Server-Sent Events (SSE)

A standard for pushing real-time updates from server to client over HTTP.

Server-Sent Events (SSE) is a standard that enables servers to push real-time updates to clients over a single HTTP connection. Unlike WebSocket, SSE is unidirectional - only the server can send data.

SSE vs WebSocket

Feature	SSE	WebSocket
Direction	Server to client	Bidirectional
Protocol	HTTP	WebSocket
Reconnection	Automatic	Manual
Browser Support	Native EventSource	Native WebSocket
Complexity	Simple	More complex

When to Use SSE

Live Feeds: News, social media updates
Notifications: Real-time alerts
Progress Updates: File upload, processing status
Stock Tickers: Price updates
Log Streaming: Real-time log viewing

Event Format

SSE uses a simple text-based format with event, data, id, and retry fields.

Advantages

Works over standard HTTP
Automatic reconnection
Event ID for resuming streams
Simple to implement

Code Examples

Node.js SSE Server

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`);
  };

  const interval = setInterval(() => {
    sendEvent({ time: new Date().toISOString() });
  }, 1000);

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

Browser EventSource

const source = new EventSource("/events");

source.onmessage = (event) => {
  const data = JSON.parse(event.data);
  console.log("Update:", data);
};

source.onerror = () => console.log("Connection error");

Related Terms

Mock Server

A fake server that simulates API responses for testing and development purposes.

API Testing

The process of verifying that APIs work correctly, securely, and perform well.

WebSocket

A protocol enabling full-duplex, real-time communication between client and server.

REST API

An architectural style for building web APIs using HTTP methods and stateless communication.