URL Shortener API with Kotlin (Part 1/2)

Create a URL shortener API with Kotlin, the micro-framework Ktor and PostgreSQL

The source code for this post can be found on this github repo

Introduction

Ktor is a brand new micro-framework created by the Jetbrains team, and running over the JVM. Jetbrains are the authors of Kotlin - which is now the official programming language for Android, and one of the most popular programming language on the JVM. Kotlin is gaining popularity on server-side and multi-platform application development.

Ktor is a framework for building asynchronous servers and clients in connected systems using the powerful Kotlin programming language.

In this article, you will learn:

  • How to design a simple URL shortener.
  • How to use the Ktor micro-framework with Kotlin
  • How to deploy a Ktor application

I have +4 years of experience using Spring, and I wanted to give a try to Ktor, which seems promising. Creating a URL shortener is an excellent way to start.

What is a URL shortener?

A URL shortener is a simple tool that takes a long URL and turns it into a very short one

Flow of URL shortening - from original URL to short URL

It is commonly used for 3 reasons:

  • Tracking clicks
  • Make URL much more concise.
  • Hide original URL

One famous freemium provider is bit.ly (see here)

In this article we will make a basic bit.ly like URL shortener. Let’s go

Ktor principles

Before starting I want to introduce the 3 main principles of Ktor.

Kotlin

Kotlin is the language used to develop on Ktor. It is an object-oriented and functional language. It is very stable and runs on the JVM. Kotlin is 100% interoperable with Java and allows you to benefit from its ecosystem (libraries, build system, etc.).

Functional programming

Ktor leverages the power of Kotlin and has a very functional approach. When writing code, everything seems obvious. It's very similar to what you can see on NodeJS. For me, coming from the Spring world, I find it very efficient to read and use.

Asynchronous

Kotlin provides asynchronous code execution, thanks to coroutines. Ktor exploits this feature to its full potential, and even if you have the impression that you are writing code in a blocking manner, this is not the case. Ktor makes your life easier.

HTTP Server

Here is a complete and simple example of how to expose an HTTP server (http://localhost:8080) with Ktor.

fun main(args: Array<String>): Unit = io.ktor.server.netty.EngineMain.main(args)
@kotlin.jvm.JvmOverloads
fun Application.module(testing: Boolean = false) {
routing {
get("/") {
call.respondText("Hello World", contentType = ContentType.Text.Plain)
}
}
}

URL Encoder

The URL encoder will translate an incoming address into a smaller URL. The idea is to provide an ID that will identify the final URL. Using a hash function is perfect for this operation. However, the operation is non-reversible, meaning you can’t retrieve the final URL by the generated identifier.

Function to transform a long URL into a shorter URL

// String extension
fun String.encodeToID(): String {
// hash String with MD5
val hashBytes = MessageDigest.getInstance("MD5").digest(this.toByteArray(Charsets.UTF_8))
// transform to human readable MD5 String
val hashString = String.format("%032x", BigInteger(1, hashBytes))
// truncate MD5 String
val truncatedHashString = hashString.take(6)
// return id
return truncatedHashString
}

We expose the function through the REST API

// Request object
data class Request(val url: String) {
fun toResponse(): Response = Response(url, url.encodeToID())
}
// Response object
data class Response(val originalURL: String, private val id: String) {
val shortURL: String = "http://localhost:8080/$id"
}
@kotlin.jvm.JvmOverloads
fun Application.module(testing: Boolean = false) {
install(ContentNegotiation) {
jackson {
enable(SerializationFeature.INDENT_OUTPUT)
propertyNamingStrategy = PropertyNamingStrategy.SNAKE_CASE
}
}
// Hash Table Response object by ID
val responseByID = mutableMapOf<String, Response>()
routing {
post("/api/v1/encode") {
// Deserialize JSON body to Request object
val request = call.receive<Request>()
// find the Response object if it already exists
val retrievedResponse = responseByID[request.url.encodeToID()]
if (retrievedResponse != null) {
// cache hit
log.debug("cache hit $retrievedResponse")
return@post call.respond(retrievedResponse)
}
// cache miss
val response = request.toResponse()
responseByID[request.url.encodeToID()] = response
log.debug("cache miss $response")
// Serialize Response object to JSON body
call.respond(response)
}
}
}

Handle identifier collision

Using a hash function makes no guarantee that it is not already being used. If it is in use, then you need to change it to another one. Note: even if the probability to have a collision is very low, you should handle this case.

