SSE — Explained with Examples

SSE (Server-Sent Events) is a one-way push technology that enables servers to stream data to clients over a single HTTP connection.

SSE stands for Server-Sent Events, defined as part of HTML5. Unlike WebSocket, SSE is unidirectional — only the server can send messages to the client.

How SSE Works

The client opens a standard HTTP connection, and the server keeps it open, sending data as events. The browser automatically parses these events and fires JavaScript events.

// Server-side (Node.js with Express)
app.get('/events', (req, res) => {
  res.writeHead(200, {
    'Content-Type': 'text/event-stream',
    'Cache-Control': 'no-cache',
    'Connection': 'keep-alive'
  });

  // Send an event every 2 seconds
  const interval = setInterval(() => {
    const data = JSON.stringify({
      time: new Date().toISOString(),
      value: Math.random()
    });
    res.write(`data: ${data}\n\n`);
  }, 2000);

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

// Client-side
const eventSource = new EventSource('/events');

eventSource.onmessage = (event) => {
  const data = JSON.parse(event.data);
  console.log('Server says:', data);
};

eventSource.onerror = (err) => {
  console.error('Connection lost, auto-reconnecting...');
};

Expected output: Client receives {"time":"2026-06-19T12:00:00Z","value":0.8472} every 2 seconds.

Real-World Analogy

SSE is like a radio broadcast. The radio station (server) transmits continuously, and you (client) tune in to receive. You can’t talk back to the station through the radio — it’s one-way. If your radio loses signal, it automatically re-tunes to the right frequency (auto-reconnect).

When to Use SSE vs WebSocket

Related Terms

WebSocket, REST, HTTP, API Gateway