diff --git a/README.md b/README.md
index 74ca525..a932eee 100644
--- a/README.md
+++ b/README.md
@@ -1,7 +1,6 @@
 # GUI State Machine API
 
-Service for the creation and modification of state machines of GUI tests based on a genetic algorithm.
-This [REST](https://en.wikipedia.org/wiki/Representational_state_transfer) service stores the NFA in a [graph database](https://en.wikipedia.org/wiki/Graph_database).
+REST service for the creation and modification of nondeterministic finite automaton of GUI tests based on a genetic algorithm.
 The service hides the actual implementation and defines a fixed interface for calls.
 Therefore, calling systems do not depend on the concrete implementation and it can be mocked easily for tests.
 
@@ -19,32 +18,20 @@ Use the command `sbt run` to start the REST service.
 Use the command `sbt assembly` to create a standalone JAR which includes all dependencies including the Scala libraries.
 The standalone JAR is generated as `target/scala-<scalaversion>/gui-state-machine-api-assembly-<version>.jar`.
 
-## Model Notation
-An [NFA](https://en.wikipedia.org/wiki/Nondeterministic_finite_automaton) represents the states of the GUI.
-The transitions are possible GUI actions.
-The model is based on the UI model from [Search-Based System Testing: High Coverage, No False Alarms][http://www.specmate.org/papers/2012-07-Search-basedSystemTesting-HighCoverageNoFalseAlarms.pdf] (section "4.5 UI Model").
-Whenever an unknown state is replaced by a newly discovered state, the model is updated.
-
-Open questions:
-
-* How is the state machine represented in the current version of retest?
-* state machine != NFA != graph? It is definitely an NFA since multiple outoing transitions are allowed.
-* REST -> HTTP what about the performance? Alternatively use a direct API in Scala? Should be possible from Java: <https://lampwww.epfl.ch/~michelou/scala/using-scala-from-java.html>
-* What kind of analysises on the state machines are required except for the next possible actions?
-* Whenever a new state is discovered, is the transition to the unknown state removed? Couldn't there be more states to be discovered? Doesn't matter?
-* EFG model?
-* How should the differences of the different versions of the state machines be stored?
-* The demo of Retest allows recording and replaying tests. The same can be done with <https://www.seleniumhq.org/projects/ide/>, so benefits are the automatic test generation/training and the difference testing?
-
-## Legacy code
-The package `de.retest.graph` contains all legacy classes.
-The package exists in version with GIT tag `retest-2.4.1`.
-
-## Definitions
-These definitions have been made by Furrer in his master's thesis.
-See section `2.3.1 Problemrepräsentation` for a detailed definition of the SUT model.
-The primary source for the definitions is [Exploring Realistic Program Behavior](https://www.st.cs.uni-
-saarland.de/publications/files/fgross-tr-2012.pdf).
+## Eclipse Support
+Use the command `sbt eclipse` to generate a project for Eclipse.
+
+## Bash Scripts for REST Calls
+The directory [scripts](./scripts) contains a number of Bash scripts which use `curl` to send REST calls to a running server.
+
+## NFA for the Representation of Tests
+A nondeterministic finite automaton represents the states of the GUI during the test.
+The actions executed by the user on the widgets are the transitions.
+If an action has not been executed, it leads to an unknown state.
+The NFA is based on the UI model from [Search-Based System Testing: High Coverage, No False Alarms][http://www.specmate.org/papers/2012-07-Search-basedSystemTesting-HighCoverageNoFalseAlarms.pdf] (section "4.5 UI Model").
+Whenever an unknown state is replaced by a newly discovered state, the NFA has to be updated.
+
+**At the moment, the following definitions are incomplete and must be adapted to the actual implementation which calls this service.**
 