// String extension (function signature has changed)
fun String.encodeToID(truncateLength: Int = 6): String {
// hash String with MD5
val hashBytes = MessageDigest.getInstance("MD5").digest(this.toByteArray(Charsets.UTF_8))
// transform to human readable MD5 String
val hashString = String.format("%032x", BigInteger(1, hashBytes))
// truncate MD5 String
val truncatedHashString = hashString.take(truncateLength)
// return id
return truncatedHashString
}
//...
@kotlin.jvm.JvmOverloads
fun Application.module(testing: Boolean = false) {
// ...
// Hash Table Response object by id
val responseByID = mutableMapOf<String, Response>()
fun getIdentifier(url: String, truncateLength: Int = 6): String {
val id = url.encodeToID()
val retrievedResponse = responseByID[id]
if (retrievedResponse?.originalURL != url) {
// collision spotted !
return getIdentifier(url, truncateLength + 1)
}
return id
}
routing {
post("/api/v1/encode") {
// Deserialize JSON body to Request object
val request = call.receive<Request>()
// find the Response object if it already exists
val id = getID(request.url)
val retrievedResponse = responseByID[id]
if (retrievedResponse != null) {
// cache hit
log.debug("cache hit $retrievedResponse")
return@post call.respond(retrievedResponse)
}
// cache miss
val response = request.toResponse()
responseByID[id] = response
log.debug("cache miss $response")
// Serialize Response object to JSON body
call.respond(response)
}
}
}

URL Decoder

Decoding the URL is the process of returning the original URL from the short URL. This is the reverse operation made by the URL Encoder

val shortURL = getShortURL(request.url)
val retrievedResponse = responseByID[shortURL]
retrievedResponse?.originalURL // return original URL or null

Redirect

When a user clicks on a short URL, the user is redirected to the final URL. HTTP protocol allows to do this naturally by returning a 302 status code and a redirection URL.

With Ktor the redirection is as simple as calling a method with the final URL as a parameter.

call.respondRedirect("https://www.qovery.com")

What we expect is that when the user visits http://localhost:8080/fbc951 he is redirected to https://www.qovery.com. If the URL is incorrect then redirect to https://www.google.com

@kotlin.jvm.JvmOverloads
fun Application.module(testing: Boolean = false) {
// ...
routing {
get("/{id}") {
val id = call.parameters["id"]
val retrievedResponse = id?.let { responseByID[it] }
if (id.isNullOrBlank() || retrievedResponse == null) {
return@get call.respondRedirect("https://www.google.com")
}
log.debug("redirect to: $retrievedResponse")
call.respondRedirect(retrievedResponse.originalURL)
}
// ...
}
}

Stats: clicks over time

Something that is really useful on products like bit.ly is the stats provided (click over time, referrers, country of visitors). Here is how to store click over time and make them available through the API

// added
data class Stat(val clicksOverTime: MutableList<Date> = mutableListOf())
// Response object (modified with Stat)
data class Response(val originalURL: String, private val id: String, val stat: Stat = Stat()) {
val shortURL: String = "http://localhost:8080/$id"
}
@kotlin.jvm.JvmOverloads
fun Application.module(testing: Boolean = false) {
install(ContentNegotiation) {
jackson {
// ...
// add this line to return Date object as ISO8601 format
disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS)
}
}
// ...
routing {
// ...
get("/api/v1/url/{id}/stat") {
val id = call.parameters["id"]
val retrievedResponse = id?.let { responseByID[it] }
if (id.isNullOrBlank() || retrievedResponse == null) {
return@get call.respond(HttpStatusCode.NoContent)
}
call.respond(retrievedResponse.stat)
}
// ...
}
}

Try the API

Run the application

$ ./gradlew run
//...
2020-03-12 09:28:08.150 [main] INFO Application - No ktor.deployment.watch patterns specified, automatic reload is not active
2020-03-12 09:28:08.606 [main] INFO Application - Responding at http://0.0.0.0:8080

