SKILL.md
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
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 modules
src/test/kotlin/
├── com/example/
│ ├── routes/
│ │ └── UserRoutesTest.kt
│ └── services/
│ └── UserServiceTest.ktApplication Entry Point
// 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
Basic Routes
// 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, "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
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
// plugins/Serialization.kt
fun Application.configureSerialization() {
install(ContentNegotiation) {
json(Json {
prettyPrint = true
isLenient = false
ignoreUnknownKeys = true
encodeDefaults = true
explicitNulls = false
})
}
}Serializable Models
@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,
)Custom Serializers
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
// 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>("Invalid or expired token"))
}
}
}
}
// Extracting user from JWT
fun ApplicationCall.userId(): String =
principal<JWTPrincipal>()
?.payload
?.getClaim("userId")
?.asString()
?: throw AuthenticationException("No userId in token")Auth Routes
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)
// plugins/StatusPages.kt
fun 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
// 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 Dependency Injection
Module Definition
// di/AppModule.kt
val 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 setup
fun Application.configureDI() {
install(Koin) {
modules(appModule)
}
}Using Koin in Routes
fun Route.userRoutes() {
val userService by inject<UserService>()
route("/users") {
get {
val users = userService.getAll()
call.respond(ApiResponse.ok(users))
}
}
}Koin for Testing
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
// Validate request data in routes
fun 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 extension
fun 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
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
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", "alice@example.com"))
}
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
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", "alice@example.com"))
}
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", "alice@example.com"))
}
response.status shouldBe HttpStatusCode.Created
}
}
})Configuration
application.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
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.