diff --git a/Backend/mocked-services/src/main/java/com/r11/tools/controller/MockController.java b/Backend/mocked-services/src/main/java/com/r11/tools/controller/MockController.java
index bd63b8b..766d1d8 100644
--- a/Backend/mocked-services/src/main/java/com/r11/tools/controller/MockController.java
+++ b/Backend/mocked-services/src/main/java/com/r11/tools/controller/MockController.java
@@ -5,19 +5,15 @@ import com.r11.tools.model.MockedMessageDto;
 import com.r11.tools.service.KlausService;
 import com.r11.tools.utilis.BusinessKey;
 import com.r11.tools.utilis.TrackingClient;
-import java.time.LocalDateTime;
-import java.util.Collections;
-import java.util.HashMap;
-import java.util.List;
-import java.util.Map;
-import java.util.Objects;
-import java.util.UUID;
 import lombok.AllArgsConstructor;
 import lombok.SneakyThrows;
 import lombok.extern.slf4j.Slf4j;
 import org.springframework.http.*;
 import org.springframework.web.bind.annotation.*;
 
+import java.time.LocalDateTime;
+import java.util.*;
+
 /**
  * Returns the homepage and provides the api for javascript async requests.
  * @author Gabriel Modzelewski
@@ -64,6 +60,32 @@ public class MockController {
         return messages;
     }
 
+    /**
+    * If provided UUID does not exist in database returns ResponseEntity with new generated UUID(if previous UUID is not provided),
+    * or old UUID(if previous UUID is provided). If provided UUID exists function returns provided UUID.
+    * @param givenUUIDValue the UUID client wants to check exsitance in database
+    * @param previousUUIDValue the previous UUID used by client(optional variable)
+    * @return ResponseEntity with UUID
+     */
+    @RequestMapping(
+            method = RequestMethod.GET ,
+            path = {"/check/{givenUUIDValue}/{previousUUIDValue}",
+                    "/check/{givenUUIDValue}"})
+    public ResponseEntity<String> checkUUID(
+            @PathVariable String givenUUIDValue
+            ,@PathVariable(required = false) String previousUUIDValue  ){
+        try{
+            UUID.fromString(givenUUIDValue);
+        } catch (IllegalArgumentException ex){
+            if (previousUUIDValue == null || previousUUIDValue.equals("")){
+                UUID newUUID = UUID.randomUUID();
+                return ResponseEntity.ok(newUUID.toString());
+            }
+            return ResponseEntity.ok(previousUUIDValue);
+        }
+        return ResponseEntity.ok(givenUUIDValue);
+    }
+
     /**
      * Accepts empty post request and creates new message in given set. The new message has default set of data,
      * which is constructed in {@link #buildDefaultMessage(UUID, int)} method.
diff --git a/Backend/mocked-services/src/main/java/com/r11/tools/repository/MockedResponseRepository.java b/Backend/mocked-services/src/main/java/com/r11/tools/repository/MockedResponseRepository.java
index 29d7477..7b12b0b 100644
--- a/Backend/mocked-services/src/main/java/com/r11/tools/repository/MockedResponseRepository.java
+++ b/Backend/mocked-services/src/main/java/com/r11/tools/repository/MockedResponseRepository.java
@@ -2,6 +2,7 @@ package com.r11.tools.repository;
 
 import com.r11.tools.model.MockedMessage;
 import java.util.List;
+import java.util.Optional;
 import java.util.UUID;
 import org.springframework.data.repository.CrudRepository;
 import org.springframework.stereotype.Repository;
@@ -19,5 +20,5 @@ public interface MockedResponseRepository extends CrudRepository<MockedMessage,
      * @param clientUUID the key-uuid of given set of messages
      * @return Optional of list of {@link com.r11.tools.model.MockedMessage}
      */
-    List<MockedMessage> findAllByClientUUID(UUID clientUUID);
+    Optional<List<MockedMessage>> findAllByClientUUID(UUID clientUUID);
 }
diff --git a/Backend/mocked-services/src/main/java/com/r11/tools/service/KlausServiceImpl.java b/Backend/mocked-services/src/main/java/com/r11/tools/service/KlausServiceImpl.java
index f3cdfaf..3c23b31 100644
--- a/Backend/mocked-services/src/main/java/com/r11/tools/service/KlausServiceImpl.java
+++ b/Backend/mocked-services/src/main/java/com/r11/tools/service/KlausServiceImpl.java
@@ -5,10 +5,6 @@ import com.r11.tools.mappers.MockedMessageMapper;
 import com.r11.tools.model.MockedMessage;
 import com.r11.tools.model.MockedMessageDto;
 import com.r11.tools.repository.MockedResponseRepository;
-import java.util.List;
-import java.util.Optional;
-import java.util.UUID;
-import java.util.stream.Collectors;
 import lombok.AllArgsConstructor;
 import lombok.SneakyThrows;
 import lombok.extern.slf4j.Slf4j;
@@ -16,6 +12,10 @@ import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
 import org.springframework.stereotype.Service;
+import java.util.List;
+import java.util.Optional;
+import java.util.UUID;
+import java.util.stream.Collectors;
 
