Long Polling vs Short Polling vs WebSockets vs SSE

When designing real-time systems, choosing the correct communication protocol depends on whether data flow is unidirectional (Server → Client) or bidirectional (Client ↔ Server), and the required latency tolerance:

1. Short Polling: The client periodically sends standard HTTP GET requests on a fixed timer (e.g., every 3 seconds). If no data has changed, the server returns an empty response.
2. Long Polling (Comet / Hanging GET): The client sends an HTTP request. The server holds the request open (does not respond) until either new data arrives or a timeout (e.g., 30s) occurs. Upon receiving a response, the client immediately initiates another long-poll request.
3. Server-Sent Events (SSE / text/event-stream): The client opens a single, long-lived HTTP connection (Content-Type: text/event-stream). The server continuously pushes UTF-8 formatted text chunks to the client unprompted. The browser's native EventSource API handles automatic reconnection, event IDs, and message parsing natively.
4. WebSockets: A persistent, bidirectional, full-duplex TCP protocol with lightweight framing (2–6 bytes), ideal for high-frequency interactive applications.