Long Polling

Long polling is a technique used in web applications to provide real-time updates to the client without the need for constant polling. It is an improvement over traditional polling methods, allowing the server to hold a request open until new data is available, significantly reducing latency and unnecessary requests.

In long polling, the client sends a request to the server, and the server keeps the request open until it has information to send back. Once the data is sent, the client immediately sends a new request, establishing a continuous connection.

Basic Flow

graph TD;
    A[Client Sends Request] --> B[Server Holds Request];
    B --> C{Data Available?};
    C -->|Yes| D[Send Data to Client];
    C -->|No| B;
    D --> A;
            

Server-side (Node.js Example)

const express = require('express');
const app = express();
let clients = [];

app.get('/poll', (req, res) => {
    clients.push(res);
});

function notifyClients(data) {
    clients.forEach(res => res.json(data));
    clients = [];
}

// Simulate data update after 5 seconds
setTimeout(() => {
    notifyClients({ message: 'New data available!' });
}, 5000);

app.listen(3000, () => {
    console.log('Server running on port 3000');
});
            

Client-side (JavaScript Example)

function poll() {
    fetch('/poll')
        .then(response => response.json())
        .then(data => {
            console.log(data);
            // Immediately call poll again
            poll();
        });
}

// Start polling
poll();
            

• Implement timeouts to prevent hanging requests.
• Use a maximum number of clients to avoid server overload.
• Optimize server response times to enhance user experience.
• Consider using WebSockets for bi-directional communication when necessary.