SSE vs Polling vs Long Polling: Real-Time Data Techniques

What Are They?

All three technologies are ways to get data from server in browser. They allow updating client in real or near real-time, especially when WebSockets aren't available.

Polling (Regular Polling)

Client periodically polls server: "Anything new?"

How Polling Works:

1. Client sends GET /data
2. Server immediately responds (even if nothing changed)
3. After X seconds client sends request again

setInterval(() => {
  fetch('/data')
    .then(res => res.json())
    .then(data => console.log(data));
}, 5000); // Request every 5 sec

Polling Cons

• Constant server load (many "empty" requests)
• Data arrives with delay
• Poor scalability

Long Polling (Long Polling)

Improved version of regular polling: request lives until new data appears.

How Long Polling Works:

1. Client makes request
2. Server waits for data to appear (or timeout expires)
3. Returns data → client immediately sends next request

function longPolling() {
  fetch('/data')
    .then(res => res.json())
    .then(data => {
      console.log(data);
      longPolling(); // Loop continues
    });
}

longPolling();

Long Polling Pros

• Fewer "empty" requests
• Data arrives almost instantly
• Doesn't require new technology (works over regular HTTP)

Long Polling Cons

• Data arrives with delay
• Poor scalability

Server-Sent Events (SSE)

SSE is persistent connection through which server sends data to client when it wants.

This is unidirectional connection: server → client only

How SSE Works

• Client creates EventSource
• Server sets text/event-stream
• Server can send messages without client request

Example (Client)

const eventSource = new EventSource('/events');

eventSource.onmessage = (event) => {
  console.log("New message:", event.data);
};

Example (Express Server)

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

  setInterval(() => {
    res.write(`data: ${JSON.stringify({ time: new Date() })}\n\n`);
  }, 3000);
});

SSE Advantages

• Simple implementation
• Works over single connection
• Ideal for notifications, chats, data streaming
• Works even through proxies (supports HTTP/2)

Comparison

What to Choose?

• Polling: if you can't use other methods or it's not critical (e.g., update every 30 sec)
• Long Polling: if data must arrive immediately, but you can't use SSE or WebSocket
• SSE: for unidirectional real-time (chats, notifications, news feeds)
• WebSocket: for bidirectional communication — games, collaborative editors, video chats, chats, etc.