Cursor rules for Kotlin development with Ktor integration

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Cursor rules for Kotlin development with Ktor integration.

Source

Synced from https://github.com/PatrickJS/awesome-cursorrules/tree/main/rules/kotlin-ktor-development-cursorrules-prompt-file.mdc.

Instruction to developer: save this file as .cursorrules and place it on the root project directory

Core Principles

• Follow SOLID, DRY, KISS, and YAGNI principles
• Adhere to OWASP security best practices
• Break tasks into smallest units and solve problems step-by-step

Technology Stack

• Framework: Kotlin Ktor with Kotlin 2.1.20+
• JDK: 21 (LTS)
• Build: Gradle with Kotlin DSL
• Dependencies: Ktor Server Core/Netty, kotlinx.serialization, Exposed, HikariCP, kotlin-logging, Koin, Kotest

Application Structure (Feature-Based)

• Organize by business features, not technical layers
• Each feature is self-contained with all related components
• Promotes modularity, reusability, and better team collaboration
• Makes codebase easier to navigate and maintain
• Enables parallel development on different features

src/main/kotlin/com/company/app/
├── common/              # Shared utilities, extensions
├── config/              # Application configuration, DI
└── features/
    ├── auth/            # Feature directory
    │   ├── models/
    │   ├── repositories/
    │   ├── services/
    │   └── routes/
    └── users/           # Another feature
        ├── ...

Test structure mirrors the feature-based organization:

src/test/kotlin/com/company/app/
├── common/
└── features/
    ├── auth/
    │   ├── models/
    │   ├── repositories/
    │   ├── services/
    │   └── routes/
    └── users/
        ├── ...

Application Logic Design

1. Route handlers: Handle requests/responses only
2. Services: Contain business logic, call repositories
3. Repositories: Handle database operations
4. Entity classes: Data classes for database models
5. DTOs: Data transfer between layers

Entities & Data Classes

• Use Kotlin data classes with proper validation
• Define Table objects when using Exposed ORM
• Use UUID or auto-incrementing integers for IDs

Repository Pattern

kotlin
interface UserRepository {
    suspend fun findById(id: UUID): UserDTO?
    suspend fun create(user: CreateUserRequest): UserDTO
    suspend fun update(id: UUID, user: UpdateUserRequest): UserDTO?
    suspend fun delete(id: UUID): Boolean
}

class UserRepositoryImpl : UserRepository {
    override suspend fun findById(id: UUID): UserDTO? = withContext(Dispatchers.IO) {
        transaction {
            Users.select { Users.id eq id }
                .mapNotNull { it.toUserDTO() }
                .singleOrNull()
        }
    }
    // Other implementations...
}

Service Layer

kotlin
interface UserService {
    suspend fun getUserById(id: UUID): UserDTO
    suspend fun createUser(request: CreateUserRequest): UserDTO
    suspend fun updateUser(id: UUID, request: UpdateUserRequest): UserDTO
    suspend fun deleteUser(id: UUID)
}

class UserServiceImpl(
    private val userRepository: UserRepository
) : UserService {
    override suspend fun getUserById(id: UUID): UserDTO {
        return userRepository.findById(id) ?: throw ResourceNotFoundException("User", id.toString())
    }
    // Other implementations...
}

Route Handlers

kotlin
fun Application.configureUserRoutes(userService: UserService) {
    routing {
        route("/api/users") {
            get("/{id}") {
                val id = call.parameters["id"]?.let { UUID.fromString(it) }
                    ?: throw ValidationException("Invalid ID format")
                val user = userService.getUserById(id)
                call.respond(ApiResponse("SUCCESS", "User retrieved", user))
            }
            // Other routes...
        }
    }
}

Error Handling

kotlin
open class ApplicationException(
    message: String,
    val statusCode: HttpStatusCode = HttpStatusCode.InternalServerError
) : RuntimeException(message)

class ResourceNotFoundException(resource: String, id: String) :
    ApplicationException("$resource with ID $id not found", HttpStatusCode.NotFound)

fun Application.configureExceptions() {
    install(StatusPages) {
        exception<ResourceNotFoundException> { call, cause ->
            call.respond(cause.statusCode, ApiResponse("ERROR", cause.message ?: "Resource not found"))
        }
        exception<Throwable> { call, cause ->
            call.respond(HttpStatusCode.InternalServerError, ApiResponse("ERROR", "An internal error occurred"))
        }
    }
}

