Long Polling: The Patient Connection

Learn HTTP long polling - a server-side technique that holds connections open until data arrives. Achieve near real-time updates with standard protocols.

January 22, 2025|networking polling http real-time comet

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Long Polling: The Patient Waiter

Like a patient waiter who stands by your table until you're ready to order, long polling holds the connection open until the server has something to deliver. This eliminates the constant back-and-forth of short polling while maintaining HTTP compatibility.

Speed:

Connection Timeout30s

Event FrequencyMedium

Key Insight: The Patient Connection

Notice how the connection stays open until data is ready? This eliminates wasted requests. When an event occurs, it is delivered instantly - no waiting for the next poll interval. Efficiency approaches 100% because every response contains actual data.

Key Concepts

Server Holds Connection: Instead of responding immediately, server waits for data
Immediate Reconnect: Client reconnects as soon as it receives a response
Near-Zero Latency: Events are delivered the moment they occur
~100% Efficiency: Every response contains actual data (or is a timeout)

The key insight: connections return data, not empty responses. No wasted bandwidth on "nothing new" replies.

How It Works

Client                          Server
  |                               |
  |-------- GET /api/data ------->|
  |                               | (Hold connection)
  |         Connection held       | (Waiting for event...)
  |         (10 seconds...)       |
  |                               | EVENT OCCURS!
  |<------- 200 OK (data) --------|
  |                               |
  |-------- GET /api/data ------->| (Immediate reconnect)
  |                               | (Hold connection)
  ... (continues) ...

Implementation

import requests
from requests.exceptions import Timeout

def long_poll(url, timeout=30):
    """Long poll with immediate reconnect."""
    while True:
        try:
            response = requests.get(url, timeout=timeout)
            if response.status_code == 200:
                handle_data(response.json())
            # Immediate reconnect - no sleep!
        except Timeout:
            # Server timeout - reconnect immediately
            continue
        except Exception as e:
            time.sleep(1)  # Brief pause on error