Scala stack to build a microservice Alexey Novakov, INNOQ 

About me 4yrs in Scala, 10yrs in Java Senior Consultant at INNOQ Data Processing, Distributed Systems and Functional Programming fan 

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Typelevel Others Lightbend Origin of the libraries 

Cats, Shapeless 

(★ GitHub Stars) ★ 

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Let’s build a typical microservice 

• HTTP-Server: Akka-HTTP • JSON Serialization: Spray-JSON or uPickle • Persistence: Slick • Logging: Scala-logging • Configuration: Typesafe Config, Pure Config, Refined • Testing: Scalatest, test-containers-scala • FP: Cats • Build Tool: SBT Tooling 

REST API Service Architecture Domain Model Postgres SQL HTTP Client 

Add: POST /api/v1/trips, body = JSON Update: PUT /api/v1/trips/, body = JSON Delete: DELETE /api/v1/trips/ Select all: GET /api/v1/trips?sort=id&page=1&pageSize=100 Select one: GET /api/v1/trips/ Ride booking API 

Models final case class Trip( id: Int, city: String, vehicle: Vehicle, price: Int, completed: Boolean, distance: Option[Int], endDate: Option[LocalDate] ) final case class Trips(trips: Seq[Trip]) object Vehicle extends Enumeration { type Vehicle = Value val Bike, Taxi, Car = Value } 

HTTP Server • Akka-HTTP • Options: 1. Core Server API (HttpRequest => HttpResponse) 2. Routing DSL (Directives) • helps to make code DRY • but requires to learn a DSL (error-prone, time consuming) 

val route: Route = pathPrefix("api" / "v1" / "trips") { concat( pathEndOrSingleSlash { get { parameters('sort.?, 'page.as[Int].?, 'pageSize.as[Int].?) { (sort, page, pageSize) => log.debug("Select all sorted by '{}'", sort) val cars = service.selectAll(page, pageSize, sort) complete(cars) } } }, val serverBinding = Http().bindAndHandle(, server.host, server.port) Routing DSL …. import akka.http.scaladsl.server.Directives._ 

JSON En/Decoding • Spray-JSON: • integrated via akka-http-spray-json library • uPickle: • String -> AST -> Case Class • integrated via de.heikoseeberger:akka-http-upickle library 

JSON Codes Spray-JSON import spray.json.DefaultJsonProtocol._ implicit val tripFormat: RootJsonFormat[Trip] = jsonFormat7(Trip) implicit val tripsFormat: RootJsonFormat[Trips] = jsonFormat1(Trips) Upickle import upickle.default._ implicit val tripRW: ReadWriter[Trip] = macroRW implicit val tripsRW: ReadWriter[Trips] = macroRW 

Usage of JSON Codecs post { entity(as[Trip]) { trip: Trip => log.debug("Create new trip '{}'", trip) val inserted = service.insert(trip) complete { toCommandResponse(inserted, CommandResult) } } } 

Data Layer trait Repository[F[_]] { def delete(id: Int) : F[Int] def update(id: Int, row: Trip): F[Int] def createSchema(): F[Unit] def insert(row: Trip): F[Int] def selectAll(page: Int, pageSize: Int, sort: String): F[Seq[Trip]] def select(id: Int): F[Option[Trip]] def sortingFields: Set[String] } 

Typical Implementation in class Trips(tag: Tag) extends Table[Trip](tag, "trips") { def id = column[Int]("id", O.AutoInc, O.PrimaryKey) def city = column[String]("city") def price = column[Int]("price") def completed = column[Boolean]("completed") override def * = (id, city, vehicle, price, completed, distance, endDate) <> (Trip.tupled, Trip.unapply) } 

def select(id: Int): Future[Option[Trip]] = db.run(trips.filter(_.id === id).take(1).result.headOption) val trips = TableQuery[Trips] class TripRepository(db: Database) extends Repository[Future] { } …. 

Generic Types HTTP Routes: RequestContext 㱺 Future[RouteResult] TripService[F[_]] Repository[F[_]] TripRepo[Future] TripService[Future] Slick (can be replaced by Doobie, etc.) Akka HTTP (can be replaced by Play, http4s, etc.) Implementation Application, (framework, library free) 

FP • We want TripService be generic in its return type: F[_] def selectAll(…): F[Trips] • F[_] - can be later set to Future, Task, IO, Id, etc. • Cats Type Classes to the rescue! 