Testing Strategies and Coverage Requirements

Test Coverage Requirements

• Minimum coverage: 80% overall code coverage required
• Critical components: 90%+ coverage for repositories, services, and validation
• Test all edge cases: Empty collections, null values, boundary conditions
• Test failure paths: Exception handling, validation errors, timeouts
• All public APIs: Must have integration tests
• Performance-critical paths: Must have benchmarking tests

Unit Testing with Kotest

kotlin
class UserServiceTest : DescribeSpec({
    describe("UserService") {
        val mockRepository = mockk<UserRepository>()
        val userService = UserServiceImpl(mockRepository)

        it("should return user when exists") {
            val userId = UUID.randomUUID()
            val user = UserDTO(userId.toString(), "Test User", "test@example.com")
            coEvery { mockRepository.findById(userId) } returns user

            val result = runBlocking { userService.getUserById(userId) }

            result shouldBe user
        }

        it("should throw exception when user not found") {
            val userId = UUID.randomUUID()
            coEvery { mockRepository.findById(userId) } returns null

            shouldThrow<ResourceNotFoundException> {
                runBlocking { userService.getUserById(userId) }
            }
        }
    }
})

Route Testing with Ktor 3.x

kotlin
class UserRoutesTest : FunSpec({
    test("GET /api/users/{id} returns 200 when user exists") {
        val mockService = mockk<UserService>()
        val userId = UUID.randomUUID()
        val user = UserDTO(userId.toString(), "Test User", "test@example.com")

        coEvery { mockService.getUserById(userId) } returns user

        testApplication {
            application {
                configureRouting()
                configureDI { single { mockService } }
            }

            client.get("/api/users/$userId").apply {
                status shouldBe HttpStatusCode.OK
                bodyAsText().let {
                    Json.decodeFromString<ApiResponse<UserDTO>>(it)
                }.data shouldBe user
            }
        }
    }
})

Key Principles for Testable Code

1. Single Responsibility: Each method should do one thing well
2. Pure Functions: Same input always produces same output
3. Dependency Injection: Constructor injection for testable components
4. Clear Boundaries: Well-defined inputs and outputs
5. Small Methods: Extract complex logic into testable helper functions

Configuration Management

kotlin
// Type-safe configuration
interface AppConfig {
    val database: DatabaseConfig
    val security: SecurityConfig
}

data class DatabaseConfig(
    val driver: String,
    val url: String,
    val user: String,
    val password: String
)

// Access in application
fun Application.configureDI() {
    val appConfig = HoconAppConfig(environment.config)

    install(Koin) {
        modules(module {
            single<AppConfig> { appConfig }
            single { appConfig.database }
        })
    }
}

Security Best Practices

kotlin
fun Application.configureSecurity() {
    install(Authentication) {
        jwt("auth-jwt") {
            // JWT configuration
        }
    }

    install(DefaultHeaders) {
        header(HttpHeaders.XContentTypeOptions, "nosniff")
        header(HttpHeaders.XFrameOptions, "DENY")
        header(HttpHeaders.ContentSecurityPolicy, "default-src 'self'")
        header("Strict-Transport-Security", "max-age=31536000; includeSubDomains")
    }
}

Health Checks & Monitoring

kotlin
fun Application.configureMonitoring() {
    val startTime = System.currentTimeMillis()

    routing {
        get("/health") {
            call.respond(mapOf("status" to "UP", "uptime" to "${(System.currentTimeMillis() - startTime) / 1000}s"))
        }

        get("/metrics") {
            call.respond(prometheusRegistry.scrape())
        }
    }

    install(MicrometerMetrics) {
        registry = PrometheusMeterRegistry(PrometheusConfig.DEFAULT)
        meterBinders = listOf(
            JvmMemoryMetrics(),
            JvmGcMetrics(),
            ProcessorMetrics(),
            JvmThreadMetrics()
        )
    }
}

Performance Tuning

• JVM Settings: -XX:+UseG1GC -XX:MaxGCPauseMillis=100 -XX:MaxRAMPercentage=75.0
• Connection Pooling: Configure HikariCP with proper sizing based on workload
• Caching: Use Caffeine for in-memory caching of frequently accessed data
• Coroutines: Use structured concurrency for asynchronous processing
• Database Queries: Optimize with proper indexing, batch operations, pagination