 ### Test Suite
 A test suite is a set of test cases.
@@ -70,49 +57,4 @@ Some suggestions on how the REST API could look like:
 * `/application/<long>` GET queries a registered GUI application
 * `/application/<long>/test-suites` GET queries all test suites for an existing GUI application
 * `/application/<long>/create-test-suite` POST registers a new test suite for an existing GUI application
-* `/application/<long>/test-suite/<long>` GET queries a registered test suite for an existing GUI application
-
-## APIs suggested in the project proposal of Surili
-* [Selenium WebDriver](http://seleniumhq.org/docs/03_webdriver.jsp)
-* [Gherkin](https://github.com/cucumber/cucumber/wiki/Gherkin)
-
-## REST Frameworks for Scala
-* Good example of a REST service with Akka: <https://github.com/ArchDev/akka-http-rest>
-* <https://doc.akka.io/docs/akka-http/current/>
-* <https://www.playframework.com/>
-* <https://www.reddit.com/r/scala/comments/6izqac/akka_http_vs_play_ws_what_is_the_current_state/>
-* <https://slides.com/leonardoregnier/deck#/>
-* <https://nordicapis.com/8-frameworks-to-build-a-web-api-in-scala/>
-* Akk HTTP test examples: <https://github.com/akka/akka-http/tree/master/akka-http-tests/src/test/scala/akka/http/scaladsl/server/directives>
-* Replaced by Akka HTTP: <http://spray.io/>
-
-If REST is too slow, Scala can still be called directly via Java.
-
-## NFA Frameworks
-TODO Add some.
-
-## Graph Databases
-[List of graph databases](https://en.wikipedia.org/wiki/Graph_database#List_of_graph_databases)
-
-### Neo4j
-* https://neo4j.com/ ([Licensing](https://neo4j.com/licensing/))
-* https://github.com/neo4j/neo4j
-* https://github.com/FaKod/neo4j-scala/ (Scala Wrapper)
-* https://github.com/seancheatham/scala-graph
-* https://neo4j.com/blog/neo4j-scala-introduction/
-
-### TinkerPop3
-* http://tinkerpop.apache.org/
-* https://github.com/apache/tinkerpop
-* https://users.scala-lang.org/t/scala-graph-combined-with-a-graph-db/2824
-* https://github.com/mpollmeier/gremlin-scala
-
-## Automatic Test Generation
-* [EvoSuite](http://www.evosuite.org/)
-* [Randoop](https://randoop.github.io/randoop/)
-* [Guitar](https://sourceforge.net/projects/guitar/)
-
-## Literature
-* [Search-Based System Testing: High Coverage, No False Alarms](https://dl.acm.org/citation.cfm?id=2336762)
-* Machine Learning and Evolutionary Computing for GUI-based Regression Testing, Master's Thesis by Daniel Kraus
-* Vergleichsstudie maschineller Testverfahren für GUI-basierte Systeme, Master's Thesis by Felix Furrer
\ No newline at end of file
+* `/application/<long>/test-suite/<long>` GET queries a registered test suite for an existing GUI application
\ No newline at end of file
diff --git a/build.sbt b/build.sbt
index aee92b6..1bb36f2 100644
--- a/build.sbt
+++ b/build.sbt
@@ -6,6 +6,7 @@ organization := "tdauth"
 
 scalaVersion := "2.12.7"
 
+libraryDependencies += "io.spray" % "spray-json_2.12" % "1.3.4"
 libraryDependencies += "com.typesafe.akka" %% "akka-http" % "10.1.5"
 libraryDependencies += "com.typesafe.akka" %% "akka-http-core" % "10.1.5"
 libraryDependencies += "com.typesafe.akka" %% "akka-stream" % "2.5.12"
diff --git a/scripts/applications.sh b/scripts/applications.sh
new file mode 100644
index 0000000..a61b8e8
--- /dev/null
+++ b/scripts/applications.sh
@@ -0,0 +1,2 @@
+#!/bin/bash
+curl -H "Content-Type: application/json" -X GET http://localhost:8888/applications
\ No newline at end of file
diff --git a/scripts/create-application.sh b/scripts/create-application.sh
new file mode 100644
index 0000000..ad08926
--- /dev/null
+++ b/scripts/create-application.sh
@@ -0,0 +1,2 @@
+#!/bin/bash
+curl -H "Content-Type: application/json" -X POST http://localhost:8888/create-application
\ No newline at end of file
diff --git a/src/main/scala/de/retest/guistatemachine/JsonFormatForIdMap.scala b/src/main/scala/de/retest/guistatemachine/JsonFormatForIdMap.scala
new file mode 100644
index 0000000..ccfcef8
--- /dev/null
+++ b/src/main/scala/de/retest/guistatemachine/JsonFormatForIdMap.scala
@@ -0,0 +1,30 @@
+package de.retest.guistatemachine
+
+import spray.json._
+import akka.http.scaladsl.marshallers.sprayjson.SprayJsonSupport._
+import spray.json.DefaultJsonProtocol._
+
+import de.retest.guistatemachine.model.Id
+import de.retest.guistatemachine.model.Map
+import scala.collection.immutable.HashMap
+
+/**
+ * Transforms a [[Map]] into a `scala.collection.immutable.Map[String, T]`, so it can be converted into valid JSON.
+ * Besides, transforms a JSON object which is a `scala.collection.immutable.Map[String, T]` back into a [[Map]].
+ * This transformer requires a JSON format for the type `K`.
+ */
+class JsonFormatForIdMap[T](implicit val jsonFormat: JsonFormat[T]) extends RootJsonFormat[Map[T]] {
+  override def write(obj: Map[T]): JsValue =
+    obj.values.map { field => (field._1.id.toString -> field._2) }.toJson
+
+  override def read(json: JsValue): Map[T] = {
+    val obj = json.asJsObject
+    if (obj.fields.isEmpty) {
+      new Map[T](new HashMap[Id, T]())
+      // TODO Fix conversation back into the Map type.
+    } else {
+      val map = json.asInstanceOf[scala.collection.immutable.Map[String, T]]
+      new Map[T](map.map { x => (Id(x._1.toLong) -> x._2) })
+    }
+  }
+}
\ No newline at end of file
diff --git a/src/main/scala/de/retest/guistatemachine/RestService.scala b/src/main/scala/de/retest/guistatemachine/RestService.scala
index ddbe996..f79fc8f 100644
--- a/src/main/scala/de/retest/guistatemachine/RestService.scala
+++ b/src/main/scala/de/retest/guistatemachine/RestService.scala
@@ -9,108 +9,57 @@ import akka.http.scaladsl.server.Directives._
 import akka.http.scaladsl.model.StatusCodes
 import akka.http.scaladsl.marshallers.sprayjson.SprayJsonSupport._
 import spray.json.DefaultJsonProtocol._
+import spray.json._
 
