seconds timeout = 15.seconds maxFrameSize = 64 * 1024 // 64 KiB — increase only if your protocol requires larger frames masking = false // Server-to-client frames are unmasked per RFC 6455; client-to-server are always masked by Ktor }}
fun Route.chatRoutes() { val connections = Collections.synchronizedSet<Connection>(LinkedHashSet())
webSocket("/chat") { val thisConnection = Connection(this) connections += thisConnection
try { send("Connected! Users online: ${connections.size}")
for (frame in incoming) { frame as? Frame.Text ?: continue val text = frame.readText() val message = ChatMessage(thisConnection.name, text)
// Snapshot under lock to avoid ConcurrentModificationException val snapshot = synchronized(connections) { connections.toList() } snapshot.forEach { conn -> conn.session.send(Json.encodeToString(message)) } } } catch (e: Exception) { logger.error("WebSocket error", e) } finally { connections -= thisConnection } }}
data class Connection(val session: DefaultWebSocketSession) { val name: String = "User-${counter.getAndIncrement()}"
companion object { private val counter = AtomicInteger(0) }}testApplication Testing
Basic Route Testing
kotlin
class UserRoutesTest : FunSpec({ test("GET /users returns list of users") { testApplication { application { install(Koin) { modules(testModule) } configureSerialization() configureRouting() }
val response = client.get("/users")
response.status shouldBe HttpStatusCode.OK val body = response.body<ApiResponse<List<UserResponse>>>() body.success shouldBe true body.data.shouldNotBeNull().shouldNotBeEmpty() } }
test("POST /users creates a user") { testApplication { application { install(Koin) { modules(testModule) } configureSerialization() configureStatusPages() configureRouting() }
val client = createClient { install(io.ktor.client.plugins.contentnegotiation.ContentNegotiation) { json() } }
val response = client.post("/users") { contentType(ContentType.Application.Json) setBody(CreateUserRequest("Alice", "[email protected]")) }
response.status shouldBe HttpStatusCode.Created } }
test("GET /users/{id} returns 404 for unknown id") { testApplication { application { install(Koin) { modules(testModule) } configureSerialization() configureStatusPages() configureRouting() }
val response = client.get("/users/unknown-id")
response.status shouldBe HttpStatusCode.NotFound } }})Testing Authenticated Routes
kotlin
class AuthenticatedRoutesTest : FunSpec({ test("protected route requires JWT") { testApplication { application { install(Koin) { modules(testModule) } configureSerialization() configureAuthentication() configureRouting() }
val response = client.post("/users") { contentType(ContentType.Application.Json) setBody(CreateUserRequest("Alice", "[email protected]")) }
response.status shouldBe HttpStatusCode.Unauthorized } }
test("protected route succeeds with valid JWT") { testApplication { application { install(Koin) { modules(testModule) } configureSerialization() configureAuthentication() configureRouting() }
val token = generateTestJWT(userId = "test-user")
val client = createClient { install(io.ktor.client.plugins.contentnegotiation.ContentNegotiation) { json() } }
val response = client.post("/users") { contentType(ContentType.Application.Json) bearerAuth(token) setBody(CreateUserRequest("Alice", "[email protected]")) }
response.status shouldBe HttpStatusCode.Created } }})Configuration
application.yaml
yaml
ktor: application: modules: - com.example.ApplicationKt.module deployment: port: 8080
jwt: secret: ${JWT_SECRET} issuer: "https://example.com" audience: "https://example.com/api" realm: "example"
database: url: ${DATABASE_URL} driver: "org.postgresql.Driver" maxPoolSize: 10Reading Config
kotlin
fun Application.configureDI() { val dbUrl = environment.config.property("database.url").getString() val dbDriver = environment.config.property("database.driver").getString() val maxPoolSize = environment.config.property("database.maxPoolSize").getString().toInt()
install(Koin) { modules(module { single { DatabaseConfig(dbUrl, dbDriver, maxPoolSize) } single { DatabaseFactory.create(get()) } }) }}Quick Reference: Ktor Patterns
| Pattern | Description |
|---|---|
route("/path") { get { } } | Route grouping with DSL |
call.receive<T>() | Deserialize request body |
call.respond(status, body) | Send response with status |
call.parameters["id"] | Read path parameters |
call.request.queryParameters["q"] | Read query parameters |
install(Plugin) { } | Install and configure plugin |
authenticate("name") { } | Protect routes with auth |
by inject<T>() | Koin dependency injection |
testApplication { } | Integration testing |
Remember: Ktor is designed around Kotlin coroutines and DSLs. Keep routes thin, push logic to services, and use Koin for dependency injection. Test with testApplication for full integration coverage.
