Suppose I want to make coffee. This involves 4 steps:

• 1a. grind coffee beans
• 1b. heat water
• 2. combine
• 3. filter

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Building a rest API with akka and spray is easy. This is how I did it: SprayApiApp:

import akka.actor.{ActorSystem, Props}
import akka.io.IO
import akka.pattern.ask
import akka.util.Timeout
import spray.can.Http
import scala.concurrent.duration._

object SprayApiApp extends App {
  //we need an ActorSystem to host our application in
  implicit val system = ActorSystem("SprayApiApp")

  //create apiActor
  val apiActor = system.actorOf(Props[ApiActor], "apiActor")

  //timeout needs to be set as an implicit val for the ask method (?)
  implicit val timeout = Timeout(5.seconds)

  //start a new HTTP server on port 8080 with apiActor as the handler
  IO(Http) ? Http.Bind(apiActor, interface = "localhost", port = 8080)
}

ApiActor:

import akka.actor.{ActorLogging, Actor}
import spray.http.MediaTypes
import spray.httpx.SprayJsonSupport._
import spray.json.DefaultJsonProtocol
import spray.routing._

object RobotProtocol extends DefaultJsonProtocol {
  //Our domain class
  case class Robot(name: String)

  //We use the default json marshalling for Robot.
  //There are multiple jsonFormat methods in DefaultJsonProtocol. Depending on how many parameters the model class has.
  //Robot has just one, so we use jsonFormat1
  implicit val RobotFormat = jsonFormat1(Robot)
}
import RobotProtocol._

class ApiActor extends Actor with HttpService with ActorLogging {
  //A list of our domain objects
  var robots = List(Robot("R2D2"), Robot("Asimo"))

  //The HttpService trait defines only one abstract member, which
  //connects the services environment to the enclosing actor or test
  def actorRefFactory = context

  //This actor only runs our route, but you could add
  //other things here, like request stream processing or timeout handling
  def receive = runRoute(apiRoute)

  //Notice that both path methods return a Route. We need to chain them together with ~
  val apiRoute: Route =
    path("robots") {
      get { //with get we will return our current list of robots
        log.info("Building get route")
        complete {
          log.info("Executing get route")
          //complete will return the result in an appropriate format
          //With SprayJsonSupport it knows how to marshall a List to json
          //With RobotFormat it knows how to marshall Robot
          robots
        }
      } ~ post { //With post we will add a robot
        log.info("Building post route")
        handleWith { robot: Robot =>  //handleWith will unmarshall the input
          log.info("Executing post route")
          robots = robot :: robots
          robot //handleWith will also marshall the result. Here we simply return the new robot.
        }
      }
    } ~ path("") { //When we go to localhost:8080/ just show a link to localhost:8080/robots
      respondWithMediaType(MediaTypes.`text/html`) { //XML is marshalled to `text/xml` by default, so we simply override here
        complete {
          [The list of robots](/robots)
        }
      }
    }
}

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When you start using Scala, it's tempting to also start using sbt. You can also use your favorite build tool: Gradle. This is my default Gradle build file:

apply plugin: 'scala'

repositories {
  mavenCentral()
}

dependencies {
  compile group: 'org.scala-lang', name: 'scala-library', version: '2.11.5'
  compile group: 'com.typesafe.akka', name: 'akka-actor_2.11', version: '2.3.9'
  compile group: 'com.typesafe.akka', name: 'akka-remote_2.11', version: '2.3.9'
  testCompile group: 'org.scalatest', name: 'scalatest_2.11', version: '2.2.4'
}

//run the akka application
task run(type: JavaExec, dependsOn: classes) {
  //object to run. (The one that extends App)
  main = 'pi.Pi'
  classpath sourceSets.main.runtimeClasspath
  classpath configurations.runtime
}

//run scala tests. These are not automatically picked up by gradle,
//so we run them like this.
task spec(dependsOn: ['testClasses'], type: JavaExec) {
  main = 'org.scalatest.tools.Runner'
  args = ['-R', 'build/classes/test', '-o']
  classpath = sourceSets.test.runtimeClasspath
}

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