 /**
  * Service for {@link com.r11.tools.controller.MockController} and {@link com.r11.tools.controller.MockController}
@@ -52,10 +52,11 @@ public class KlausServiceImpl implements KlausService {
      * @return List of {@link MockedMessageDto}
      */
     @Override
-    public List<MockedMessageDto> getAllMockedResponses(UUID clientUUID) {
-        return mockedResponseRepository.findAllByClientUUID(clientUUID).stream()
+    public List<MockedMessageDto> getAllMockedResponses(UUID clientUUID){
+        Optional<List<MockedMessage>> listOptional = mockedResponseRepository.findAllByClientUUID(clientUUID);
+        return listOptional.map(mockedMessages -> mockedMessages.stream()
                 .map(mockedMessageMapper::mockedMessageToMockedMessageDto)
-                .collect(Collectors.toList());
+                .collect(Collectors.toList())).orElse(List.of());
     }
 
     /**
diff --git a/Backend/mocked-services/src/main/resources/static/html/mock.html b/Backend/mocked-services/src/main/resources/static/html/mock.html
index 4e15ac5..e62775b 100644
--- a/Backend/mocked-services/src/main/resources/static/html/mock.html
+++ b/Backend/mocked-services/src/main/resources/static/html/mock.html
@@ -27,6 +27,16 @@
                     </div>
                         <label for="editable">Editable</label>
                         <input type="checkbox" onchange="changeEditionOfUUID(this)" name="editable" id="editable" value="false"/>
+
+                        <div class="hiddable" id="uuid-validation-strategy">
+                            <label style="margin-bottom: 1%;display: block;"> UUID generation strategy:</label>
+                            <input type="radio" checked name="uuid-validation-type" value="new" id="generateNew"/>
+                            <label for="generateNew">Generate new UUID</label>
+                            <br>
+                            <input type="radio" name="uuid-validation-type" value="restore" id="restore"/>
+                            <label for="generateNew">Restore previous UUID</label>
+                        </div>
+
                 </div>
                 <div>
                     <!-- h2 -->
@@ -253,6 +263,16 @@
         </div>
     </div>
     <div id="overlay"></div>
+    <div id="modal-uuidChanged" class="modal">
+        <div class="header">
+            <div>Change UUID info<i class="r-exclamation"></i></div>
+            <button onclick="window.location.reload();">&times;</button>
+        </div>
+        <div id="changeUUIDSuccess" class="body hiddable uuid-modal-body">Your message UUID has been changed successfully.</div>
+        <div id="newUUID" class="body hiddable uuid-modal-body">You provided wrong UUID! <br> New UUID has been generated!</div>
+        <div id="restoredUUID" class="body hiddable uuid-modal-body">You provided wrong UUID! <br> Old UUID has been restored!</div>
+        <div id="noChgUUID" class="body hiddable uuid-modal-body">You doesn't provide any change to UUID!</div>
+    </div>
     <div id="modal-confirm" class="modal">
         <div class="header">
             <div>Message saved<i class="r-exclamation"></i></div>
diff --git a/Backend/mocked-services/src/main/resources/static/js/datatransfer.js b/Backend/mocked-services/src/main/resources/static/js/datatransfer.js
index df3df98..8a375ae 100644
--- a/Backend/mocked-services/src/main/resources/static/js/datatransfer.js
+++ b/Backend/mocked-services/src/main/resources/static/js/datatransfer.js
@@ -102,6 +102,7 @@ const idToDisplay = function(){
 }
 
 function refreshData(){
+    $("#uuid-input").val(clientUUID);
     fillMessageList();
     console.log("List initiated");
     let id = idToDisplay();
@@ -247,10 +248,60 @@ function copyUUIDToClipboard(){
     navigator.clipboard.writeText( document.getElementById('uuid-input').value );
 }
 
