Long Polling

Long Polling is a client-server communication technique used to achieve real-time updates by reducing unnecessary network requests and server load compared to short polling. It allows a client to wait for updates from the server without repeatedly sending requests at fixed intervals. Instead, the server holds the client's request open until there is new data available to send or a timeout occurs.

Long polling bridges the gap between traditional polling methods and real-time communication technologies like WebSockets. It is often used when real-time updates are required, but the infrastructure for WebSockets or other advanced methods is unavailable.

How Long Polling Works

1. Client Sends a Request:

   • The client sends an HTTP request to the server to check for updates (similar to short polling).

2. Server Holds the Request:

   • The server does not immediately respond. Instead, it keeps the connection open until:
     • New data is available to send to the client.
     • A predefined timeout period is reached.

3. Server Sends a Response:

   • When new data becomes available, the server sends the data as a response to the client.

4. Client Reconnects:

   • After receiving the response, the client immediately sends another request to the server to wait for more updates.

5. Cycle Repeats:

   • This process continues, ensuring that the client is always ready to receive new data as soon as it becomes available.

Key Characteristics of Long Polling

1. Reduced Redundant Requests:

   • Unlike short polling, where requests are sent at regular intervals regardless of new data, long polling only sends requests when necessary.

2. Real-Time Updates:

   • Provides near real-time updates to the client without requiring a persistent connection like WebSockets.

3. Timeout Mechanism:

   • If no new data is available within the timeout period, the server responds with an empty response, and the client reconnects.

4. Stateless Communication:

   • Each long polling request is independent, making it a stateless protocol. However, the server can maintain context if needed.

5. Compatibility:

   • Works over standard HTTP/HTTPS protocols, making it compatible with most web servers and firewalls.

Advantages of Long Polling

1. Real-Time Behavior:

   • Achieves near real-time updates without requiring a persistent connection like WebSockets.

2. Reduced Overhead:

   • Reduces unnecessary requests compared to short polling by only responding when there is new data.

3. Compatibility:

   • Works with standard HTTP protocols and does not require special configurations on the server or client.

4. Simplicity:

   • Easier to implement than WebSockets, especially for applications already using HTTP.

5. Firewall/Proxy Friendly:

   • Since it uses standard HTTP, it is less likely to face issues with firewalls or proxies compared to WebSockets.

Disadvantages of Long Polling

1. Increased Latency:

   • Compared to WebSockets, there can be a slight delay as the client must re-establish the connection after each response.

2. Resource Intensive:

   • Keeping connections open for long periods can strain the server, especially in high-traffic applications.

3. Scalability Challenges:

   • Servers need to manage many simultaneous open connections, which can become a bottleneck.

4. Not Fully Real-Time:

   • While faster than short polling, it is not as instantaneous as WebSockets because of the need to reconnect after each response.

When to Use Long Polling

Long polling is suitable for scenarios where:

1. Real-Time Updates Are Needed:

   • Near real-time updates are required, but full duplex communication (e.g., WebSockets) is unnecessary or unsupported.
   • Example: Chat applications, live sports score updates.

2. No Persistent Connection is Feasible:

   • The infrastructure does not support WebSockets or server-sent events.
   • Example: Systems behind strict firewalls or proxies.

3. Simplicity is Preferred:

   • Applications with moderate traffic where implementing WebSockets is overkill.
   • Example: Internal tools or dashboards.

Examples of Long Polling

1. Chat Applications:

   • A messaging app where the client polls the server for new messages as soon as the previous request is completed.

2. Stock Market Updates:

   • A stock trading platform that sends price updates to the client whenever new data is available.

3. Live Notifications:

   • Applications like email or task management systems that send updates when new notifications arrive.

4. IoT Devices:

   • Devices polling a server for configuration changes or commands.

Comparison: Long Polling vs Short Polling

Feature	Long Polling	Short Polling
Request Frequency	Requests are sent only after receiving a response or timeout.	Requests are sent at regular intervals, regardless of updates.
Server Behavior	The server holds the request until data is available or a timeout occurs.	The server responds immediately, even if no new data exists.
Latency	Lower latency compared to short polling.	Higher latency due to fixed request intervals.
Efficiency	More efficient, as unnecessary requests are minimized.	Less efficient, as most requests return no new data.
Use Case	Suitable for near real-time updates.	Suitable for periodic updates without strict timing needs.

Comparison: Long Polling vs WebSockets

Feature	Long Polling	WebSockets
Connection Type	Multiple HTTP requests are used (one for each polling cycle).	Persistent connection using a single TCP socket.
Latency	Higher latency compared to WebSockets due to reconnections.	Very low latency as data is pushed instantly.
Scalability	High server load due to simultaneous open connections.	More efficient for large-scale real-time applications.
Compatibility	Works with standard HTTP/HTTPS protocols.	May face issues with firewalls or proxies.
Use Case	Ideal for moderate real-time needs.	Suitable for highly interactive real-time applications.

Long Polling in Action: Example

Imagine a real-time notification system:

1. A user opens a web application, and the client sends a long polling request to the server.
2. The server checks for new notifications. If no notifications are available, the server holds the request for 30 seconds.
3. When a new notification arrives, the server sends it as a response.
4. The client processes the notification and immediately sends a new request to the server to wait for the next update.
5. If no new notifications are received within the 30-second timeout, the server responds with an empty response, and the client reconnects.