move code to src/test

* so that it compiles and tests pass
* fix some additional snip references in getting started

akka-docs/src/test/scala/docs/cluster/FactorialBackend.scala

@@ -0,0 +1,46 @@
+package scala.docs.cluster
+
+import scala.annotation.tailrec
+import scala.concurrent.Future
+import com.typesafe.config.ConfigFactory
+import akka.actor.Actor
+import akka.actor.ActorLogging
+import akka.actor.ActorSystem
+import akka.actor.Props
+import akka.pattern.pipe
+
+//#backend
+class FactorialBackend extends Actor with ActorLogging {
+
+  import context.dispatcher
+
+  def receive = {
+    case (n: Int) =>
+      Future(factorial(n)) map { result => (n, result) } pipeTo sender()
+  }
+
+  def factorial(n: Int): BigInt = {
+    @tailrec def factorialAcc(acc: BigInt, n: Int): BigInt = {
+      if (n <= 1) acc
+      else factorialAcc(acc * n, n - 1)
+    }
+    factorialAcc(BigInt(1), n)
+  }
+
+}
+//#backend
+
+object FactorialBackend {
+  def main(args: Array[String]): Unit = {
+    // Override the configuration of the port when specified as program argument
+    val port = if (args.isEmpty) "0" else args(0)
+    val config = ConfigFactory.parseString(s"akka.remote.netty.tcp.port=$port").
+      withFallback(ConfigFactory.parseString("akka.cluster.roles = [backend]")).
+      withFallback(ConfigFactory.load("factorial"))
+
+    val system = ActorSystem("ClusterSystem", config)
+    system.actorOf(Props[FactorialBackend], name = "factorialBackend")
+
+    system.actorOf(Props[MetricsListener], name = "metricsListener")
+  }
+}