From Mobile to Backend with Kotlin and Ktor - FOSDEM 2022

As mobile developers, we often see the backend world as something magical that “does things”. But what if I tell you that you can easily build a backend even if mobile is your thing?

With this talk, I want to show how it is possible to bring your mobile knowledge (and shift it a little bit) to build a backend with Kotlin and Ktor. I will show how to structure the project, set up Dependency Injection, connect to a database and test everything to have a working backend ready to be deployed.

Transcript

1. Marco Gomiero
   FOSDEM
   From Mobile to
   Backend with
   Kotlin and Ktor
   

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2. FOSDEM - @marcoGomier
   Marco Gomiero
   👨💻 Android Engineer @ TIER 🛴 🇩🇪 🇮🇹
   

   Google Developer Expert for Kotlin
   > Twitter: @marcoGomier
   

   > Github: prof18
   

   > Website: marcogomiero.com
   

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3. FOSDEM - @marcoGomier
   🤔 Why?
   

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4. FOSDEM - @marcoGomier
   • Side project
   
   
   • Understand how things works “on the other side”
   
   
   • Help you teammates
   🤔 Why?
   

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5. FOSDEM - @marcoGomier
   * based on a true story
   

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6. FOSDEM - @marcoGomier
   Ktor
   https://ktor.io/
   

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7. FOSDEM - @marcoGomier
   Ktor
   • Kotlin
   
   
   • Lightweight and flexible
   
   
   • Asynchronous with Coroutines
   
   
   • Unopinionated
   

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8. FOSDEM - @marcoGomier
   Unopinionated
   • Whatever architecture
   
   
   • Whatever pattern
   Knowledge Transfer
   

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9. FOSDEM
   @marcoGomier
   Architecture
   

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10. FOSDEM - @marcoGomier
    Android
    Activity/Fragment
    ViewModel
    Repository
    Local Data Source Remote Data Source
    Application
    

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11. FOSDEM - @marcoGomier
    Resource*
    Repository
    Local Data Source Remote Data Source
    Application
    * or Controller
    Ktor
    

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12. FOSDEM - @marcoGomier
    Resource
    Repository
    Local Data Source Remote Data Source
    Application
    Activity/Fragment
    ViewModel
    Repository
    Local Data Source Remote Data Source
    Application
    

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13. FOSDEM - @marcoGomier
    Domain
    Data
    Resource
    Repository
    Local Data Source Remote Data Source
    Application
    Activity/Fragment
    ViewModel
    Repository
    Local Data Source Remote Data Source
    Application
    Presentation
    Application
    

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14. FOSDEM - @marcoGomier
    Application
    class MyApp : Application() {
    
    
    override fun onCreate() {
    
    
    super.onCreate()
    
    
    if (BuildConfig.DEBUG) {
    
    
    Timber.plant(Timber.DebugTree())
    
    
    }
    
    
    initAnalytics()
    
    
    initCrashReporting()
    
    
    initRandomLib()
    
    
    }
    
    
    }
    

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15. FOSDEM - @marcoGomier
    fun Application.module() {
    
    
    install(Koin) {
    
    
    slf4jLogger()
    
    
    modules(koinModules)
    
    
    }
    
    
    setupConfig()
    
    
    setupDatabase()
    
    
    install(ContentNegotiation) {
    
    
    json()
    
    
    }
    
    
    install(CallLogging) {
    
    
    level = Level.INFO
    
    
    }
    
    
    install(Locations)
    
    
    routing {
    
    
    setupEndpoint()
    
    
    }
    
    
    }
    Application
    

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16. FOSDEM - @marcoGomier
    Plugin
    https://ktor.io/docs/plugins.html
    • Add a specific feature to your backend
    
    
    • Highly customisable
    
    
    • No plugin activated by default
    

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17. FOSDEM - @marcoGomier
    Plugin
    https://ktor.io/docs/
    

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18. FOSDEM - @marcoGomier
    Domain
    Data
    Presentation
    Application
    

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19. FOSDEM - @marcoGomier
    Presentation
    class MainActivity : AppCompatActivity() {
    
    
    override fun onCreate(savedInstanceState: Bundle?) {
    
    
    super.onCreate(savedInstanceState)
    
    
    setContentView(R.layout.activity_main)
    
    
    ...
    