Then execute the commands

# generate a short URL
$ curl -X POST -d '{"url": "https://www.qovery.com"}' -H "Content-type: application/json" "http://localhost:8080/api/v1/encode"
{
"original_url": "https://www.qovery.com",
"stat": {
"clicks_over_time": []
},
"short_url": "http://localhost:8080/fbc951"
}
# generate 4 fake clicks
$ curl -X GET 'http://localhost:8080/fbc951'
$ curl -X GET 'http://localhost:8080/fbc951'
$ curl -X GET 'http://localhost:8080/fbc951'
$ curl -X GET 'http://localhost:8080/fbc951'
# show stat
$ curl -X GET 'http://localhost:8080/api/v1/url/fbc951/stat'
{
"clicks_over_time": [
"2020-03-11T21:10:52.354+0000",
"2020-03-11T21:10:54.093+0000",
"2020-03-11T21:12:34.987+0000",
"2020-03-11T21:12:37.223+0000"
]
}

Connect to a PostgreSQL database with Exposed

By storing the data in memory, we lose all the data every time the application restart. Which is problematic for running in production. To make the data persistent we will store it in a PostgreSQL database. We will have to add 1 new dependency - Exposed. Exposed (with Hikari Connection Pool) is a lightweight SQL library on top of JDBC driver for Kotlin. With exposed it is possible to access databases in two flavours: typesafe SQL wrapping DSL and lightweight Data Access Objects (DAO).

Add the dependencies to your build.gradle (or POM.xml)

repositories {
jcenter()
}
dependencies {
// Connection Pool and PostgreSQL driver
implementation("com.zaxxer:HikariCP:3.4.2")
implementation("org.postgresql:postgresql:42.2.11")
// Exposed
implementation("org.jetbrains.exposed:exposed-core:0.22.1")
implementation("org.jetbrains.exposed:exposed-dao:0.22.1")
implementation("org.jetbrains.exposed:exposed-jdbc:0.22.1")
implementation("org.jetbrains.exposed:exposed-java-time:0.22.1")
}

We need to have 2 distincts tables, one containing all the final URLs with their correspond identifier

object ResponseTable : Table("response") {
val id = varchar("id", 32)
val originalURL = varchar("original_url", 2048)
override val primaryKey: PrimaryKey = PrimaryKey(id)
}

And a second one with all the clicking points

object ClickOverTimeTable : Table("click_over_time") {
val id = integer("id").autoIncrement()
val clickDate = datetime("click_date")
val response = reference("response_id", onDelete = ReferenceOption.CASCADE, refColumn = ResponseTable.id)
override val primaryKey: PrimaryKey = PrimaryKey(id)
}

We need to create the tables as defined above programmatically

fun initDatabase() {
val config = HikariConfig().apply {
jdbcUrl = "jdbc:postgresql://127.0.0.1:5432/exposed"
username = "exposed"
password = "exposed"
driverClassName = "org.postgresql.Driver"
}
Database.connect(HikariDataSource(config))
transaction {
// create tables if they do not exist
SchemaUtils.createMissingTablesAndColumns(RequestTable, ClickOverTimeTable)
}
}
@kotlin.jvm.JvmOverloads
fun Application.module(testing: Boolean = false) {
initDatabase()
// ...
}

We have to replace the Hash Table used to store the data by the PostgreSQL database (see the final code here)

Deploy in the Cloud with Qovery

Qovery is going to help us to deploy the final application in the Cloud without the need to configure the CI/CD, network, security, load balancing, database and all the DevOps tasks

Qovery is a Container as a Service platform for developer - developers can deploy their application in the Cloud in just a few seconds

  • Sign in to the Qovery web interface.

    Qovery Sign-up page

  • Configuration

    To deploy on Qovery, two files are mandatories

    .qovery.yml - a very simple way to declare the dependencies that you need (e.g: PostgreSQL) and where you want to run it (here on AWS and eu-west-3 / Paris)

    application:
    name: api
    project: url-shortener
    publicly_accessible: true
    databases:
    - type: postgresql
    version: "11.5"
    name: my-pql-db
    routers:
    - name: main
    routes:
    - application_name: api
    paths:
    - /*

    Dockerfile - to build and run your application on Qovery

    FROM openjdk:8-jre-alpine
    ENV APPLICATION_USER ktor
    RUN adduser -D -g '' $APPLICATION_USER
    RUN mkdir /app
    RUN chown -R $APPLICATION_USER /app
    USER $APPLICATION_USER
    COPY ./build/libs/ktor-url-shortener.jar /app/ktor-url-shortener.jar
    WORKDIR /app
    CMD ["java", "-server", "-XX:+UnlockExperimentalVMOptions", "-XX:+UseCGroupMemoryLimitForHeap", "-XX:InitialRAMFraction=2", "-XX:MinRAMFraction=2", "-XX:MaxRAMFraction=2", "-XX:+UseG1GC", "-XX:MaxGCPauseMillis=100", "-XX:+UseStringDeduplication", "-jar", "ktor-url-shortener.jar"]

    Connect to PostgreSQL

    Qovery add dynamically all required environment variables to connect to the database at the runtime of the container.

