HTML Server-Sent Events (SSE)

SSE (Server-Sent Events) allow servers to push real-time updates to the browser over a single HTTP connection. Unlike WebSockets, it is one-way (server → client). Unlike normal HTTP requests, where the browser repeatedly asks for new data, with SSE the server automatically sends updates whenever new data is available.

Use Cases:

News websites (live headlines)
Stock/crypto updates (real-time prices)
Weather alerts
Social media notifications
Live sports scores

How SSE Works (Client + Server)

SSE is based on a one-way communication model:

Client (browser) → Creates a connection using EventSource.
Server → Sends events/messages in text/event-stream format.

👉 Think of it like radio broadcasting: once connected, you keep receiving updates automaticall

Client-Side (Browser) Code

You use the EventSource API in JavaScript to listen for events.

<!DOCTYPE html>
<html>
<head>
  <title>Server-Sent Events Example</title>
</head>
<body>
  <h2>🔴 Live Updates from Server</h2>
  <div id="updates"></div>

  <script>
    // Create connection to server
    const eventSource = new EventSource("server.php");

    // When a new message arrives
    eventSource.onmessage = function(event) {
      document.getElementById("updates").innerHTML += "<p>" + event.data + "</p>";
    };

    // When connection is established
    eventSource.onopen = function() {
      console.log("Connected to server ✅");
    };

    // When an error occurs
    eventSource.onerror = function() {
      console.log("Error ❌ or lost connection");
    };
  </script>
</body>
</html>

Server-Side Code Examples

(A) SSE with PHP

<?php
header("Content-Type: text/event-stream");
header("Cache-Control: no-cache");
header("Connection: keep-alive");

// Send updates every 3 seconds
while (true) {
    echo "data: Current server time: " . date("h:i:s") . "\n\n";
    ob_flush();
    flush();
    sleep(3);
}
?>

Content-Type: text/event-stream → tells browser this is SSE data.
Each message starts with data: and ends with two newlines.
flush() ensures the data is sent immediately.

(B) SSE with Node.js (Express)

const express = require("express");
const app = express();

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

  // Send an event every 3 seconds
  setInterval(() => {
    res.write(`data: Server time: ${new Date().toLocaleTimeString()}\n\n`);
  }, 3000);
});

app.listen(3000, () => console.log("SSE server running on http://localhost:3000"));

/events → SSE endpoint.
res.write("data: ... \\n\\n") → sends messages to the client.
Client connects via new EventSource("<http://localhost:3000/events>");.

Client connects using:

const eventSource = new EventSource("http://localhost:3000/events");

Custom Events in SSE

You can send custom events instead of only onmessage.

Server (PHP Example):

echo "event: priceUpdate\n";
echo "data: Bitcoin Price: $40,000\n\n";
flush();

Client:

eventSource.addEventListener("priceUpdate", function(event) {
  console.log("Custom Event → " + event.data);
});

Closing an SSE Connection

When updates are no longer needed:

eventSource.close();
console.log("Connection closed ❌");

Error Handling in SSE

SSE automatically tries to reconnect if the connection is lost, but you should still handle errors.

eventSource.onerror = function() {
  console.log("⚠️ Lost connection, retrying...");
};

Advantages & Limitations

Advantages:

Simple & lightweight compared to WebSockets.
Built-in auto-reconnect.
Works over normal HTTP/HTTPS.

Limitations:

One-way only (server → client).
Cannot send binary data (only text).
Not supported in Internet Explorer.

Real-World Applications of SSE

News apps → Live headlines
Finance → Real-time stock/crypto updates
Social media → Notifications
Sports → Live scores
Weather → Instant weather alerts