FP: Cats Usage def selectAll(page: Option[Int], pageSize: Option[Int], sort: Option[String]): F[Trips] = { ….. repo .selectAll(pageN, size, sort) // <— F[Seq[Trip]] .map(Trips) } import cats.Functor import cats.syntax.functor._ class TripService[F[_]](repo: Repository[F])(implicit F: Functor[F]) { 

Scala Logging • It wraps slf4j and requires logging backend library like logback • Usage: class MyClass extends LazyLogging { logger.debug("This is very convenient ;-)”) … } “com.typesafe.scala-logging:scala-logging" if (logger.isDebugEnabled) logger.debug(s"Some $expensive message!") • check-enabled-idiom is applied automatically by Scala macros 

Configuration: HOCON foo { bar = 10 baz = 12 } foo.bar =10 foo.baz =12 Or Example: Human-Optimized Config Object Notation Typesafe Config library 

server { host = localhost port = 8080 } storage { host = localhost port = 5432 dbName = trips url = "jdbc:postgresql://"${storage.host}":"${storage.port}"/"${storage.dbName} driver = "org.postgresql.Driver" user = “trips" password = "trips" } application.conf 

import pureconfig.loadConfig import pureconfig.generic.auto._ val path = … // path to application.conf def load: Either[ConfigReaderFailures, Server] = { loadConfig[Server](Paths.get(path), “server”) } “com.github.pureconfig:pureconfig" final case class Server( host: String = "localhost", port: Int = 8080 ) 

“eu.timepit:refined-pureconfig" import eu.timepit.refined.types.net.UserPortNumber import eu.timepit.refined.types.string.NonEmptyString import eu.timepit.refined.pureconfig._ [info] Compiling 2 Scala sources to /Users/aa/dev/git/akka-crud-service/target/scala-2.12/classes ... [error] /Users/aa/dev/git/akka-crud-service/src/main/scala/org/alexeyn/configs.scala:39:84: Left predicate of (!(808 < 1024) && !(808 > 49151)) failed: Predicate (808 < 1024) did not fail. [error] final case class Server(host: NonEmptyString = "localhost", port: UserPortNumber = 808) final case class Server( host: NonEmptyString = "localhost", port: UserPortNumber = 808 ) 

Custom Types type ConnectionTimeout = Int Refined Interval.OpenClosed[W.`0`.T, W.`100000`.T] type MaxPoolSize = Int Refined Interval.OpenClosed[W.`0`.T, W.`100`.T] type JdbcUrl = String Refined MatchesRegex[W.`"""jdbc:\\w+://\\w+:[0-9]{4,5}/\\w+"""`.T] final case class JdbcConfig( url: JdbcUrl, connectionTimeout: ConnectionTimeout, maximumPoolSize: MaxPoolSize ) 

Testing • Scalatest + akka-testkit: • path: HTTP Routes Application • test-containers-scala + … • to test integration • path: HTTP Routes Application Postgres Container • Note mock libraries like Mockito are not really needed in Scala projects. Thanks to FP 

private val stubRepo = createStubRepo private val service = new TripService[Future](stubRepo) private val routes = CommandRoutes.routes(service) "CommandRoutes" should { "insert new trip and return its id" in { val request: HttpRequest = RequestsSupport.insertRequest(berlin) request ~> routes ~> check { val count = entityAs[CommandResult].count count should ===(1) } } Testing Application 

class E2ETest extends WordSpec … with ForAllTestContainer override val container = PostgreSQLContainer("postgres:10.4") lazy val cfg: Config = ConfigFactory.load( ConfigFactory .parseMap( Map( "port" -> container.mappedPort(5432), "url" -> container.jdbcUrl, "user" -> container.username, "password" -> container.password ).asJava).atKey("storage") ) lazy val mod = new Module(cfg, createSchema = false) 

Container is started by Scalatest 

SBT Plugins plugins.sbt: addSbtPlugin("com.typesafe.sbt" % "sbt-native-packager" % “x.y.z”) build.sbt: enablePlugins(JavaAppPackaging, AshScriptPlugin) FROM openjdk:8-jre-alpine WORKDIR /opt/docker ADD --chown=daemon:daemon opt /opt USER daemon ENTRYPOINT ["/opt/docker/bin/akka-crud-service"] CMD [] Generates busybox compatible script $ sbt docker:publishLocal 

Useful SBT Plugins 1. sbt-release addSbtPlugin(“com.github.gseitz" % "sbt-release" % “x.y.z") Easy to bump version and commit to Git. 1.2.1-SNAPSHOT -> 1.2.1 -> 1.2.2-SNAPSHOT 2. sbt-scalafmt addSbtPlugin("com.geirsson" % "sbt-scalafmt" % “x.y.z") Can check & fail the build, format sources 

Dependency Injection val db = Database.forConfig("storage", cfg) // class TripRepository(db: Database) val repo = wire[TripRepository] // class TripService[F[_]](repo: Repository[F]) val service = wire[TripService[Future]] // class QueryRoutes(service: TripService[Future]) val routes = concat(wire[QueryRoutes].routes, wire[CommandRoutes].routes) import com.softwaremill.macwire._ 

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More Info 1. Scalar 2018 whiteboard voting results! https://blog.softwaremill.com/scalar-2018-whiteboard-voting-results-c6f50f8fb16d 2. Scala Developer Survey Results 2018 (link) 3. Scaladex: https://index.scala-lang.org 4. Source Code: https://github.com/novakov-alexey/akka-crud-service 5. Some Scala posts: https://medium.com/se-notes-by-alexey-novakov Twitter: @alexey_novakov