Kotlin Ktor Patterns

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.kt

Application 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()

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

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

Custom Serializers

object InstantSerializer : KSerializer {
    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("Invalid or expired token"))
            }
        }
    }
}

// Extracting user from JWT
fun ApplicationCall.userId(): String =
    principal()
        ?.payload
        ?.getClaim("userId")
        ?.asString()
        ?: throw AuthenticationException("No userId in token")

Auth Routes

fun Route.authRoutes() {
    val authService by inject()

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

        post("/register") {
            val request = call.receive()
            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 { call, cause ->
            call.respond(
                HttpStatusCode.BadRequest,
                ApiResponse.error("Invalid request body: ${cause.message}"),
            )
        }

        exception { call, cause ->
            call.respond(
                HttpStatusCode.BadRequest,
                ApiResponse.error(cause.message ?: "Bad request"),
            )
        }

        exception { call, _ ->
            call.respond(
                HttpStatusCode.Unauthorized,
                ApiResponse.error("Authentication required"),
            )
        }

        exception { call, _ ->
            call.respond(
                HttpStatusCode.Forbidden,
                ApiResponse.error("Access denied"),
            )
        }

        exception { call, cause ->
            call.respond(
                HttpStatusCode.NotFound,
                ApiResponse.error(cause.message ?: "Resource not found"),
            )
        }

        exception { call, cause ->
            call.application.log.error("Unhandled exception", cause)
            call.respond(
                HttpStatusCode.InternalServerError,
                ApiResponse.error("Internal server error"),
            )
        }

        status(HttpStatusCode.NotFound) { call, status ->
            call.respond(status, ApiResponse.error("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 { DatabaseFactory.create(get()) }

    // Repositories
    single { ExposedUserRepository(get()) }
    single { 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()

    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 { mockk() }
        single { UserService(get()) }
    }

    private val repository by inject()
    private val service by inject()

    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()

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

        // Validate
        require(request.name.isNotBlank()) { "Name is required" }
        require(request.name.length (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>>()
            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: ${J

…

## Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

- **Author:** [affaan-m](https://github.com/affaan-m)
- **Source:** [affaan-m/ECC](https://github.com/affaan-m/ECC)
- **License:** MIT
- **Homepage:** https://ecc.tools

Install and usage instructions live in the source repository linked above.