What is SSE?

Server-Sent Events (SSE) is a standard for pushing updates from server to client over HTTP. Unlike WebSocket, it's unidirectional (server to client only).

SSE vs WebSocket

Aspect	SSE	WebSocket
Direction	Server → Client only	Bidirectional
Protocol	HTTP	WebSocket
Reconnection	Automatic	Manual
Browser support	Native EventSource	Native WebSocket
Complexity	Simple	More complex

How SSE Works

Client                              Server
   │                                   │
   │─── GET /events ──────────────────►│
   │    Accept: text/event-stream      │
   │                                   │
   │◄── HTTP 200 ─────────────────────│
   │    Content-Type: text/event-stream│
   │                                   │
   │◄── data: message 1 ──────────────│
   │◄── data: message 2 ──────────────│
   │◄── data: message 3 ──────────────│
   │    (connection stays open)        │

Event Format

event: update
id: 123
retry: 5000
data: {"price": 150.25}

event: notification
data: New message received

Field	Description
event	Event type (optional)
id	Event ID for reconnection
retry	Reconnection time in ms
data	Event payload

Client Example (JavaScript)

const source = new EventSource('/api/events');

source.onmessage = (event) => {
  console.log('Message:', event.data);
};

source.addEventListener('update', (event) => {
  const data = JSON.parse(event.data);
  console.log('Update:', data);
});

source.onerror = () => {
  console.log('Connection error, will auto-reconnect');
};

Server Example (Node.js)

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

  const sendEvent = (data) => {
    res.write(`data: ${JSON.stringify(data)}\n\n`);
  };

  // Send updates
  const interval = setInterval(() => {
    sendEvent({ time: new Date() });
  }, 1000);

  req.on('close', () => {
    clearInterval(interval);
  });
});

Use Cases

Use Case	Why SSE
Live feeds	News, social media updates
Notifications	Push alerts to users
Progress updates	File upload, long tasks
Stock tickers	Price updates
Log streaming	Real-time logs

When to Use SSE vs WebSocket

Requirement	Choose
Server → Client only	SSE
Bidirectional needed	WebSocket
Simple implementation	SSE
Binary data	WebSocket
Auto-reconnection	SSE
Low latency bidirectional	WebSocket

Interview Tips

Explain unidirectional nature
Compare with WebSocket and polling
Describe auto-reconnection with event IDs
Know the event format (event, id, data)
Give use cases: notifications, live updates

Server-Sent Events

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