An example of an asynchronous service using futures to be run on an event loop

event-loop-future-example.scala

import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.Future


class FileApp(val fileCounter: FileCounter) extends Service {

  private def countWordsV1(number: Int): Future[Int] = fileCounter.countWordsInFile(s"${number}a")
    .flatMap(c1 => fileCounter.countWordsInFile(s"${number}b").map(c2 => c1 + c2))

  private def countWordsV2(number: Int): Future[Int] = {
    for {
      c1 <- fileCounter.countWordsInFile(s"${number}a")
      c2 <- fileCounter.countWordsInFile(s"${number}b")
    } yield c1 + c2
  }

  def apply(line: String): Future[String] = Future.unit.map { _ =>
    // NB: the following line might throw an exception
    // so we're doing the conversion inside `map`
    // to wrap any exception in a failed future
    line.toInt
  }.flatMap(i => countWordsV2(i)).map(_.toString)
}

trait FileCounter {
  def countWordsInFile(id: String): Future[Int]
}

/**
 * The service that you "plug in" to the event loop.
 * The event loop calls the `apply()` function when a new line is ready
 * and
 */
trait Service {
  def apply(line: String): Future[String]
}