-import java.util.LinkedList
-
-// domain model
-// TODO #1 Id should use Long and the REST paths as well. Use concurrent hash maps with the IDs and generate new IDs for new items.
-final case class Id(id: Int)
-final case class GuiApplication() {
-  val testSuites = TestSuites()
-}
-final case class GuiApplications() {
-  val applications = new LinkedList[GuiApplication]
-}
-final case class TestSuite()
-final case class TestSuites() {
-  val testSuites = new LinkedList[TestSuite]
-}
+import de.retest.guistatemachine.persistence.Persistence
+import de.retest.guistatemachine.model.GuiApplications
+import de.retest.guistatemachine.model.GuiApplication
+import de.retest.guistatemachine.model.TestSuite
+import de.retest.guistatemachine.model.TestSuites
+import de.retest.guistatemachine.model.Id
+import de.retest.guistatemachine.persistence.Persistence
 
 trait RestService {
   implicit val system: ActorSystem
   implicit val materializer: ActorMaterializer
 
-  // database
-  val guiApplications = GuiApplications()
-
-  def addApplication(): Id = {
-    val apps = guiApplications.applications
-    apps.synchronized {
-      apps.add(new GuiApplication)
-      Id(apps.size() - 1)
-    }
-  }
-  def getApplication(id: Id): Option[GuiApplication] = {
-    val apps = guiApplications.applications
-    apps.synchronized {
-      val index = id.id
-      if (index >= 0 && index < apps.size()) Some(apps.get(index)) else None
-    }
-  }
-
-  def getTestSuites(applicationId: Id): Option[TestSuites] = {
-    val app = getApplication(applicationId)
-    app match {
-      case Some(x) => Some(x.testSuites)
-      case None => None
-    }
-  }
-
-  def addTestSuite(applicationId: Id): Option[Id] = {
-    val app = getApplication(applicationId)
-    app match {
-      case Some(x) => {
-        val testSuites = x.testSuites.testSuites
-        testSuites.synchronized {
-          testSuites.add(new TestSuite)
-          Some(Id(testSuites.size() - 1))
-        }
-      }
-      case None => None
-    }
-  }
-  def getTestSuite(applicationId: Id, testSuiteId: Id): Option[TestSuite] = {
-    val app = getApplication(applicationId)
-    app match {
-      case Some(x) => {
-        val testSuites = x.testSuites.testSuites
-        testSuites.synchronized {
-          val index = testSuiteId.id
-          if (index >= 0 && index < testSuites.size()) Some(testSuites.get(index)) else None
-        }
-      }
-      case None => None
-    }
-  }
-
   // formats for unmarshalling and marshalling
   implicit val idFormat = jsonFormat1(Id)
-  implicit val applicationFormat = jsonFormat0(GuiApplication)
-  implicit val applicationsFormat = jsonFormat0(GuiApplications)
   implicit val testSuiteFormat = jsonFormat0(TestSuite)
-  implicit val testSuitesFormat = jsonFormat0(TestSuites)
+  implicit val hashMapFormatTestSuites = new JsonFormatForIdMap[TestSuite]
+  implicit val testSuitesFormat = jsonFormat1(TestSuites)
+  implicit val applicationFormat = jsonFormat1(GuiApplication)
+  implicit val hashMapFormatApplications = new JsonFormatForIdMap[GuiApplication]
+  implicit val applicationsFormat = jsonFormat1(GuiApplications)
 
