Server-Sent Events: Real-Time Updates Without WebSockets

WebSockets are bidirectional — great for chat. But if you only need server→client updates (live feeds, progress bars, notifications), Server-Sent Events are simpler, cheaper, and work over plain HTTP.

When to Use SSE vs WebSockets

	SSE	WebSockets
Direction	Server → Client only	Bidirectional
Protocol	HTTP	WS
Auto-reconnect	Built in	Manual
Load balancer friendly	Yes (with care)	Requires sticky sessions
Use case	Live feeds, progress, notifications	Chat, gaming, collaboration

Server Implementation

app.get('/api/events', (req, res) => {
  // Set SSE headers
  res.setHeader('Content-Type', 'text/event-stream');
  res.setHeader('Cache-Control', 'no-cache');
  res.setHeader('Connection', 'keep-alive');
  res.flushHeaders();

  // Helper to send events
  const send = (event: string, data: unknown) => {
    res.write(`event: ${event}\ndata: ${JSON.stringify(data)}\n\n`);
  };

  // Send initial state
  send('connected', { userId: req.user.id });

  // Subscribe to events
  const unsubscribe = eventBus.on('notification', (payload) => {
    if (payload.userId === req.user.id) {
      send('notification', payload);
    }
  });

  // Keep-alive ping every 30s
  const ping = setInterval(() => res.write(':ping\n\n'), 30000);

  // Cleanup on disconnect
  req.on('close', () => {
    unsubscribe();
    clearInterval(ping);
  });
});

Client Implementation

function useSSE(url: string) {
  const [events, setEvents] = useState<Event[]>([]);

  useEffect(() => {
    const sse = new EventSource(url, { withCredentials: true });

    sse.addEventListener('notification', (e) => {
      const data = JSON.parse(e.data);
      setEvents(prev => [...prev, data]);
    });

    sse.onerror = () => {
      // EventSource auto-reconnects — onerror fires on reconnect attempts too
      console.log('SSE reconnecting...');
    };

    return () => sse.close();
  }, [url]);

  return events;
}

SSE for AI Streaming

SSE is how LLM APIs stream tokens — the exact pattern Anthropic and OpenAI use:

// Stream Claude response to the browser
app.post('/api/chat', async (req, res) => {
  res.setHeader('Content-Type', 'text/event-stream');
  res.flushHeaders();

  const stream = client.messages.stream({
    model: 'claude-sonnet-4-6',
    max_tokens: 1024,
    messages: req.body.messages,
  });

  for await (const chunk of stream) {
    if (chunk.type === 'content_block_delta') {
      res.write(`data: ${JSON.stringify({ text: chunk.delta.text })}\n\n`);
    }
  }

  res.write('data: [DONE]\n\n');
  res.end();
});

SSE-based AI streaming, real-time notifications, and live updates are part of the full-stack patterns in the AI SaaS Starter Kit.

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