    To list all of them

    $ qovery application env list
    SCOPE | KEY | VALUE
    BUILT_IN | QOVERY_JSON_B64 | <base64>
    BUILT_IN | QOVERY_BRANCH_NAME | with_postgresql
    BUILT_IN | QOVERY_IS_PRODUCTION | false
    BUILT_IN | QOVERY_DATABASE_MY_PQL_DB_NAME | my-pql-db
    BUILT_IN | QOVERY_DATABASE_MY_PQL_DB_TYPE | POSTGRESQL
    BUILT_IN | QOVERY_DATABASE_MY_PQL_DB_VERSION | 11.5
    BUILT_IN | QOVERY_DATABASE_MY_PQL_DB_CONNECTION_URI | <hidden>
    BUILT_IN | QOVERY_DATABASE_MY_PQL_DB_CONNECTION_URI_WITHOUT_CREDENTIALS | <hidden>
    BUILT_IN | QOVERY_DATABASE_MY_PQL_DB_HOST | <hidden>
    BUILT_IN | QOVERY_DATABASE_MY_PQL_DB_FQDN | <hidden>
    BUILT_IN | QOVERY_DATABASE_MY_PQL_DB_PORT | <hidden>
    BUILT_IN | QOVERY_DATABASE_MY_PQL_DB_USERNAME | <hidden>
    BUILT_IN | QOVERY_DATABASE_MY_PQL_DB_PASSWORD | <hidden>
    BUILT_IN | QOVERY_DATABASE_MY_PQL_DB_DATABASE | postgres
    BUILT_IN | QOVERY_APPLICATION_API_HOSTNAME | <hidden>
    BUILT_IN | QOVERY_APPLICATION_API_HOST | <hidden>

    To use them

    fun initDatabase() {
    val config = HikariConfig().apply {
    jdbcUrl = "jdbc:${System.getenv("QOVERY_DATABASE_MY_PQL_DB_CONNECTION_URI_WITHOUT_CREDENTIALS")}"
    username = System.getenv("QOVERY_DATABASE_MY_PQL_DB_USERNAME")
    password = System.getenv("QOVERY_DATABASE_MY_PQL_DB_PASSWORD")
    driverClassName = "org.postgresql.Driver"
    }
    Database.connect(HikariDataSource(config))
    transaction {
    // create tables if they do not exist
    SchemaUtils.createMissingTablesAndColumns(RequestTable, ClickOverTimeTable)
    }
    }

    Deploy

    Deploying your app with Qovery is as simple as commit and push your code.

    $ git add .qovery.yml Dockerfile
    $ git commit -m "add .qovery.yml and Dockerfile files"
    $ git push -u origin master

    To get the public URL

    $ qovery status
    BRANCH NAME | STATUS | ENDPOINTS | APPLICATIONS | DATABASES | BROKERS | STORAGE
    with_postgresql | running | https://qavcgggy6g6dlkbj-main-gtw.qovery.io | 1 | 1 | 0 | 0
    APPLICATION NAME | STATUS | ENDPOINT | DATABASES | BROKERS | STORAGE
    api | running | https://ete6bq97amj9n82c-qavcgggy6g6dlkbj-app.qovery.io | 1 | 0 | 0
    DATABASE NAME | STATUS | TYPE | VERSION | ENDPOINT | PORT | USERNAME | PASSWORD | APPLICATIONS
    my-pql-db | running | POSTGRESQL | 11.5 | <hidden> | <hidden> | <hidden> | <hidden> | api

    Conclusion

    We have seen that creating an URL shortener API with Ktor and Kotlin is extremely simple. Connecting the application to PostgreSQL is very easy with the Exposed library. In just a few lines of code, the service is fully functional and can be deployed in production very quickly with the help of Qovery. In the next part, we will see how to create a web interface connecting to this API to convert our URLs without using the curl command.

    Part 2: bind a web interface to the API - [link coming soon]

    Tutorial