Kotlin for Backend

Hi!

linkedin.com/in/cosminstefan

cosmin@greenerpastures.ro

Goal for today?

We've all heard
of Kotlin

But why?

A quick recap

Very powerful

General Purpose

Easy to learn

Great tooling (by JetBrains)

JVM compatible

Open Source (Apache 2.0)

Platform targeting

Android (Kotlin/JVM)

Server (Kotlin/JVM)
iOS (Kotlin/Native)
Browser (Kotlin/JS)

Fine... Kotlin...
What now?

Quite a few options!

Many Server-side Frameworks

Spring
Vert.x

Spark

Http4K

Ktor

© Jon Kenfield

Ktor

Framework ​for building asynchronous servers and clients in connected systems using Kotlin

Written in pure Kotlin

Jetbrain's official framework

Strong backers:

Lightweight & unopinionated

Very fast boot & routing

Performance:

Backed by Coroutines

Suspending API via NIO

Asynchronous I/O:

Servlet containers (Tomcat, Jetty)

Standalone (Docker, Fat JAR)

Hosting (AppEngine, Heroku)

Unopinionated deploy:

Authentication

CORS

Serialization

Powerful features:

Sessions

Templating

Compression

Static content

HSTS


Web Sockets

And even more features:

Auto-reload

Metrics

Logging

And many more ...

Project Setup

Ktor

start.ktor.io / IntelliJ Plugin


Base project structure

Bootstrapping:

start.ktor.io

Base project

Gradle Build Setup


Pre-Configured dependencies
 

Ready to run

fun main(args: Array<String>): Unit = io.ktor.server.cio.EngineMain.main(args)

fun Application.module(testing: Boolean = false) {
    install(ContentNegotiation) { ... }

    install(Compression) { ... }

    install(AutoHeadResponse)

    install(CallLogging) { ... }

    install(CORS) { ... }

    routing {
        get("/") {
            call.respondText("Hello World!", contentType = ContentType.Text.Plain)
        }

        get("/html-dsl") {
            call.respondHtml {
                body {
                    h1 { +"HTML" }
                    ul {
                        for (n in 1..10) {
                            li { +"$n" }
                        }
                    }
                }
            }
        }

        get("/json/gson") {
            call.respond(mapOf("Hello" to "World"))
        }
    }
}

Easy-win Features

Ktor

Defaults to SLF4J & Logback


Auto Call Result Logging

Logging:


fun Application.module(testing: Boolean = false) {
    
    log.debug("App has initialized at: {}", Instant.now())

    install(CallLogging) {
        level = Level.INFO
        filter { call -> call.request.path().startsWith("/") }
    }
    
    ...
}

Fast feedback loop cycle


Simple setup: watch list

Autoreload:


# In application.conf

ktor {
    deployment {
        port = 8080
        watch = [ main, module1, module2 ]
    }
    
    …
}

Then just build the app!

Compress outgoing content


gzip, deflate, identity

Compression:


fun Application.module(testing: Boolean = false) {
    
    install(Compression) {
        gzip {
            priority = 1.0
        }
        deflate {
            priority = 10.0
            minimumSize(1024) // condition
        }
    }
    
    ...
}

Automatic content conversion

 

Based on Content-Type and Accept headers

Content negotiation:


fun Application.module(testing: Boolean = false) {
    
    install(ContentNegotiation) { 
        register(ContentType.Application.Json, JacksonConverter())
    }

    ...
    
    routing {
    
        get("/data") {
            call.respond(MyDataClass("Hello", "World"))
        }
        
        post("/data") {
            val myRequest = call.receive<MyRequestClass>()
        }
        
    }
}

Automatic handling

Opens API for JavaScript calls

CORS Support:


fun Application.module(testing: Boolean = false) {

    install(CORS) {
    
    	method(HttpMethod.Patch)
    	method(HttpMethod.Options)
        
        header(HttpHeaders.XForwardedProto)
        
        host("example.org")
        host("example.com", subDomains = listOf("www"))
    }
    
    ...
}

Build HTML responses


Freemarker, Mustache, Thymeleaf, Veolcity, KotlinX

Templating:

fun Application.module(testing: Boolean = false) {

    install(Mustache) {
        mustacheFactory = DefaultMustacheFactory("templates")
    }
   
    routing {        
        get("/html-mustache") {
            val todo = Todo("Prepare presentation")
            call.respond(MustacheContent("todos.hbs", mapOf("todo" to todo)))
        }
        
        get("/html-dsl") {
            call.respondHtml {
                body {
                    h1 { +"HTML" }
                    ul {
                        for (n in 1..10) {
                            li { +"$n" }
                        }
                    }
                }
            }
        }
        ...
    }
}

Routing

Ktor

Support for simplifying and structuring
request handling

Valid paths for requests


Functions that process the requests

Main roles:


fun Application.module(testing: Boolean = false) {

    routing {
    
        get("/") {
            call.respondText("Hello World!")
        }

        route("/api/todos") {

            // Final path: /apis/todos
            get {
                val todosList: List<Todo> = dataRepository.loadTodos()
                call.respond(todosList)
            }

