Long Polling
Long Polling is a server-push technology that enables servers to deliver information to clients. In this approach, the client initiates a request to the server, and the server holds the request open until new data is available. Once new data arrives, the server responds to the pending request and sends the data to the client. After receiving the response, the client immediately initiates another request, repeating this cycle. The main advantage is its simplicity in implementation and good compatibility with older browsers. However, it has a drawback: each data update requires re-establishing the connection, which increases latency and server load.
Example: In an online chat application using Long Polling, the client sends an HTTP request to wait for server messages. If no new messages arrive within 10 seconds, the server returns an empty response, and the client immediately sends another request to wait.
WebSockets
WebSockets is a network communication protocol that enables full-duplex communication over a single connection. It simplifies and enhances data exchange between clients and servers. Once a WebSocket connection is established, both the server and client can send data to each other at any time from either end. WebSockets are particularly well-suited for applications requiring real-time interaction.
Example: In a stock market ticker display system, using WebSockets allows real-time stock price updates to be pushed to the client without requiring frequent page refreshes or reconnections.
Server-Sent Events (SSE)
Server-Sent Events (SSE) is a technology that enables servers to send updates to clients, designed for establishing unidirectional connections to the server. After the client establishes a connection, it can only receive data from the server. SSE is highly effective for simple one-to-many broadcasts, such as real-time news headlines or blog post updates.
Example: On a news website, editors can push updates of the latest news to all online readers, while the readers' browsers passively receive the information without manual refreshes.
Comet
Comet is an umbrella term for techniques that use Long Polling to enable servers to push data to clients. It simulates server-push functionality, primarily leveraging JavaScript and HTTP persistent connections. Comet is designed to create more dynamic web applications, allowing servers to send data to clients in real-time without additional client requests. It can be implemented through various methods, such as iframes or script tags.
Example: In a real-time multiplayer game, where the server needs to continuously push status updates of other players to all clients, Comet technology facilitates this real-time data push.
Each of these technologies has specific use cases and trade-offs. Selecting the right technology depends on the application's requirements and implementation complexity.