+
+async function fetchUUIDCheck(givenUUID , strategy){
+    var newUUID = "UUID" ;
+    url = host + "/api/mock/check/";
+
+    switch(strategy){
+        case "new":{
+            await fetch(url + givenUUID, { method : "GET" })
+                .then ( response => response.text() )
+                .then ( data => {
+                    newUUID = data;
+                    console.log("newUUID: "+newUUID);
+                } )
+            break;
+        }
+        case "restore":{
+            await fetch(url + givenUUID + "/" + clientUUID , { method: "GET" })
+                .then (response => response.text() )
+                .then (data => {
+                    newUUID = data;
+                    console.log("restoreUUID: "+newUUID);
+                } )
+            break;
+        }
+    }
+    return newUUID ;
+}
+
 function changeUUID(element){
-    clientUUID = element.value ;
-    checkUuid();
-    setCookie();
+
+    const uuidStrategy = $('input[name="uuid-validation-type"]:checked').val();
+    const givenUUID = element.value ;
+
+    if( givenUUID == clientUUID ){
+        uuidChangeModalDisplay("noChg");
+        return;
+    }
+
+    var newUUID = fetchUUIDCheck(givenUUID , uuidStrategy);
+    var changeMessage = uuidStrategy;
+    newUUID
+    .then( data => {
+        if (givenUUID == data) { 
+            changeMessage = "success";
+        }
+        clientUUID = data;
+        
+        uuidChangeModalDisplay(changeMessage);
+        document.cookie = C_UUID + '=' + data ;
+    } )
+    $("#uuid-input").attr("disabled", true);
+    $("#editable").attr("checked", false);
+    loadCookies();
+    refreshData();
 }
 
 
diff --git a/Backend/mocked-services/src/main/resources/static/js/modal.js b/Backend/mocked-services/src/main/resources/static/js/modal.js
index 3ea450c..f743055 100644
--- a/Backend/mocked-services/src/main/resources/static/js/modal.js
+++ b/Backend/mocked-services/src/main/resources/static/js/modal.js
@@ -7,6 +7,7 @@ var methodToCall = {
 const overlay = $('#overlay');
 const savedModal = $('#modal-confirm');
 const dataLossModal = $('#modal-query');
+const uuidChangeModal = $('#modal-uuidChanged')
 const dataLossModalYes = dataLossModal.children().eq(2).children().eq(0);
 const dataLossModalNo = dataLossModal.children().eq(2).children().eq(1);
 const allModals = $('.modal');
@@ -23,6 +24,37 @@ const dataLossModalDisplay = function(){
     showModal(dataLossModal);
 }
 
+const uuidChangeModalDisplay = function(addidionalMessage){
+
+    switch(addidionalMessage){
+        case "success":{
+            console.log("success");
+            $(".uuid-modal-body").removeClass("active");
+            $("#changeUUIDSuccess").addClass("active");
+            break;
+        }
+        case "new":{
+            console.log("new UUID");
+            $(".uuid-modal-body").removeClass("active");
+            $("#newUUID").addClass("active");
+            break;
+        }
+        case "restore":{
+            console.log("restoredUUID");
+            $(".uuid-modal-body").removeClass("active");
+            $("#restoredUUID").addClass("active");
+            break;
+        }
+        case "noChg":{
+            console.log("No changes");
+            $(".uuid-modal-body").removeClass("active");
+            $("#noChgUUID").addClass("active");
+            break;
+        }
+    }
+    showModal(uuidChangeModal);
+}
+
 function setMethodToCall(name, id){
     methodToCall.name = name;
     methodToCall.id = id;
diff --git a/Backend/mocked-services/src/main/resources/static/js/uianimation.js b/Backend/mocked-services/src/main/resources/static/js/uianimation.js
index d80f6db..0586a85 100644
--- a/Backend/mocked-services/src/main/resources/static/js/uianimation.js
+++ b/Backend/mocked-services/src/main/resources/static/js/uianimation.js
@@ -3,12 +3,14 @@ var selectMenu = $("#selectMenuContent");
 var advancedTab = $("#advanced");
 var basicID = $("#basicItemData")
 var advancedID = $("#advancedItemData");
+var advancedUUIDOptions = $("#uuid-validation-strategy");
 var focusedField = false;
 function changeAdvancedVisibility(){
     if(advancedVisibility){
         selectMenu.removeClass('active');
         advancedTab.removeClass('active');
         advancedID.removeClass('active');
+        advancedUUIDOptions.removeClass('active');
         basicID.addClass('active');
         advancedVisibility = false;
     }
@@ -16,6 +18,7 @@ function changeAdvancedVisibility(){
         selectMenu.addClass('active');
         advancedTab.addClass('active');
         advancedID.addClass('active');
+        advancedUUIDOptions.addClass('active');
         basicID.removeClass('active');
         advancedVisibility = true;
     }