            // Final path: /apis/todos/items
            post("/items") {
                val item = call.receive<Todo>()
                dataRepository.addTodo(item)
            }
        }
    }
}

route defines a base path for children


APIs can be separated

Keeping a clean structure:

fun Routing.todosApi() {

    val dataRepository: DataRepository = ... // inject repository

    route("/todos") {

        // Resolves to /todos
        get {
            val todosList: List<Todo> = dataRepository.loadTodos()
            call.respond(todosList)
        }

        route("/items") {
            // Resolves to /todos/items
            post {
                val item = call.receive<Todo>()
                dataRepository.addTodo(item)
            }

            // Resolves to /todos/items/{id}
            delete("/{id}") {
                val itemId = call.parameters["id"]
                dataRepository.removeTodo(itemId)
            }
        }
    }
}
fun Application.module() {
    
    routing {
        route("/api/v1") {
            // Install TODOs API module (Final path: /api/v1/todos)
            todosApi()
            
            // Install User Profile API module (Final path: /api/v1/profile)
            profileApi()
        }
        
        // Install main HTML Web app module
        webApp()
    }
}

Dependency Injection

Koin

A pragmatic lightweight dependency injection framework for Kotlin

No proxy

 

No code generation

 

No reflection

Performance:

Define modules


Lazy inject at
consumption location

Main principles:

class AuthenticationService {
    ...
}

class DataRepository(val authenticationService: AuthenticationService) {
    ...
}

val mainModule = module {

    single { AuthenticationService() }
    
    single { DataRepository(get()) }
}


fun Application.module(testing: Boolean = false) {
    
    install(Koin) {
        // Use SLF4J Koin Logger
        slf4jLogger()
        // Declare used modules
        modules(mainModule)
    }
    ...

    routing {
        // Lazy inject DataRepository
        val dataRepository: DataRepository by inject()

        get("/data") {
            call.respond(dataRepository.loadData())
        }
        ...
        
    }
}

SQL Framework

Exposed

A lightweight SQL library for Kotlin, by Jetbrains

Typesafe SQL wrapping DSL


Lightweight data access objects (DAOs)

Layers of database access:

Typesafe


Full querying support

Comprehensive features:

PostgreSQL, MySQL, H2


MariaDB, Oracle, SQLite, SQL Server

Currently supported dialects:

object Users : Table("users") {
    val id = integer("id").autoIncrement().primaryKey()
    val name = varchar("name", 128)
}

object Todos : Table("todos") {
    val id = integer("id").autoIncrement().primaryKey()
    val description = varchar("description", 256)
    val reminderAt = datetime("reminder_at")
    val userId = integer("user_id").references(Users.id, ReferenceOption.CASCADE)
}
class TodosRepository {

    init {
        Database.connect("jdbc:h2:mem:todos", driver = "org.h2.Driver")
        
        transaction {
            addLogger(Slf4jSqlDebugLogger)
            SchemaUtils.create(Users, Todos)
        }
    }
    ...
}
class TodosRepository {
    
    ...
    fun loadTodos(): List<String> = transaction {

        // SELECT * FROM TODOS WHERE TODOS.USER_ID = {userId}
        Todos.select { Todos.userId eq userId }.map { it[Todos.description] }

        // SELECT TODOS.DESCRIPTION FROM TODOS
        Todos.slice(Todos.description).selectAll().map { it[Todos.description] }
    }
}
class TodosRepository {
    
    ...
    fun addTodo(userId: Int, description: String): Int = transaction {

        // INSERT INTO TODOS (DESCRIPTION, REMINDER_AT, USER_ID) VALUES ('Do stuff', NULL, 1)
        val todoId = Todos.insert {
            it[Todos.description] = "Do stuff"
            it[Todos.userId] = userId
        } get Todos.id

        return@transaction todoId
    }
}

Deploying to the cloud

AppEngine

Project on Google Cloud

 

gcloud cli

Pre-requisites:

buildscript {
    ext.appengine_version = '1.9.60'
    ext.appengine_plugin_version = '1.3.4'

    repositories {
        jcenter()
    }
    dependencies {
        ...
        classpath "com.google.cloud.tools:appengine-gradle-plugin:$appengine_plugin_version"
    }
}

apply plugin: 'kotlin'
apply plugin: 'war'
apply plugin: 'com.google.cloud.tools.appengine'

sourceSets {
    main.kotlin.srcDirs = [ 'src/main/kotlin' ]
}

...

dependencies {
    ....

    providedCompile "com.google.appengine:appengine:$appengine_version"
}

task run(dependsOn: appengineRun)
# Run the app locally, on http://localhost:8080
./gradlew :google-appengine-standard:appengineRun

# Setup the project on Google Cloud
gcloud app create

# Deploy the application to the cloud, to https://demo-ktor.appspot.com
gradle :google-appengine-standard:appengineDeploy

What to keep in mind?

Thank you!

Kotlin for Backend

By Cosmin Stefan

Made with Slides.com

Kotlin for Backend

Kotlin is a great fit for developing server-side applications, allowing you to write concise and expressive code while maintaining full compatibility with existing Java-based technology stacks and a smooth learning curve. We’ll be going over the process of building an entire backend API using Kotlin, with the right tools, from dependency injection to routing to database access.

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