    }
    
    
    }
    

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20. FOSDEM - @marcoGomier
    fun Route.jokeEndpoint() {
    
    
    val jokeRepository by inject()
    
    
    get {
    
    
    call.respond(jokeRepository.getRandomJoke())
    
    
    }
    
    
    post { apiCallParams
    ->
    
    val name = apiCallParams.name
    
    
    jokeRepository.watch(name)
    
    
    call.respond("Ok")
    
    
    }
    
    
    }
    Presentation
    

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21. FOSDEM - @marcoGomier
    Domain
    Data
    Presentation
    Application
    

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22. FOSDEM - @marcoGomier
    Domain
    class JokeRepositoryImpl(
    
    
    private val jokeLocalDataSource: JokeLocalDataSource
    
    
    ) : JokeRepository {
    
    
    override suspend fun getRandomJoke(): JokeDTO {
    
    
    // ...
    
    }
    
    
    }
    

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23. FOSDEM - @marcoGomier
    Domain
    Data
    Presentation
    Application
    

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24. FOSDEM - @marcoGomier
    Room
    class LocalDatasource(
    
    
    private val db: RoomDatabase
    
    
    ) {
    
    
    suspend fun getAllowedApps(): List {
    
    
    return db.allowedAppDAO().getAllAllowedApps()
    
    
    }
    
    
    }
    @Entity(tableName = "allowed_app")
    
    
    data class AllowedApp(
    
    
    @PrimaryKey val packageName: String,
    
    
    @ColumnInfo val appName: String
    
    
    )
    @Dao
    
    
    interface AllowedAppDAO {
    
    
    @Query("SELECT * FROM allowed_app")
    
    
    suspend fun getAllAllowedApps(): List
    
    
    }
    Data
    

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25. FOSDEM - @marcoGomier
    Exposed
    https://github.com/JetBrains/Exposed
    

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26. FOSDEM - @marcoGomier
    Exposed
    class JokeLocalDataSourceImpl : JokeLocalDataSource {
    
    
    override suspend fun getAllJokes(): List {
    
    
    val joke = newSuspendedTransaction {
    
    
    val query = JokeTable.selectAll()
    
    
    Joke.wrapRows(query).toList()
    
    
    }
    
    
    }
    
    
    }
    object JokeTable: IdTable(name = "joke") {
    
    
    val createdAt = datetime("created_at")
    
    
    val updatedAt = datetime("updated_at")
    
    
    val value = text("value")
    
    
    override val id: Column>>
    = varchar("joke_id", 255).entityId()
    
    
    override val primaryKey: PrimaryKey = PrimaryKey(id)
    
    
    }
    class Joke(id: EntityID): Entity(id) {
    
    
    companion object: EntityClass(JokeTable)
    
    
    var createdAt by JokeTable.createdAt
    
    
    var updatedAt by JokeTable.updatedAt
    
    
    var value by JokeTable.value
    
    
    }
    Data
    

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27. FOSDEM - @marcoGomier
    EmailThreadTable.join(EmailFolderTable, JoinType.INNER, additionalConstraint = {
    
    
    (EmailThreadTable.id eq EmailFolderTable.threadId).and(
    
    
    EmailThreadTable.account eq EmailFolderTable.accountId
    
    
    )
    
    
    })
    
    
    .select { EmailThreadTable.account eq account.id.value and (EmailFolderTable.folderId eq folderId) }
    
    
    .orderBy(EmailThreadTable.lastEmailDate, SortOrder.DESC)
    
    
    .limit(n = pageSize)
    
    
    

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28. FOSDEM
    @marcoGomier
    Dependency Injection
    

