Long Polling vs Short Polling

Polling is the simplest mechanism for a client to receive updates from a server. In a standard HTTP request-response model, the server cannot initiate communication -- the client must ask for new data. Short polling and long polling are two strategies for the client to discover new data as quickly as possible while working within the constraints of HTTP. Understanding both patterns, their trade-offs, and their place relative to WebSockets and Server-Sent Events is important for system design because many real-world systems use polling either as a primary mechanism or as a fallback.

Short polling is the most straightforward approach: the client sends an HTTP request to the server at a fixed interval (e.g., every 5 seconds), and the server responds immediately with the current state or an empty response if nothing has changed. The client processes the response and waits for the interval to elapse before polling again. This is trivial to implement -- it is just a timer and a standard HTTP request -- but wasteful. In a typical application, the vast majority of poll responses contain no new data. If 1 million clients poll every 5 seconds, the server handles 200,000 requests per second, most returning empty responses. The polling interval also introduces a latency floor: on average, new data is delayed by half the polling interval, and in the worst case by the full interval.

Long polling (also known as the Comet pattern) improves on short polling by eliminating most empty responses. The client sends an HTTP request, and instead of responding immediately, the server holds the request open until either: (1) new data is available, in which case the server responds with the data, or (2) a timeout expires (typically 30-60 seconds), in which case the server responds with an empty body. Upon receiving a response (either with data or a timeout), the client immediately sends a new long-poll request. This approach delivers data with near-zero latency (as fast as the server can detect and respond) and generates far fewer empty responses than short polling. However, long polling ties up a server connection (and often a thread) for each waiting client, creating scalability challenges for high-concurrency scenarios.

The choice between polling, long polling, WebSockets, and SSE depends on update frequency, infrastructure constraints, and scalability requirements. Short polling is appropriate for low-frequency updates (every 30-60 seconds) where simplicity is paramount -- dashboards, status pages, and periodic data refreshes. Long polling is appropriate for near-real-time updates over standard HTTP infrastructure, especially as a fallback when WebSockets are blocked by corporate firewalls. WebSockets are superior for high-frequency bidirectional communication (chat, gaming, live trading). SSE is the best choice for server-to-client-only push over standard HTTP. Many production systems implement a hierarchy: WebSocket as primary, long polling as fallback, and short polling as a last resort.

Imagine waiting for a package at a receptionist desk. Short polling is like walking up to the desk every 5 minutes and asking 'has my package arrived?' Most trips, the answer is 'not yet,' and you have wasted the trip. Long polling is like walking up and saying 'I will wait here until my package arrives or 30 minutes pass.' The receptionist holds you at the desk and hands you the package the moment it arrives, or sends you away after 30 minutes to come back and wait again. Long polling gets you the package faster with fewer wasted trips, but you are occupying a spot at the desk the entire time.