Mastering Polling Techniques in Node.js: Short Polling vs. Long Polling

Introduction

In backend engineering, polling is a common technique for handling request-response patterns in cases where regular updates are needed. Unlike the usual approach where a client sends a request and gets an immediate response, polling works by repeatedly asking the server for updates. This makes it a good fit for applications that need real-time data or frequent updates, like social media feeds, notifications, and monitoring tools.

In this article, we’ll break down what polling is and cover two main types: short polling and long polling. With some examples in Node.js, we’ll look at how each approach works, along with the pros and cons, to help you decide which option might be right for your project.

What is Polling?

Polling is a method where a client sends repeated requests to a server at specified intervals to check for new data. In a typical request-response (req/res) model, the client initiates a request, and the server responds immediately. With polling, however, the server may or may not have new information to deliver. Polling ensures that the client is regularly informed of any updates, although it may involve sending requests when no new data is available.

Types of Polling: Short Polling and Long Polling

In the context of backend systems, two common forms of polling exist:

• Short Polling: The client sends requests at regular intervals without waiting for new data.

• Long Polling: The client waits for the server to respond with new data, reducing the frequency of unnecessary requests.

Each has its own applications, benefits, and limitations, which we’ll explore next.

What is Short Polling?

Short polling, or periodic polling, involves sending requests at regular intervals, such as every few seconds, to check if new data is available. This is a simpler approach, as the client doesn’t rely on the server to notify it about new data. Instead, the client repeatedly checks the server on its own.

Short Polling: Pros and Cons

Pros:

• Simple to Implement: Short polling is straightforward to set up and requires no special configuration on the server-side.

• Consistent: Clients receive updates at regular intervals, making it predictable for time-sensitive applications.

Cons:

• High Server Load: Since requests are sent regularly, even when no new data is available, short polling can create unnecessary traffic and put strain on server resources.

• Latency: There’s always a delay between requests, meaning clients might not receive updates as soon as they’re available.

Short Polling Code Example (Node.js)

Here’s a basic example of how short polling can be implemented in Node.js:

const axios = require('axios');

// Function to check for updates every 5 seconds
function checkForUpdates() {
    axios.get('http://localhost:3000/data')
        .then(response => {
            console.log('Received data:', response.data);
        })
        .catch(error => {
            console.error('Error fetching data:', error);
        });
}

// Polling every 5 seconds
setInterval(checkForUpdates, 5000);

In this example, the client calls the server every 5 seconds to check for updates. If new data is available, it gets logged to the console.

What is Long Polling?

Long polling is a more optimized approach. Instead of sending regular requests regardless of data availability, the client sends a request to the server and waits. If the server has new data, it responds immediately. If not, the server holds the request open until new data becomes available or a timeout occurs. Once the client receives a response, it immediately sends another request to keep the connection open.

Long Polling: Pros and Cons

Pros:

• Reduced Server Load: Long polling minimizes the number of unnecessary requests, as the server only responds when new data is available.

• Lower Latency: Clients receive updates as soon as new data is available, ensuring they are always up-to-date.

Cons:

• Connection Overhead: Maintaining open connections can be resource-intensive, especially if there are many clients.

• Implementation Complexity: Long polling requires more handling on both the server and client sides to maintain and manage open requests.

Long Polling Code Example (Node.js)

Below is an example of how long polling can be implemented in Node.js:

// Server side 
// Server.js
const express = require('express');
const app = express();
const PORT = 3000;

// Sample data
let data = null;

// Endpoint for client requests
app.get('/data', (req, res) => {
    if (data) {
        res.send(data); // Send data if available
        data = null; // Reset data after sending
    } else {
        // Hold the request open until new data is available
        setTimeout(() => {
            res.send('No new data');
        }, 10000); // Timeout after 10 seconds
    }
});

// Simulate new data arriving every 15 seconds
setInterval(() => {
    data = { timestamp: Date.now(), message: 'New data available!' };
}, 15000);

app.listen(PORT, () => {
    console.log(`Server running on http://localhost:${PORT}`);
});

//Client side
const axios = require('axios');

// Long polling function
function longPoll() {
    axios.get('http://localhost:3000/data')
        .then(response => {
            console.log('Received data:', response.data);
            longPoll(); // Call again immediately to continue long polling
        })
        .catch(error => {
            console.error('Error fetching data:', error);
            setTimeout(longPoll, 5000); // Retry after delay if error occurs
        });
}

// Start long polling
longPoll();

In this example, the server waits up to 10 seconds before responding if no new data is available, allowing clients to keep their connections open without frequent requests. Once the client receives data, it immediately initiates another request to maintain the polling.

Conclusion

Polling is a versatile technique to keep clients updated with new data. Short polling is a straightforward but resource-intensive approach, ideal for less frequent updates or non-critical data. Long polling offers a more efficient solution for real-time applications, reducing server load and providing faster updates. However, it requires careful implementation to handle open connections effectively.

Choosing between short polling and long polling depends on the application’s specific requirements, such as update frequency, data importance, and server resource availability. By mastering these polling techniques in Node.js, developers can build responsive, real-time applications tailored to diverse user needs.