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29. FOSDEM - @marcoGomier https://insert-koin.io/docs/quickstart/ktor
    Koin
    

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30. FOSDEM - @marcoGomier
    val appModule = module {
    
    
    single { HelloRepositoryImpl() }
    
    
    factory { MySimplePresenter(get()) }
    
    
    }
    

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31. FOSDEM - @marcoGomier
    class MyApplication : Application() {
    
    
    override fun onCreate() {
    
    
    super.onCreate()
    
    
    
    
    startKoin {
    
    
    androidLogger()
    
    
    androidContext(this@MyApplication)
    
    
    modules(appModule)
    
    
    }
    
    
    }
    
    
    }
    

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32. FOSDEM - @marcoGomier
    class MainActivity : AppCompatActivity() {
    
    
    val firstPresenter: MySimplePresenter by inject()
    
    
    override fun onCreate(savedInstanceState: Bundle?) {
    
    
    super.onCreate(savedInstanceState)
    
    
    setContentView(R.layout.activity_main)
    
    
    ...
    
    }
    
    
    }
    

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33. FOSDEM - @marcoGomier
    val appModule = module {
    
    
    single { JokeLocalDataSourceImpl() }
    
    
    single { JokeRepositoryImpl(get()) }
    
    
    }
    

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34. FOSDEM - @marcoGomier
    fun Application.module() {
    
    
    install(Koin) {
    
    
    slf4jLogger()
    
    
    modules(koinModules)
    
    
    }
    
    
    ...
    
    }
    

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35. FOSDEM - @marcoGomier
    fun Route.jokeEndpoint() {
    
    
    val jokeRepository by inject()
    
    
    get {
    
    
    call.respond(jokeRepository.getRandomJoke())
    
    
    }
    
    
    post { apiCallParams
    ->
    
    val name = apiCallParams.name
    
    
    jokeRepository.watch(name)
    
    
    call.respond("Ok")
    
    
    }
    
    
    }
    

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36. FOSDEM
    @marcoGomier
    Logging
    

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37. FOSDEM - @marcoGomier
    Timber
    class MyApplication : Application() {
    
    
    override fun onCreate() {
    
    
    super.onCreate()
    
    
    if (BuildConfig.DEBUG) {
    
    
    Timber.plant(Timber.DebugTree())
    
    
    } else {
    
    
    //
    Log to somewhere else
    
    
    }
    
    
    }
    
    
    }
    https://github.com/JakeWharton/timber
    

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38. FOSDEM - @marcoGomier https://github.com/JakeWharton/timber
    Timber.d(“Message")
    
    
    Timber.v("Message")
    
    
    Timber.i("Message")
    
    
    Timber.w("Message")
    
    
    Timber.wtf("Message")
    
    
    Timber.e(exception)
    Timber
    

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39. FOSDEM - @marcoGomier
    SLF4J
    http://www.slf4j.org/index.html
    routing {
    
    
    get("/api/v1") {
    
    
    call.application.environment.log.info("Hello from /api/v1!")
    
    
    }
    
    
    }
    fun Application.module(testing: Boolean = false) {
    
    
    log.info("Hello from module!")
    
    
    }
    

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40. FOSDEM - @marcoGomier
    val logger = LoggerFactory.getLogger(MyClass
    ::
    class.java)
    inline fun T.getLogger(): Logger {
    
    
    return LoggerFactory.getLogger(T
    ::
    class.java)
    
    
    }
    class MyClass {
    
    
    private val logger = getLogger()
    
    
    fun main() {
    
    
    logger.info("Hello World")
    
    
    }
    
    
    }
    

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41. FOSDEM - @marcoGomier
    logger.trace("Message")
    
    
    logger.debug("Message")
    
    
    logger.info("Message")
    
    
    logger.warn("Message")
    
    
    logger.error("Message")
    SLF4J
    

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42. FOSDEM - @marcoGomier
    logback.xml
    

