Auto-Reconnection & Acknowledgments

~11 min · library, reconnect, ack

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Reconnection is built-in

Socket.IO ships with exponential backoff + jitter reconnection out of the box. You wrote it by hand in Track 2; here you set five options and it just works. Reasonable defaults for most apps: max attempts = Infinity, base delay 1s, cap 5-30s, jitter 0.5.

Reconnect events

Listen on socket.io.on('reconnect') for successful reconnections, reconnect_attempt for retries, reconnect_failed for permanent failures. Use these to drive UI states like "Connecting..." overlays and "Offline" badges.

Acknowledgements as confirmation

Returning a value (or list, for multi-arg) from a server event handler becomes the ack payload on the client side. This collapses the correlation-ID dance from Track 5 into one line of code.

Code

Reconnection options·javascript

const socket = io('https://api.example.com', {
  auth: { token },
  reconnection: true,
  reconnectionAttempts: Infinity,
  reconnectionDelay: 1_000,        // start delay
  reconnectionDelayMax: 30_000,    // cap
  randomizationFactor: 0.5,        // ±50% jitter
});

socket.io.on('reconnect_attempt', (n) => {
  console.log('reconnect attempt', n);
  setBanner('Reconnecting...');
});

socket.io.on('reconnect', (n) => {
  console.log('reconnected after', n, 'attempts');
  setBanner(null);
});

socket.io.on('reconnect_failed', () => {
  console.log('reconnect failed permanently');
  setBanner('Offline. Refresh to retry.');
});

Server returns ack data·python

@sio.on('chat:message')
async def chat(sid, data):
    saved = await db.save_message(data)
    return {'status': 'ok', 'id': saved.id, 'ts': saved.created_at}

External links

Socket.IO: Connection state recovery

Exercise

Wire reconnection options + the three reconnect_* events to a 'Status: Online / Reconnecting / Offline' badge in your UI. Kill the server for 30 seconds, watch the badge cycle through states, and verify it lands back on Online when you restart.