Design WhatsApp/Messenger, Real-time Chat System Design Interview

@ServerEndpoint("/chat")
public class ChatEndpoint {
    private static Map<String, Session> sessions = new ConcurrentHashMap<>();
    
    @OnOpen
    public void onOpen(Session session) {
        String userId = authenticate(session);
        sessions.put(userId, session);
        presenceService.setOnline(userId, getServerId());
    }
    
    @OnMessage
    public void onMessage(String message, Session session) {
        ChatMessage msg = parse(message);
        messageService.send(msg);
    }
    
    @OnClose
    public void onClose(Session session) {
        String userId = getUser(session);
        sessions.remove(userId);
        presenceService.setOffline(userId);
    }
}

1. User A sends message to User B
2. WebSocket server receives message
3. Store in database (Cassandra)
4. Lookup User B's WebSocket server
5. Route message to that server
6. Deliver to User B
7. Send "delivered" receipt to User A

CREATE TABLE messages (
  chat_id UUID,
  message_id TIMEUUID,
  sender_id BIGINT,
  content TEXT,
  media_url TEXT,
  status TEXT,
  created_at TIMESTAMP,
  PRIMARY KEY (chat_id, message_id)
) WITH CLUSTERING ORDER BY (message_id DESC);

// Message stored once
INSERT INTO group_messages (group_id, message_id, content, sender)
// Each member queries their groups
SELECT * FROM group_messages 
WHERE group_id IN (user's groups)
ORDER BY message_id DESC

// Set online with TTL
SET user:123:online "server-1" EX 30
// Heartbeat every 25 seconds to refresh
EXPIRE user:123:online 30
// Check if online
EXISTS user:123:online

1. Client uploads to CDN
2. Get CDN URL
3. Send message with media_url
4. Recipient downloads from CDN