Long Polling vs WebSockets — How to Achieve Real-Time Communication day 55 of system design

Learn the key differences between Long Polling and WebSockets, how they work, and when to use each for real-time applications — with Python examples

Whether you’re playing an online game or chatting with a friend — updates appear in real-time without ever hitting “refresh.”

Behind these seamless experiences lies a crucial engineering decision: how to push real-time updates from servers to clients.

The traditional HTTP model was built around request–response:

“Client asks, server answers.”

But in real-time systems, the server needs to talk first — and more often.

This is where Long Polling and WebSockets come in — two popular methods to achieve real-time communication on the web.

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🧠 1. Why Traditional HTTP Isn’t Enough

HTTP follows a client-driven request–response model:

1. The client (browser/app) sends a request to the server.
2. The server processes the request and responds.
3. The connection closes.

This works fine for static or on-demand content, but for live data:

• ❌ The server can’t push updates to the client.
• ❌ HTTP is stateless, so there’s no persistent connection.
• ❌ You’d need constant polling to get new data.

To build truly real-time experiences — like live chat, multiplayer games, or financial tickers — we need a way for the server to instantly notify clients of updates.

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⏳ 2. Long Polling

Long Polling is a clever hack that simulates real-time communication over standard HTTP.

Instead of sending requests every second (like regular polling), the client sends a request and waits — keeping the connection open until the server has something new to send.

⚙️ How It Works

1. Client sends a request and waits for new data.
2. The server holds the connection open until it has data or a timeout occurs.
3. If new data arrives → server responds immediately.
4. If timeout occurs → server sends a minimal response.
5. The client immediately reopens a new connection.

This creates a near-continuous loop that feels real-time.

✅ Pros

• Simple to implement (standard HTTP).
• Works everywhere — across proxies, firewalls, and browsers.

❌ Cons

• Slight latency after each update (client must reconnect).
• Server overhead (many open “hanging” connections).

💡 Use Cases

• Simple chat apps or comment feeds.
• Notification systems (e.g., “new email” alerts).
• Legacy systems that can’t use WebSockets.

💻 Example (Python)

python
import requests
import time

def long_poll():
    while True:
        try:
            response = requests.get("http://localhost:5000/updates", timeout=60)
            if response.status_code == 200 and response.text.strip():
                print("New data:", response.json())
            else:
                print("No new data, reconnecting...")
        except requests.exceptions.Timeout:
            print("Timeout reached, reconnecting...")
        except Exception as e:
            print("Error:", e)
            time.sleep(5)
        finally:
            # Immediately re-establish connection
            continue

if __name__ == "__main__":
    long_poll()

1. WebSockets

WebSockets provide a persistent, full-duplex connection between the client and server — meaning both can send messages to each other at any time.

This removes the overhead of repeatedly opening and closing HTTP connections.

How It Works

Handshake:
The client sends an HTTP request with Upgrade: websocket.

Connection Upgrade:
The server switches from HTTP → WebSocket (ws:// or wss://).

Persistent Channel:
Both client and server can now exchange messages freely until the connection closes.

✅ Pros

Extremely low latency.

Less network overhead (single persistent connection).

Scales well for frequent or high-volume updates.

❌ Cons

Slightly more complex setup (client + server must support it).

Some firewalls/proxies may block WebSocket traffic.

Managing reconnections adds implementation complexity.

💡 Use Cases

Real-time chat and collaboration tools (Slack, Google Docs).

Multiplayer online games.

Live dashboards (sports, finance, IoT).

Example of the above use cases

import asyncio
import websockets
import json

async def connect():
    uri = "ws://localhost:6789"
    async with websockets.connect(uri) as websocket:
        await websocket.send(json.dumps({"message": "Hello Server!"}))
        print("Connected to server and sent greeting.")

        try:
            async for message in websocket:
                data = json.loads(message)
                print("Received:", data)
        except websockets.ConnectionClosed:
            print("Connection closed. Reconnecting...")
            await asyncio.sleep(2)
            await connect()

 4. Choosing the Right Approach

if __name__ == "__main__":
    asyncio.run(connect())

Factor	Long Polling	WebSockets
Implementation	Simple (HTTP-based)	Requires setup
Performance	Higher latency	Near-zero latency
Scalability	Limited for many clients	Scales efficiently
Compatibility	Works everywhere	May need proxy support
Use Case	Notifications, light updates	Real-time apps, games

🧩 5. Alternatives Worth Considering

1. Server-Sent Events (SSE)

One-way communication: server → client.

Lightweight and simple for push notifications or news feeds.

1. MQTT

Publish–subscribe protocol used in IoT.

Designed for lightweight, device-to-server messaging.

1. Socket.io

Abstraction layer over WebSockets (and Long Polling fallback).

Handles reconnections, fallbacks, and cross-browser quirks automatically.

Final Thoughts

While both Long Polling and WebSockets achieve “real-time” communication, the right choice depends on your project’s needs:

Choose Long Polling when simplicity and broad compatibility matter.

Choose WebSockets when performance, scalability, and bidirectional communication are key.

Either way, both are essential tools in building modern, dynamic, and interactive web experiences.