-  val route: Route =
+  /**
+   * Creates the complete route for the REST service with all possible paths.
+   */
+  def getRoute(persistence: Persistence): Route =
     get {
-      path("applications") {
-        complete(guiApplications)
+      pathSingleSlash {
+        complete("GUI State Machine API")
       } ~
-        pathPrefix("application" / IntNumber) { id =>
-          val app = getApplication(Id(id))
+        path("applications") {
+          complete(persistence.getApplications())
+        } ~
+        pathPrefix("application" / LongNumber) { id =>
+          println("Getting application with ID " + id)
+          val app = persistence.getApplication(Id(id))
           app match {
             case Some(x) => complete(x)
             case None => complete(StatusCodes.NotFound)
           }
         } ~
-        pathPrefix("application" / IntNumber / "test-suites") { id =>
-          val testSuites = getTestSuites(Id(id))
+        pathPrefix("application" / LongNumber / "test-suites") { id =>
+          val testSuites = persistence.getTestSuites(Id(id))
           testSuites match {
             case Some(x) => complete(x)
             case None => complete(StatusCodes.NotFound)
           }
         } ~
-        pathPrefix("application" / IntNumber / "test-suite" / IntNumber) { (appId, suiteId) =>
-          val suite = getTestSuite(Id(appId), Id(suiteId))
+        pathPrefix("application" / LongNumber / "test-suite" / LongNumber) { (appId, suiteId) =>
+          val suite = persistence.getTestSuite(Id(appId), Id(suiteId))
           suite match {
             case Some(x) => complete(x)
             case None => complete(StatusCodes.NotFound)
@@ -119,12 +68,12 @@ trait RestService {
     } ~
       post {
         path("create-application") {
-          val id = addApplication()
+          val id = persistence.addApplication()
           complete(id)
         } ~
-          pathPrefix("application" / IntNumber / "create-test-suite") { appId =>
+          pathPrefix("application" / LongNumber / "create-test-suite") { appId =>
             {
-              val id = addTestSuite(Id(appId))
+              val id = persistence.addTestSuite(Id(appId))
               complete(id)
             }
           }
diff --git a/src/main/scala/de/retest/guistatemachine/WebServer.scala b/src/main/scala/de/retest/guistatemachine/WebServer.scala
index cc1ec57..fe07191 100644
--- a/src/main/scala/de/retest/guistatemachine/WebServer.scala
+++ b/src/main/scala/de/retest/guistatemachine/WebServer.scala
@@ -5,16 +5,22 @@ import scala.io.StdIn
 import akka.actor.ActorSystem
 import akka.http.scaladsl.Http
 import akka.stream.ActorMaterializer
+import de.retest.guistatemachine.persistence.Persistence
 
 object WebServer extends App with RestService {
+  final val HOST = "localhost"
+  final val PORT = 8888
+
   implicit val system = ActorSystem("gui-state-machine-api-system")
   implicit val materializer = ActorMaterializer()
   // needed for the future flatMap/onComplete in the end
   implicit val executionContext = system.dispatcher
 
-  val bindingFuture = Http().bindAndHandle(route, "localhost", 8080)
+  val persistence = new Persistence
+
+  val bindingFuture = Http().bindAndHandle(getRoute(persistence), HOST, PORT)
 
