feat: enhance chat service with Redis pub/sub for multi-instance message handling, add instance ID for deduplication

backend/chat/src/main.go

@@ -41,8 +41,17 @@ var (
 	// Rate limit config (env-overridable)
 	rateLimitMax    = 10
 	rateLimitWindow = 10 * time.Second
+
+	// Instance ID for multi-instance pub/sub deduplication
+	instanceID = fmt.Sprintf("chat-%s", generateShortID())
 )
 
+func generateShortID() string {
+	b := make([]byte, 6)
+	rand.Read(b)
+	return hex.EncodeToString(b)
+}
+
 type clientInfo struct {
 	Username string
 	Room     string
@@ -2161,7 +2170,7 @@ func handleMetrics(w http.ResponseWriter, r *http.Request) {
 
 	fmt.Fprintln(w, "# HELP linearvoid_chat_build_info Static build info for chat service.")
 	fmt.Fprintln(w, "# TYPE linearvoid_chat_build_info gauge")
-	fmt.Fprintln(w, "linearvoid_chat_build_info{service=\"chat\",database_driver=\""+databaseDriverName+"\"} 1")
+	fmt.Fprintln(w, "linearvoid_chat_build_info{service=\"chat\",database_driver=\""+databaseDriverName+"\",instance_id=\""+instanceID+"\"} 1")
 }
 
 // broadcast sends a message to all connected clients in a room and saves to DB
@@ -2172,12 +2181,23 @@ func broadcast(msg Message) {
 	// Persist to configured SQL backend.
 	db.Create(&msg)
 
+	// Local fan-out
 	for client := range clients {
 		if err := client.WriteJSON(msg); err != nil {
 			client.Close()
 			delete(clients, client)
+			delete(clientMeta, client)
 		}
 	}
+
+	// Redis pub/sub for multi-instance fan-out
+	if redisClient != nil && msg.Room != "" {
+		payload, _ := json.Marshal(map[string]any{
+			"instance": instanceID,
+			"msg":      msg,
+		})
+		redisClient.Publish(context.Background(), fmt.Sprintf("room:%s:messages", msg.Room), payload)
+	}
 }
 
 // handleConnection handles WebSocket connections for chat (public rooms, private DMs, moderation)
@@ -2333,6 +2353,47 @@ func pushChatHistory(conn *websocket.Conn, room string, limit int) {
 // dmsSent tracks total private messages for Prometheus metrics (set from handleMetrics).
 var dmsSent int64
 
+// subscribeRoomMessages listens for Redis pub/sub messages and fans out locally.
+func subscribeRoomMessages() {
+	if redisClient == nil {
+		return
+	}
+	pubsub := redisClient.PSubscribe(context.Background(), "room:*:messages")
+	defer pubsub.Close()
+
+	log.Printf("Redis pub/sub subscriber started (instance %s)", instanceID)
+
+	for {
+		msg, err := pubsub.ReceiveMessage(context.Background())
+		if err != nil {
+			log.Printf("Redis pub/sub error: %v — retrying in 5s", err)
+			time.Sleep(5 * time.Second)
+			pubsub = redisClient.PSubscribe(context.Background(), "room:*:messages")
+			continue
+		}
+
+		var envelope struct {
+			Instance string  `json:"instance"`
+			Msg      Message `json:"msg"`
+		}
+		if err := json.Unmarshal([]byte(msg.Payload), &envelope); err != nil {
+			continue
+		}
+
+		// Skip messages originating from this instance
+		if envelope.Instance == instanceID {
+			continue
+		}
+
+		// Fan out to local clients
+		mutex.Lock()
+		for client := range clients {
+			client.WriteJSON(envelope.Msg)
+		}
+		mutex.Unlock()
+	}
+}
+
 // handlePrivateDM routes private messages to the specific user pair
 func handlePrivateDM(msg Message) {
 	msg.Type = "private"
@@ -2640,6 +2701,11 @@ func main() {
 	initRedis()
 	initDB()
 
+	// Subscribe to Redis pub/sub for multi-instance message fan-out
+	go subscribeRoomMessages()
+
+	log.Printf("Instance ID: %s", instanceID)
+
 	http.HandleFunc("/ws", handleConnection)
 	http.HandleFunc("/api/admin/login", handleAdminLogin)
 	http.HandleFunc("/api/admin/logout", handleAdminLogout)