diff --git a/src/main/java/chat/ChatServiceImpl.java b/src/main/java/chat/ChatServiceImpl.java
index 4e1de13bb242b1cb48e2313187da54a0acb18372..b851ee09c49660ffd5ce0ea2ac73ea3920cb5321 100644
--- a/src/main/java/chat/ChatServiceImpl.java
+++ b/src/main/java/chat/ChatServiceImpl.java
@@ -15,8 +15,11 @@ public class ChatServiceImpl extends ChatServiceGrpc.ChatServiceImplBase {
     @Override public StreamObserver<ChatServiceOuterClass.ChatMessage> chat(final StreamObserver<ChatServiceOuterClass.ChatMessage> responseObserver){
 
         //the stream used to communicate with a specific client is stored in a hash set (avoiding duplicates)
-        observers.add(responseObserver);
+        synchronized (observers) {
 
+            observers.add(responseObserver);
+
+        }
         //it returns the stream that will be used by the clients to send messages.
         //the client will write on this stream
         return new StreamObserver<ChatServiceOuterClass.ChatMessage>() {
@@ -30,8 +33,17 @@ public class ChatServiceImpl extends ChatServiceGrpc.ChatServiceImplBase {
 
                 System.out.println("[MESSAGE RECEIVED] Received a message from "+from+": "+message);
 
+
+                HashSet<StreamObserver> copy;
+
+                synchronized (observers) {
+
+                    copy = new HashSet<>(observers);
+
+                }
+
                 //iterating on all the streams to communicate with all the clients
-                for(StreamObserver<ChatServiceOuterClass.ChatMessage> observer: observers){
+                for(StreamObserver<ChatServiceOuterClass.ChatMessage> observer: copy){
 
                     //we exclude the one which is sending the message
                     if(!observer.equals(responseObserver))
@@ -45,13 +57,20 @@ public class ChatServiceImpl extends ChatServiceGrpc.ChatServiceImplBase {
             //if there is an error (client abruptly disconnect) we remove the client.
             public void onError(Throwable throwable) {
 
-                observers.remove(responseObserver);
+                synchronized (observers) {
+
+                    observers.remove(responseObserver);
 
+                }
             }
 
             //if the client explicitly terminated, we remove it from the hashset.
             public void onCompleted() {
-                observers.remove(responseObserver);
+                synchronized (observers) {
+
+                    observers.remove(responseObserver);
+
+                }
             }
         };
     }