Intermittent message loss: messages not received on client despite being sent from server

[ankithedau]

Problem

We're experiencing an intermittent issue where messages sent from the server via Socket.IO sometimes don't appear on the client widget, even though:

• Messages are confirmed to have been sent from the server/conversations
• The socket connection appears to be active (socket.connected === true)
• The issue occurs during active conversations (not just on initial connection)
• Messages stop appearing mid-conversation, then may resume later after page refresh

Environment

• Socket.IO version: Latest (via CDN)
• Transport: WebSocket with polling fallback

Configuration

const socketConfig = {
  path: '/sockets-live-chat/socket.io',
  transports: ['websocket', 'polling'],
  reconnection: true,
  reconnectionAttempts: 10,
  reconnectionDelay: 2000,
  reconnectionDelayMax: 10000,
  timeout: 30000,
  connectionStateRecovery: true,
  pingTimeout: 60000,
  pingInterval: 25000,
  forceNew: true,
};

Current Implementation

We're using a custom event sendMessage with acknowledgment:

socket.on('sendMessage', (message: SocketMessage, callback) => {
  // Process message
  handleMessage(message);
  // Acknowledge MESSAGE events
  if (typeof callback === 'function' && message.event === 'MESSAGE') {
    callback({ ack: true });
  }
});

Questions

1. Could messages be silently dropped during reconnection attempts? Even when socket.connected === true, could there be a brief window where messages are lost?

2. Is there a race condition with event listeners? If the socket reconnects automatically, could the event listener be temporarily inactive or not properly reattached?

3. Should we be checking additional connection states? Beyond socket.connected, are there other properties we should verify before processing messages?

4. Connection state recovery: With connectionStateRecovery: true, could there be scenarios where messages sent during recovery are lost?

What We've Tried

• Always acknowledging MESSAGE events to prevent server timeouts
• Checking both singleton socket and actual socket connection status
• Keeping message handlers active during reconnection
• Adding error handling for message processing

Expected Behavior

All messages sent from the server should be received and processed by the client, even during reconnection attempts or connection state changes.

Actual Behavior

Messages intermittently stop appearing on the client widget, even though:

• The socket connection shows as active
• Messages are confirmed sent from the server

Additional Context

This is a live chat widget where message reliability is critical. The issue occurs randomly during active conversations, making it difficult to reproduce.