What is HTTP Long Polling?

HTTP long polling is a technique used in web development to enable servers to push information to a client whenever there is new data available, without the client having to send a request each time. Long polling is a variation of the traditional polling technique to reduce latency and improve real-time data updates, making the communication between a client (such as a web browser) and a server more efficient than standard polling.

How HTTP Long Polling Works

1. Client Request: The client makes an HTTP request to the server.

2. Server Holding: Instead of responding immediately if there is no data available, the server holds the request open, waiting for new data to become available.

3. Response on Event: If new data appears or an event occurs, the server responds to the held request with the new information.

4. Close and Reopen: After the client receives the new data, it immediately sends another request, restarting the process. This results in the client always having a pending HTTP request, which the server can respond to as soon as new data is available.

5. Timeout: If no new data is available after a certain timeout, the server sends a response back to close the connection. The client then immediately makes a new request, and the cycle repeats.

Advantages of HTTP Long Polling

• Reduced Latency: Unlike traditional polling, where clients might poll the server frequently only to find there is no new data, long polling ensures that responses are sent as soon as new data is available, reducing overall latency.
• Efficient Communication: This approach minimizes the number of requests needed when compared to standard polling because new requests are sent only after the previous request has been answered.
• Simplicity: Long polling can be implemented relatively easily on top of existing infrastructure without requiring special protocols or technologies, making it accessible for many web applications.

Disadvantages of HTTP Long Polling

• Resource Utilization: Long polling can still be resource-intensive on the server because each client holds a connection open for a long time, which could lead to resource exhaustion under high load.
• Scalability Issues: Managing a large number of concurrent connections can be challenging and may require additional server-side management and infrastructure adjustments.
• Complexity in Connection Management: Handling timeouts, network errors, and retries can complicate the server and client-side code.

Comparison to Other Techniques

• Traditional Polling: In traditional polling, the client repeatedly sends HTTP requests at regular intervals to check for updates, which can result in higher latency and more network traffic.
• WebSockets: WebSockets provide a full-duplex communication channel that allows for more interactive communication between the client and server. Unlike long polling, WebSockets maintain a single persistent connection and allow for real-time, bi-directional communication without the overhead of repeatedly opening and closing connections.
• Server-Sent Events (SSE): SSEs are another alternative where the server can push updates to the client over a single long-held HTTP connection. Unlike long polling, SSEs are specifically designed for handling server-push events and do not require the client to re-establish a connection after receiving data.

Use Cases

HTTP long polling is suitable for applications where it is crucial to receive updates in a timely manner but where the frequency of updates does not justify maintaining a WebSocket connection. Examples include:

• Chat applications where new messages need to be displayed as soon as they are available.
• Live dashboards that update periodically with new data.
• Notification services where users must be alerted to events or changes in status.

Conclusion

While HTTP long polling is an improvement over traditional polling in terms of reducing latency and network overhead, it comes with its own set of challenges regarding server resource utilization and complexity. Newer technologies like WebSockets or Server-Sent Events may offer better performance and scalability depending on the application requirements. However, long polling remains a viable option, especially when upgrading or modifying existing applications without the infrastructure for more complex communication protocols.