comet in web development
Comet is a web application model in which a web server sends data to the client without the client having to request it (or poll for it) explicitly. It's a technique that enables web pages to be updated by the server with new information as soon as it becomes available, making it possible to build web applications that need to display real-time data such as chat apps, live sports scores, stock tickers, and more.
Comet is an umbrella term for multiple techniques that enable servers to send data to the client once an initial client connection has been established. Unlike traditional web request/response models where the client always initiates communication, Comet establishes a more persistent connection for pushing data from server to client.
How Comet Works
Comet can be implemented using several different approaches, each with its own trade-offs. The two primary techniques are long polling and streaming:
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Long Polling: The client sends a request to the server, just like a normal HTTP request, but instead of responding immediately with available data, the server holds the request open until new data becomes available or a timeout occurs. Once the response is sent to the client, the client immediately sends another request, and the process repeats. This creates a near-continuous connection where the server can send data to the client at any time.
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Streaming: The client opens a connection to the server, which is kept open indefinitely. The server continuously sends data to the client over this single, long-lived connection. This could be achieved through various means, including using an iframe class='youtube-video' or the HTML5 EventSource API. The server decides how much and when to send data, which can be more efficient than long polling in scenarios where data is frequently updated.
Comet in Modern Web Development
While Comet was a significant advancement in enabling real-time web applications, the development of WebSockets and newer HTML5 APIs has provided more efficient, standardized methods for real-time communication between browsers and servers. WebSockets, in particular, establish a full-duplex communication channel over a single long-lived connection, making them more efficient than Comet techniques for real-time web applications.
However, understanding Comet is still valuable as it represents the evolution of web technologies striving to achieve real-time communication. It also serves as a foundation for understanding the limitations and challenges that led to the development and widespread adoption of WebSockets and other real-time technologies in modern web applications.
Use Cases
Despite the emergence of newer technologies, Comet-based techniques might still be used in cases where WebSockets cannot be implemented due to browser support issues, corporate firewalls, or proxy servers that do not support WebSocket connections. In such scenarios, Comet techniques like long polling serve as a viable fallback to achieve real-time communication.