Using Rx to update the configuration stream. No variables to hold the state.

CircularConfiguration.scala

package com.example.cosa

import rx.lang.scala.subjects.{BehaviorSubject, PublishSubject}

object ConfigurationExample1 extends App {

  val poleSubject = PublishSubject[String]()
  val configurations = BehaviorSubject(Map.empty[Pole, Config])

  configurations.subscribe(x => println("Configurations: " + x))

  poleSubject
    .map({
      case x if x.startsWith("L") => Config(LeftPole, "Lefty! " + x)
      case x => Config(RightPole, "Poling to the right... " + x)
    })
    .withLatestFrom(configurations) { case (config, configMap) =>
      println("Receiving configuration for " + config.pole)
      configMap.get(config.pole).foreach(item => println("Mapping over existing config to unsubscribe"))
      configMap + (config.pole -> config)
    }
    .subscribe(configurations)

  poleSubject.onNext("L01")
  poleSubject.onNext("R02")
  poleSubject.onNext("L99")

  sealed trait Pole
  case object LeftPole extends Pole
  case object RightPole extends Pole

  case class Config(pole: Pole, data: String)

}

/**
  * Output:
  * Configurations: Map()
  * Receiving configuration for LeftPole
  * Configurations: Map(LeftPole -> Config(LeftPole,Lefty! L01))
  * Receiving configuration for RightPole
  * Configurations: Map(LeftPole -> Config(LeftPole,Lefty! L01), RightPole -> Config(RightPole,Poling to the right... R02))
  * Receiving configuration for LeftPole
  * Mapping over existing config to unsubscribe
  * Configurations: Map(LeftPole -> Config(LeftPole,Lefty! L99), RightPole -> Config(RightPole,Poling to the right... R02))
  */