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43. FOSDEM - @marcoGomier
    logback.xml
    
    
    
    
    
    
    
    
    
    %d{YYYY-MM-dd HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n
    
    pattern>
    
    
    
    encoder>
    
    
    
    appender>
    
    
    
    
    
    ${LOG_DEST}/ktor-chuck-norris-sample.log
    
    file>
    
    
    
    
    
    
    
    ${LOG_DEST}/ktor-chuck-norris-sample.%d{yyyy-MM-dd}.log
    
    fileNamePattern>
    
    
    
    
    ${LOG_MAX_HISTORY}
    
    maxHistory>
    
    
    3GB
    
    totalSizeCap>
    
    
    
    rollingPolicy>
    
    
    
    
    
    %d{YYYY-MM-dd HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n
    
    pattern>
    
    
    
    encoder>
    
    
    
    appender>
    
    
    
    
    
    />
    
    />
    
    
    root>
    
    
    />
    
    />
    
    />
    
    
    configuration>
    
    
    

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44. FOSDEM
    @marcoGomier
    Secrets
    

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45. FOSDEM - @marcoGomier
    local.properties
    tmdbKey="private_key"
    

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46. FOSDEM - @marcoGomier
    val String.byProperty: String?
    
    
    get() {
    
    
    val local = java.util.Properties()
    
    
    val localProperties: File = rootProject.file("local.properties")
    
    
    if (localProperties.exists()) {
    
    
    localProperties.inputStream().use { local.load(it) }
    
    
    return local.getProperty(this)
    
    
    }
    
    
    return null
    
    
    }
    tmdbKey="private_key"
    buildConfigField("String", "TMDB_KEY", "\"${"tmdbKey".byProperty
    ?:
    "")}\"")
    build.gradle.kts
    

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47. FOSDEM - @marcoGomier
    tmdbKey="private_key"
    buildConfigField("String", "TMDB_KEY", "\"${"tmdbKey".byProperty
    ?:
    "")}\"")
    val myKey: String = BuildConfig.TMDB_KEY
    NetworkApiInterceptor.kt
    

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48. FOSDEM - @marcoGomier
    application.conf
    

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49. FOSDEM - @marcoGomier
    application.conf
    ktor {
    
    
    deployment {
    
    
    port = 8080
    
    
    port = ${?PORT}
    
    
    }
    
    
    application {
    
    
    modules = [com.prof18.ktor.chucknorris.sample.ApplicationKt.module]
    
    
    }
    
    
    server {
    
    
    isProd = false
    
    
    }
    
    
    database {
    
    
    driverClass = "com.mysql.cj.jdbc.Driver"
    
    
    url = "jdbc:mysql:
    /
    /
    localhost:3308/chucknorris?useUnicode=true&characterEncoding=UTF-8"
    
    
    user = "root"
    
    
    password = "password"
    
    
    maxPoolSize = 3
    
    
    }
    
    
    }
    

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50. FOSDEM - @marcoGomier
    fun Application.setupConfig() {
    
    
    val appConfig by inject()
    
    
    //
    Server
    
    
    val serverObject = environment.config.config("ktor.server")
    
    
    val isProd = serverObject.property("isProd").getString().toBoolean()
    
    
    appConfig.serverConfig = ServerConfig(isProd)
    
    
    }
    data class ServerConfig(
    
    
    val isProd: Boolean
    
    
    )
    class AppConfig {
    
    
    lateinit var serverConfig: ServerConfig
    
    
    //
    Place here other configurations
    
    
    }
    

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51. FOSDEM - @marcoGomier
    fun Application.module() {
    
    
    ...
    
    
    
    setupConfig()
    
    
    val appConfig by inject()
    
    
    ...
    
