Kotlin is not only for mobile development but also for backend (it could be used everywhere actually!). In JetBrains, we build Ktor framework for backend development such as website, API, microservice. In this talk, I will introduce what Ktor is, how to integrated with Exposed SQL library, demonstrate how to build a RESTful API service in just a few lines of code. After listening to this talk, you will learn how to build API with Ktor rapidly.

1. Build an API
service using Ktor
rapidly
—
Shengyou Fan ( )
HKOSCon
2020/06/13

2. Hello!
—
I’m
• Developer Advocate
• Backend Developer
• Open Source Community Organizer

3. • General-purpose
• Static typing
• OOP + FP
• Developed by
JetBrains
• Open Source
(Apache 2.0)
https://kotlinlang.org/
Kotlin
Programing Language
—

4. Kotlin Multiplatform
—
Browser
Kotlin/JS
Server
Kotlin/JVM
iOS
Kotlin/Native
Android
Kotlin/JVM

5. Ktor Framework
—
https://ktor.io/
• Web Framework
• Asynchronous
• Servers + Clients
• Developed by
JetBrains
• Open Source
(Apache 2.0)

6. API service
—
Server
Database
Client
Kotlin/JVM

7. Ktor Project Generator
—
https://plugins.jetbrains.com/plugin/10823-ktor

8. API Endpoint
—
GET {URL}/api/tasks
###
POST {URL}/api/tasks
###
GET {URL}/api/tasks/{UUID}
###
PATCH {URL}/api/tasks/{UUID}
###
DELETE {URL}/api/tasks/{UUID}

9. API Design
—
{
"data": [
{
"id": "...UUID...",
"title": "...",
"description": "...",
"completed": true
}
]
}

10. data class TaskResponse (
val id:UUID,
val title: String,
val description: String,
val completed: Boolean
)
Data Class Design
—

11. Application Call in Ktor
—
Client
Ktor App
HTTP Server
Feature
Feature
Feature

12. Install Gson
—
// build.gradle
dependencies {
//...
// Jackson
implementation "io.ktor:ktor-gson:$ver"
}

13. Install Feature
—
install(ContentNegotiation) {
gson {
}
}

14. Routing
—
routing {
}

15. HTTP Method
—
routing {
get("/api/tasks") {
}
}

16. Return JSON
—
get("/api/tasks") {
// ...
call.respond(mapOf("data" to tasks))
}

17. Exposed
Kotlin SQL Framework
—
https://github.com/JetBrains/Exposed

18. Install Exposed
—
// gradle.properties
exposed_version=0.21.1
h2_version=1.4.200
// build.gradle
dependencies {
//...
// Exposed SQL Library
implementation "org.jetbrains.exposed:exposed-core:$ver"
implementation "org.jetbrains.exposed:exposed-dao:$ver"
implementation "org.jetbrains.exposed:exposed-jdbc:$ver"
implementation "org.jetbrains.exposed:exposed-jodatime:$ver"
// H2 database
implementation "com.h2database:h2:$ver"
}

19. object Tasks : UUIDTable() {
val title = varchar("title", 255)
val description = text("description")
var completed = bool("completed")
val createdAt = datetime("created_at")
val updatedAt = datetime("updated_at")
}
class Task(id: EntityID<UUID>) : UUIDEntity(id) {
companion object : UUIDEntityClass<Task>(Tasks)
var title by Tasks.title
var description by Tasks.description
var completed by Tasks.completed
var createdAt by Tasks.createdAt
var updatedAt by Tasks.updatedAt
}
Define Table & Entity
—

20. Connect to Database
—
Database.connect(
url = "...",
driver = "..."
)

21. Create Schema and
Initial Data
—
transaction {
SchemaUtils.create(Tasks)
}
transaction {
for (i in 1..10) {
val task = Task.new {
title = "Task $i"
description = "..."
completed = false
createdAt = DateTime.now()
updatedAt = DateTime.now()
}
}
}

22. val tasks = transaction {
Task.all().map {
TaskResponse(
it.id.value,
it.title,
it.description,
it.completed,
it.createdAt.toString("..."),
it.updatedAt.toString("...")
)
}
}
Read
—

23. Write
—
post("/api/tasks") {
val request = call.receive<TaskRequest>()
val data = transaction {
val task = Task.new {
title = request.title
description = request.description
completed = false
createdAt = DateTime.now()
updatedAt = DateTime.now()
}
// ...
}
call.respond(mapOf("data" to data))
}

24. Update
—
patch("/api/tasks/{id}/complete") {
val id = UUID.fromString(call.parameters[“id"])
val data = transaction {
val task = Task.findById(id)
if (task != null) {
task.completed = true
} else {
throw ModelNotFoundException()
}
}
call.respond(mapOf("data" to data))
}

25. Delete
—
delete("/api/tasks/{id}/delete") {
val id = UUID.fromString(call.parameters[“id"])
transaction {
val task = Task.findById(id)
if (task != null) {
task.delete()
} else {
throw ModelNotFoundException()
}
}
call.respond(HttpStatusCode.NoContent)
}

26. Deploy
—
// build.gradle
plugins {
id 'com.google.cloud.tools.jib' version '2.2.0'
}
$ gradle jib --image=<MY IMAGE>
https://github.com/GoogleContainerTools/jib

27. Recap
—
• Ktor Introduction
• Application Call Request & Response
• Routing
• API (JSON)
• Exposed DB

28. Shengyou Fan ( )
shengyou.fan@jetbrains.com
Q&A
—
Build an API service
using Ktor rapidly