-  println(s"Server online at http://localhost:8080/\nPress RETURN to stop...")
+  println(s"Server online at http://${HOST}:${PORT}/\nPress RETURN to stop...")
   StdIn.readLine() // let it run until user presses return
   bindingFuture
     .flatMap(_.unbind()) // trigger unbinding from the port
diff --git a/src/main/scala/de/retest/guistatemachine/furrermodel/GuiApplication.scala b/src/main/scala/de/retest/guistatemachine/furrermodel/GuiApplication.scala
new file mode 100644
index 0000000..39797e6
--- /dev/null
+++ b/src/main/scala/de/retest/guistatemachine/furrermodel/GuiApplication.scala
@@ -0,0 +1,13 @@
+package de.retest.guistatemachine.furrermodel
+
+/**
+ * The tested GUI application with an initial state and a number of test suites.
+ *
+ * @param id This ID is for the REST API only.
+ */
+class GuiApplication(val id : Long, initialState: State, testSuites: Seq[TestSuite]) {
+
+  def getTestSuites: Seq[TestSuite] = testSuites
+
+  def getInitialState: State = initialState
+}
\ No newline at end of file
diff --git a/src/main/scala/de/retest/guistatemachine/model/GuiWidget.scala b/src/main/scala/de/retest/guistatemachine/furrermodel/GuiWidget.scala
similarity index 75%
rename from src/main/scala/de/retest/guistatemachine/model/GuiWidget.scala
rename to src/main/scala/de/retest/guistatemachine/furrermodel/GuiWidget.scala
index 116d90e..23fb920 100644
--- a/src/main/scala/de/retest/guistatemachine/model/GuiWidget.scala
+++ b/src/main/scala/de/retest/guistatemachine/furrermodel/GuiWidget.scala
@@ -1,4 +1,4 @@
-package de.retest.guistatemachine.model
+package de.retest.guistatemachine.furrermodel
 
 /**
  * A visible widget on a [[GuiWindow]].
diff --git a/src/main/scala/de/retest/guistatemachine/model/GuiWindow.scala b/src/main/scala/de/retest/guistatemachine/furrermodel/GuiWindow.scala
similarity index 79%
rename from src/main/scala/de/retest/guistatemachine/model/GuiWindow.scala
rename to src/main/scala/de/retest/guistatemachine/furrermodel/GuiWindow.scala
index c9dc92f..1ae5c8c 100644
--- a/src/main/scala/de/retest/guistatemachine/model/GuiWindow.scala
+++ b/src/main/scala/de/retest/guistatemachine/furrermodel/GuiWindow.scala
@@ -1,4 +1,4 @@
-package de.retest.guistatemachine.model
+package de.retest.guistatemachine.furrermodel
 
 /**
  * A visible window which the user can interact with.
diff --git a/src/main/scala/de/retest/guistatemachine/model/State.scala b/src/main/scala/de/retest/guistatemachine/furrermodel/State.scala
similarity index 79%
rename from src/main/scala/de/retest/guistatemachine/model/State.scala
rename to src/main/scala/de/retest/guistatemachine/furrermodel/State.scala
index b294cd7..409da29 100644
--- a/src/main/scala/de/retest/guistatemachine/model/State.scala
+++ b/src/main/scala/de/retest/guistatemachine/furrermodel/State.scala
@@ -1,4 +1,4 @@
-package de.retest.guistatemachine.model
+package de.retest.guistatemachine.furrermodel
 
 class State {
   private val windows = Set[GuiWindow]()
diff --git a/src/main/scala/de/retest/guistatemachine/model/TestCase.scala b/src/main/scala/de/retest/guistatemachine/furrermodel/TestCase.scala
similarity index 87%
rename from src/main/scala/de/retest/guistatemachine/model/TestCase.scala
rename to src/main/scala/de/retest/guistatemachine/furrermodel/TestCase.scala
index b1bdd64..5792913 100644
--- a/src/main/scala/de/retest/guistatemachine/model/TestCase.scala
+++ b/src/main/scala/de/retest/guistatemachine/furrermodel/TestCase.scala
@@ -1,4 +1,4 @@
-package de.retest.guistatemachine.model
+package de.retest.guistatemachine.furrermodel
 
