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
2. FOSDEM - @marcoGomier Marco Gomiero 👨💻 Android Engineer @ TIER 🛴 🇩🇪 🇮🇹 
 Google Developer Expert for Kotlin > Twitter: @marcoGomier 
 > Github: prof18 
 > Website: marcogomiero.com
3. FOSDEM - @marcoGomier 🤔 Why?
4. FOSDEM - @marcoGomier • Side project • Understand how things works “on the other side” • Help you teammates 🤔 Why?
5. FOSDEM - @marcoGomier * based on a true story
6. FOSDEM - @marcoGomier Ktor https://ktor.io/
7. FOSDEM - @marcoGomier Ktor • Kotlin • Lightweight and flexible • Asynchronous with Coroutines • Unopinionated
8. FOSDEM - @marcoGomier Unopinionated • Whatever architecture • Whatever pattern Knowledge Transfer
9. FOSDEM @marcoGomier Architecture
10. FOSDEM - @marcoGomier Android Activity/Fragment ViewModel Repository Local Data Source Remote Data Source Application
11. FOSDEM - @marcoGomier Resource* Repository Local Data Source Remote Data Source Application * or Controller Ktor
12. FOSDEM - @marcoGomier Resource Repository Local Data Source Remote Data Source Application Activity/Fragment ViewModel Repository Local Data Source Remote Data Source Application
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
14. FOSDEM - @marcoGomier Application class MyApp : Application() { override fun onCreate() { super.onCreate() if (BuildConfig.DEBUG) { Timber.plant(Timber.DebugTree()) } initAnalytics() initCrashReporting() initRandomLib() } }
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
16. FOSDEM - @marcoGomier Plugin https://ktor.io/docs/plugins.html • Add a specific feature to your backend • Highly customisable • No plugin activated by default
17. FOSDEM - @marcoGomier Plugin https://ktor.io/docs/
18. FOSDEM - @marcoGomier Domain Data Presentation Application
19. FOSDEM - @marcoGomier Presentation class MainActivity : AppCompatActivity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) ... } }
20. FOSDEM - @marcoGomier fun Route.jokeEndpoint() { val jokeRepository by inject<JokeRepository>() get<JokeEndpoint.Random> { call.respond(jokeRepository.getRandomJoke()) } post<JokeEndpoint.Watch> { apiCallParams -> val name = apiCallParams.name jokeRepository.watch(name) call.respond("Ok") } } Presentation
21. FOSDEM - @marcoGomier Domain Data Presentation Application
22. FOSDEM - @marcoGomier Domain class JokeRepositoryImpl( private val jokeLocalDataSource: JokeLocalDataSource ) : JokeRepository { override suspend fun getRandomJoke(): JokeDTO { // ... } }
23. FOSDEM - @marcoGomier Domain Data Presentation Application
24. FOSDEM - @marcoGomier Room class LocalDatasource( private val db: RoomDatabase ) { suspend fun getAllowedApps(): List<AllowedApp> { 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<AllowedApp> } Data
25. FOSDEM - @marcoGomier Exposed https://github.com/JetBrains/Exposed
26. FOSDEM - @marcoGomier Exposed class JokeLocalDataSourceImpl : JokeLocalDataSource { override suspend fun getAllJokes(): List<Joke> { val joke = newSuspendedTransaction { val query = JokeTable.selectAll() Joke.wrapRows(query).toList() } } } object JokeTable: IdTable<String>(name = "joke") { val createdAt = datetime("created_at") val updatedAt = datetime("updated_at") val value = text("value") override val id: Column<EntityID<String >> = varchar("joke_id", 255).entityId() override val primaryKey: PrimaryKey = PrimaryKey(id) } class Joke(id: EntityID<String>): Entity<String>(id) { companion object: EntityClass<String, Joke>(JokeTable) var createdAt by JokeTable.createdAt var updatedAt by JokeTable.updatedAt var value by JokeTable.value } Data
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)
28. FOSDEM @marcoGomier Dependency Injection
29. FOSDEM - @marcoGomier https://insert-koin.io/docs/quickstart/ktor Koin
30. FOSDEM - @marcoGomier val appModule = module { single<HelloRepository> { HelloRepositoryImpl() } factory { MySimplePresenter(get()) } }
31. FOSDEM - @marcoGomier class MyApplication : Application() { override fun onCreate() { super.onCreate() startKoin { androidLogger() androidContext(this@MyApplication) modules(appModule) } } }
32. FOSDEM - @marcoGomier class MainActivity : AppCompatActivity() { val firstPresenter: MySimplePresenter by inject() override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) ... } }
33. FOSDEM - @marcoGomier val appModule = module { single<JokeLocalDataSource> { JokeLocalDataSourceImpl() } single<JokeRepository> { JokeRepositoryImpl(get()) } }
34. FOSDEM - @marcoGomier fun Application.module() { install(Koin) { slf4jLogger() modules(koinModules) } ... }
