Server-Sent Events, Polling and Long Polling: What They Are and When to Use

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.