 class TestCase(initialState: State) {
   private val actions = Seq[UIAction]()
diff --git a/src/main/scala/de/retest/guistatemachine/furrermodel/TestSuite.scala b/src/main/scala/de/retest/guistatemachine/furrermodel/TestSuite.scala
new file mode 100644
index 0000000..2ea30c7
--- /dev/null
+++ b/src/main/scala/de/retest/guistatemachine/furrermodel/TestSuite.scala
@@ -0,0 +1,9 @@
+package de.retest.guistatemachine.furrermodel
+
+class TestSuite {
+  private val cases = Set[TestCase]()
+
+  def size = cases.size
+
+  def length = cases.foldLeft(0)((size, c) => size + c.length)
+}
\ No newline at end of file
diff --git a/src/main/scala/de/retest/guistatemachine/furrermodel/UIAction.scala b/src/main/scala/de/retest/guistatemachine/furrermodel/UIAction.scala
new file mode 100644
index 0000000..87510cc
--- /dev/null
+++ b/src/main/scala/de/retest/guistatemachine/furrermodel/UIAction.scala
@@ -0,0 +1,5 @@
+package de.retest.guistatemachine.furrermodel
+
+trait UIAction {
+
+}
\ No newline at end of file
diff --git a/src/main/scala/de/retest/guistatemachine/model/UIPath.scala b/src/main/scala/de/retest/guistatemachine/furrermodel/UIPath.scala
similarity index 93%
rename from src/main/scala/de/retest/guistatemachine/model/UIPath.scala
rename to src/main/scala/de/retest/guistatemachine/furrermodel/UIPath.scala
index d09b847..8477557 100644
--- a/src/main/scala/de/retest/guistatemachine/model/UIPath.scala
+++ b/src/main/scala/de/retest/guistatemachine/furrermodel/UIPath.scala
@@ -1,4 +1,4 @@
-package de.retest.guistatemachine.model
+package de.retest.guistatemachine.furrermodel
 
 import scala.annotation.tailrec
 
diff --git a/src/main/scala/de/retest/guistatemachine/model/GuiApplication.scala b/src/main/scala/de/retest/guistatemachine/model/GuiApplication.scala
index 9961e6e..f7a5189 100644
--- a/src/main/scala/de/retest/guistatemachine/model/GuiApplication.scala
+++ b/src/main/scala/de/retest/guistatemachine/model/GuiApplication.scala
@@ -1,13 +1,3 @@
 package de.retest.guistatemachine.model
 
-/**
- * The tested GUI application with an initial state and a number of test suites.
- *
- * @param id This ID is for the REST API only.
- */
-class GuiApplication(val id : Long, initialState: State, testSuites: Seq[TestSuite]) {
-
-  def getTestSuites: Seq[TestSuite] = testSuites
-
-  def getInitialState: State = initialState
-}
\ No newline at end of file
+final case class GuiApplication(testSuites: TestSuites)
\ No newline at end of file
diff --git a/src/main/scala/de/retest/guistatemachine/model/GuiApplications.scala b/src/main/scala/de/retest/guistatemachine/model/GuiApplications.scala
new file mode 100644
index 0000000..9da2b30
--- /dev/null
+++ b/src/main/scala/de/retest/guistatemachine/model/GuiApplications.scala
@@ -0,0 +1,6 @@
+package de.retest.guistatemachine.model
+
+final case class GuiApplications(applications: Map[GuiApplication]) {
+  // TODO Generate IDs in a better way. Maybe random numbers until one unused element is found?
+  def generateId: Id = this.synchronized { if (applications.values.isEmpty) Id(0) else Id(applications.values.keySet.max.id + 1) }
+}
\ No newline at end of file
diff --git a/src/main/scala/de/retest/guistatemachine/model/Id.scala b/src/main/scala/de/retest/guistatemachine/model/Id.scala
new file mode 100644
index 0000000..a02f591
--- /dev/null
+++ b/src/main/scala/de/retest/guistatemachine/model/Id.scala
@@ -0,0 +1,7 @@
+package de.retest.guistatemachine.model
+
+final case class Id(val id: Long) extends Ordered[Id] {
+  import scala.math.Ordered.orderingToOrdered
+
+  override def compare(that: Id): Int = this.id compare that.id
+}
\ No newline at end of file
diff --git a/src/main/scala/de/retest/guistatemachine/model/Map.scala b/src/main/scala/de/retest/guistatemachine/model/Map.scala
new file mode 100644
index 0000000..b67cf77
--- /dev/null
+++ b/src/main/scala/de/retest/guistatemachine/model/Map.scala
@@ -0,0 +1,10 @@
+package de.retest.guistatemachine.model
+
+/**
+ * This custom type allows storing values using [[Id]] as key.
+ * [[JsonFormatForIdMap]] implements marshalling and unmarshalling for JSON for this type.
+ * We cannot extend immutable maps in Scala, so we have to keep it as field.
+ */
+case class Map[T](var values: scala.collection.immutable.Map[Id, T]) {
+
+}
\ No newline at end of file
diff --git a/src/main/scala/de/retest/guistatemachine/model/TestSuite.scala b/src/main/scala/de/retest/guistatemachine/model/TestSuite.scala
index 435f304..27b104f 100644
--- a/src/main/scala/de/retest/guistatemachine/model/TestSuite.scala
+++ b/src/main/scala/de/retest/guistatemachine/model/TestSuite.scala
@@ -1,9 +1,3 @@
 package de.retest.guistatemachine.model
 
