batch.scala

import java.util.concurrent.ConcurrentLinkedQueue
import scala.annotation.tailrec
import scala.concurrent.{ExecutionContext, Future, Promise}
implicit val ec =  ExecutionContext.global

val data = Map(
  "getBff 1" -> 3,
  "getBff 2" -> 4,
  "getFN 1" -> "Harry",
  "getFN 2" -> "Biff",
  "getLN 3" -> "Jones",
  "getLN 4" -> "Smith")

case class Query[A](s: String) {
  private val p = Promise[A]()
  def future = p.future
  def fulfill(): Unit = p.success(data(s).asInstanceOf[A])
}
val queries = new ConcurrentLinkedQueue[Query[_]]
def query[A](cmd: String): Future[A] = {
  val q = Query(cmd)
  queries.add(q)
  q.future
}

import scala.annotation.tailrec
@tailrec
def runBatch(): Unit = {
  val q = queries.poll()
  if (q ne null) {
    q.fulfill()
    runBatch()
  }
}

def getBff(i: Int) = query[Int](s"getBff $i")
def getFirstName(i: Int) = query[String](s"getFN $i")
def getLastName(i: Int): Future[String] = query[String](s"getLN $i")

def par[A,B](f1: Future[A], f2: Future[B]): Future[(A,B)] = for(a<-f1;b<-f2) yield(a,b)

val x = Future.traverse(Seq(1,2))(i =>
  for {
    bff <- getBff(i)
    fn <- getFirstName(i)
    // (bff, fn) <- par(getBff(i), getFirstName(i))
    ln <- getLastName(bff)
  } yield s"$fn $ln")