35. FOSDEM - @marcoGomier fun Route.jokeEndpoint() { val jokeRepository by inject<JokeRepository>() get<JokeEndpoint.Random> { call.respond(jokeRepository.getRandomJoke()) } post<JokeEndpoint.Watch> { apiCallParams -> val name = apiCallParams.name jokeRepository.watch(name) call.respond("Ok") } }
36. FOSDEM @marcoGomier Logging
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
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
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!") }
40. FOSDEM - @marcoGomier val logger = LoggerFactory.getLogger(MyClass :: class.java) inline fun <reified T> T.getLogger(): Logger { return LoggerFactory.getLogger(T :: class.java) } class MyClass { private val logger = getLogger() fun main() { logger.info("Hello World") } }
41. FOSDEM - @marcoGomier logger.trace("Message") logger.debug("Message") logger.info("Message") logger.warn("Message") logger.error("Message") SLF4J
42. FOSDEM - @marcoGomier logback.xml
43. FOSDEM - @marcoGomier logback.xml <configuration> <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender"> <encoder> <pattern>%d{YYYY-MM-dd HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n </ pattern> </ encoder> </ appender> <appender name="FILE" class="ch.qos.logback.core.rolling.RollingFileAppender"> <file>${LOG_DEST}/ktor-chuck-norris-sample.log </ file> <rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy"> <!-- daily rollover --> <fileNamePattern>${LOG_DEST}/ktor-chuck-norris-sample.%d{yyyy-MM-dd}.log </ fileNamePattern> <!-- keep 90 days' worth of history capped at 3GB total size --> <maxHistory>${LOG_MAX_HISTORY} </ maxHistory> <totalSizeCap>3GB </ totalSizeCap> </ rollingPolicy> <encoder> <pattern>%d{YYYY-MM-dd HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n </ pattern> </ encoder> </ appender> <root level="info"> <appender-ref ref="STDOUT" /> <appender-ref ref="FILE" /> </ root> <logger name="org.eclipse.jetty" level="INFO" /> <logger name="io.netty" level="INFO" /> <logger name="org.quartz" level="INFO" /> </ configuration>
44. FOSDEM @marcoGomier Secrets
45. FOSDEM - @marcoGomier local.properties tmdbKey="private_key"
46. FOSDEM - @marcoGomier val String.byProperty: String? get() { val local = java.util.Properties() val localProperties: File = rootProject.file("local.properties") if (localProperties.exists()) { localProperties.inputStream().use { local.load(it) } return local.getProperty(this) } return null } tmdbKey="private_key" buildConfigField("String", "TMDB_KEY", "\"${"tmdbKey".byProperty ?: "")}\"") build.gradle.kts
47. FOSDEM - @marcoGomier tmdbKey="private_key" buildConfigField("String", "TMDB_KEY", "\"${"tmdbKey".byProperty ?: "")}\"") val myKey: String = BuildConfig.TMDB_KEY NetworkApiInterceptor.kt
48. FOSDEM - @marcoGomier application.conf
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 } }
50. FOSDEM - @marcoGomier fun Application.setupConfig() { val appConfig by inject<AppConfig>() // 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 }
51. FOSDEM - @marcoGomier fun Application.module() { ... setupConfig() val appConfig by inject<AppConfig>() ... }
52. FOSDEM - @marcoGomier java -jar ktor-backend.jar -config=/config-folder/application.conf
53. FOSDEM @marcoGomier Testing
54. FOSDEM - @marcoGomier • Unit Test -> Just regular Kotlin Unit tests • androidTest -> TestEngine Testing https://ktor.io/docs/testing.html
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
56. FOSDEM @marcoGomier Deploy
57. FOSDEM - @marcoGomier Deploy • aar • apk • aab
58. FOSDEM - @marcoGomier https://github.com/johnrengelman/shadow ./gradlew assembleRelease ./gradlew bundleRelease
59. FOSDEM - @marcoGomier Deploy • Fat JAR • Executable JVM application • WAR • GraalVM https://ktor.io/docs/deploy.html
60. FOSDEM - @marcoGomier • Fat JAR • Executable JVM application • WAR • GraalVM https://ktor.io/docs/deploy.html Deploy
61. FOSDEM - @marcoGomier build.gradle.kts https://github.com/johnrengelman/shadow plugins { ... id("com.github.johnrengelman.shadow") version "7.0.0" }
62. FOSDEM - @marcoGomier https://github.com/johnrengelman/shadow tasks { shadowJar { manifest { attributes(Pair("Main-Class", "io.ktor.server.netty.EngineMain")) } } } build.gradle.kts
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
64. FOSDEM - @marcoGomier https://github.com/johnrengelman/shadow ./gradlew shadowJar
65. FOSDEM - @marcoGomier
66. FOSDEM @marcoGomier Conclusions
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
68. FOSDEM - @marcoGomier https://github.com/prof18/ktor-chuck-norris-sample
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/
70. Marco Gomiero FOSDEM Thank you! > Twitter: @marcoGomier 
 > Github: prof18 
 > Website: marcogomiero.com