-class TestSuite {
-  private val cases = Set[TestCase]()
-
-  def size = cases.size
-
-  def length = cases.foldLeft(0)((size, c) => size + c.length)
-}
\ No newline at end of file
+final case class TestSuite()
\ No newline at end of file
diff --git a/src/main/scala/de/retest/guistatemachine/model/TestSuites.scala b/src/main/scala/de/retest/guistatemachine/model/TestSuites.scala
new file mode 100644
index 0000000..52c855c
--- /dev/null
+++ b/src/main/scala/de/retest/guistatemachine/model/TestSuites.scala
@@ -0,0 +1,6 @@
+package de.retest.guistatemachine.model
+
+final case class TestSuites(testSuites: Map[TestSuite]) {
+  // TODO Generate IDs in a better way. Maybe random numbers until one unused element is found?
+  def generateId: Id = this.synchronized { if (testSuites.values.isEmpty) Id(0) else Id(testSuites.values.keySet.max.id + 1) }
+}
\ No newline at end of file
diff --git a/src/main/scala/de/retest/guistatemachine/model/UIAction.scala b/src/main/scala/de/retest/guistatemachine/model/UIAction.scala
deleted file mode 100644
index ea12ee5..0000000
--- a/src/main/scala/de/retest/guistatemachine/model/UIAction.scala
+++ /dev/null
@@ -1,5 +0,0 @@
-package de.retest.guistatemachine.model
-
-trait UIAction {
-
-}
\ No newline at end of file
diff --git a/src/main/scala/de/retest/guistatemachine/persistence/Persistence.scala b/src/main/scala/de/retest/guistatemachine/persistence/Persistence.scala
new file mode 100644
index 0000000..73c7d51
--- /dev/null
+++ b/src/main/scala/de/retest/guistatemachine/persistence/Persistence.scala
@@ -0,0 +1,71 @@
+package de.retest.guistatemachine.persistence
+
+import scala.collection.immutable.HashMap
+
+import de.retest.guistatemachine.model.GuiApplication
+import de.retest.guistatemachine.model.GuiApplications
+import de.retest.guistatemachine.model.Id
+import de.retest.guistatemachine.model.TestSuite
+import de.retest.guistatemachine.model.TestSuites
+import de.retest.guistatemachine.model.Map
+
+/**
+ * Allows concurrent access to the persistence of the resources.
+ * The actual persistence layer is hidden by this class.
+ */
+class Persistence {
+  // database
+  private val guiApplications = GuiApplications(Map[GuiApplication](new HashMap[Id, GuiApplication]))
+
+  def getApplications(): GuiApplications = guiApplications
+
+  def addApplication(): Id = {
+    val apps = guiApplications
+    apps.synchronized {
+      val id = apps.generateId
+      apps.applications.values = apps.applications.values + (id -> new GuiApplication(TestSuites(Map[TestSuite](new HashMap[Id, TestSuite]))))
+      id
+    }
+  }
+  def getApplication(id: Id): Option[GuiApplication] = {
+    val apps = guiApplications
+    apps.synchronized {
+      if (apps.applications.values.contains(id)) Some(apps.applications.values(id)) else { None }
+    }
+  }
+
+  def getTestSuites(applicationId: Id): Option[TestSuites] = {
+    val app = getApplication(applicationId)
+    app match {
+      case Some(x) => Some(x.testSuites)
+      case None => None
+    }
+  }
+
+  def addTestSuite(applicationId: Id): Option[Id] = {
+    val app = getApplication(applicationId)
+    app match {
+      case Some(x) => {
+        val testSuites = x.testSuites
+        testSuites.synchronized {
+          val id = testSuites.generateId
+          testSuites.testSuites.values = testSuites.testSuites.values + (id -> TestSuite())
+          Some(id)
+        }
+      }
+      case None => None
+    }
+  }
+  def getTestSuite(applicationId: Id, testSuiteId: Id): Option[TestSuite] = {
+    val app = getApplication(applicationId)
+    app match {
+      case Some(x) => {
+        val testSuites = x.testSuites
+        testSuites.synchronized {
+          if (testSuites.testSuites.values.contains(testSuiteId)) Some(testSuites.testSuites.values(testSuiteId)) else None
+        }
+      }
+      case None => None
+    }
+  }
+}
\ No newline at end of file
diff --git a/src/test/scala/de/retest/guistatemachine/RestServiceSpec.scala b/src/test/scala/de/retest/guistatemachine/RestServiceSpec.scala
index d60a892..861b055 100644
--- a/src/test/scala/de/retest/guistatemachine/RestServiceSpec.scala
+++ b/src/test/scala/de/retest/guistatemachine/RestServiceSpec.scala
@@ -9,16 +9,32 @@ import akka.http.scaladsl.model.HttpEntity
 import akka.http.scaladsl.model.ContentTypes
 import akka.http.scaladsl.model.HttpCharset
 import akka.http.scaladsl.marshallers.sprayjson.SprayJsonSupport._
+import de.retest.guistatemachine.model.GuiApplications
+import de.retest.guistatemachine.model.Id
+import de.retest.guistatemachine.model.GuiApplication
+import de.retest.guistatemachine.model.TestSuites
+import de.retest.guistatemachine.persistence.Persistence
+import akka.http.scaladsl.model.MediaType
+import akka.http.scaladsl.model.MediaTypes
 