    }
    
    
    

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52. FOSDEM - @marcoGomier
    java -jar ktor-backend.jar -config=/config-folder/application.conf
    

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53. FOSDEM
    @marcoGomier
    Testing
    

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54. FOSDEM - @marcoGomier
    • Unit Test
    ->
    Just regular Kotlin Unit tests
    
    
    • androidTest
    ->
    TestEngine
    Testing
    https://ktor.io/docs/testing.html
    

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55. FOSDEM - @marcoGomier
    @Test
    
    
    fun testRequests() = withTestApplication(module(testing = true)) {
    
    
    with(handleRequest(HttpMethod.Get, "/")) {
    
    
    assertEquals(HttpStatusCode.OK, response.status())
    
    
    assertEquals("Hello from Ktor Testable sample application", response.content)
    
    
    }
    https://ktor.io/docs/testing.html
    

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56. FOSDEM
    @marcoGomier
    Deploy
    

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57. FOSDEM - @marcoGomier
    Deploy
    • aar
    
    
    • apk
    
    
    • aab
    

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58. FOSDEM - @marcoGomier https://github.com/johnrengelman/shadow
    ./gradlew assembleRelease
    ./gradlew bundleRelease
    

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59. FOSDEM - @marcoGomier
    Deploy
    • Fat JAR
    
    
    • Executable JVM application
    
    
    • WAR
    
    
    • GraalVM
    https://ktor.io/docs/deploy.html
    

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60. FOSDEM - @marcoGomier
    • Fat JAR
    
    
    • Executable JVM application
    
    
    • WAR
    
    
    • GraalVM
    https://ktor.io/docs/deploy.html
    Deploy
    

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61. FOSDEM - @marcoGomier
    build.gradle.kts
    https://github.com/johnrengelman/shadow
    plugins {
    
    
    ...
    
    id("com.github.johnrengelman.shadow") version "7.0.0"
    
    
    }
    

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62. FOSDEM - @marcoGomier https://github.com/johnrengelman/shadow
    tasks {
    
    
    shadowJar {
    
    
    manifest {
    
    
    attributes(Pair("Main-Class", "io.ktor.server.netty.EngineMain"))
    
    
    }
    
    
    }
    
    
    }
    build.gradle.kts
    

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63. FOSDEM - @marcoGomier https://github.com/johnrengelman/shadow
    tasks {
    
    
    shadowJar {
    
    
    manifest {
    
    
    attributes(Pair("Main-Class", "io.ktor.server.netty.EngineMain"))
    
    
    }
    
    
    }
    
    
    }
    application {
    
    
    mainClass.set("io.ktor.server.netty.EngineMain")
    
    
    }
    build.gradle.kts
    

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64. FOSDEM - @marcoGomier https://github.com/johnrengelman/shadow
    ./gradlew shadowJar
    

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65. FOSDEM - @marcoGomier
    

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66. FOSDEM
    @marcoGomier
    Conclusions
    

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67. FOSDEM - @marcoGomier
    • Ktor is flexible and unopinionated
    ->
    Knowledge Transfer
    
    
    • Mobile knowledge can be adapted
    
    
    • Effective scaling and deploying can be hard
    
    
    • “Going to the other side” enrich your dev vision
    Conclusions
    

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68. FOSDEM - @marcoGomier https://github.com/prof18/ktor-chuck-norris-sample
    

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69. Bibliography / Useful Links
    • https:
    //
    ktor.io/
    
    
    • https:
    //
    ktor.io/learn/
    
    
    • https:
    //
    ktor.io/docs/welcome.html
    
    
    • https:
    //
    github.com/JetBrains/Exposed/wiki
    
    
    • https:
    //
    www.marcogomiero.com/posts/2021/ktor-project-structure/
    
    
    • https:
    //
    www.marcogomiero.com/posts/2021/ktor-logging-on-disk/
    
    
    • https:
    //
    www.marcogomiero.com/posts/2021/ktor-in-memory-db-testing/
    

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70. Marco Gomiero
    FOSDEM
    Thank you!
    
    
    > Twitter: @marcoGomier
    

    > Github: prof18
    

    > Website: marcogomiero.com
    

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