 class RestServiceSpec extends WordSpec with Matchers with ScalatestRouteTest with RestService {
 
-  lazy val sut = route
+  val sut = getRoute(new Persistence)
 
   "The service" should {
+    "show the default text for the GET request with the path /" in {
+      Get("/") ~> sut ~> check {
+        val r = responseAs[String]
+        r shouldEqual "GUI State Machine API"
+      }
+    }
+
     "return an empty list for the GET request with the path /applications" in {
       Get("/applications") ~> sut ~> check {
+        handled shouldEqual true
+        mediaType shouldEqual MediaTypes.`application/json`
         val r = responseAs[GuiApplications]
-        r.applications.size shouldEqual 0
+        r.applications.values.size shouldEqual 0
       }
     }
 
@@ -28,34 +44,25 @@ class RestServiceSpec extends WordSpec with Matchers with ScalatestRouteTest wit
       }
     }
 
-    /*
-    TODO #1 Somehow the current state of the REST service is not stored after creating one application.
     "return an empty application for the GET request with the path /application/0" in {
       Get("/applications/0") ~> sut ~> check {
+        // TODO Print response here
         val r = responseAs[GuiApplication]
-        r.testSuites.testSuites.size shouldEqual 0
+        r.testSuites.testSuites.values.size shouldEqual 0
       }
     }
 
     "return an empty list for the GET request with the path /application/0/test-suites" in {
       Get("/applications/0/test-suites") ~> sut ~> check {
         val r = responseAs[TestSuites]
-        r.testSuites.size shouldEqual 0
+        r.testSuites.values.size shouldEqual 0
       }
     }
-    */
 
     "allow POST for path /application/0/create-test-suite" in {
       Post("/application/0/create-test-suite") ~> sut ~> check {
         responseAs[Id] shouldEqual Id(0)
       }
     }
-
-    "not find any root path" in {
-      Get() ~> Route.seal(sut) ~> check {
-        status shouldEqual StatusCodes.NotFound
-        responseAs[String] shouldEqual "The requested resource could not be found."
-      }
-    }
   }
 }
diff --git a/src/test/scala/de/retest/guistatemachine/persistence/PersistenceSpec.scala b/src/test/scala/de/retest/guistatemachine/persistence/PersistenceSpec.scala
new file mode 100644
index 0000000..3efaf75
--- /dev/null
+++ b/src/test/scala/de/retest/guistatemachine/persistence/PersistenceSpec.scala
@@ -0,0 +1,22 @@
+package de.retest.guistatemachine.persistence
+
+import org.scalatest.Matchers
+import org.scalatest.WordSpec
+import de.retest.guistatemachine.model.Id
+
+class PersistenceSpec extends WordSpec with Matchers {
+  val sut = new Persistence
+
+  "The pesistence" should {
+    "allow adding one test suite" in {
+      sut.getApplications().applications.values.size shouldEqual 0
+      sut.addApplication().id shouldEqual 0
+      sut.getApplications().applications.values.size shouldEqual 1
+      sut.getTestSuites(Id(0)).get.testSuites.values.size shouldEqual 0
+      sut.addTestSuite(Id(0)).get.id shouldEqual 0
+      sut.getTestSuites(Id(0)).get.testSuites.values.size shouldEqual 1
+      val s = sut.getTestSuite(Id(0), Id(0))
+      s.isEmpty shouldEqual false
+    }
+  }
+}
\ No newline at end of file