Comet - Pushing Data over Http

As described in this wiki article, Comet is a term, coined by Alex Russell, to describe the technologies for pushing data over Http. What is Comet? Comet is a web architecture allowing a web server to push data to a web browser using Http protocol. It is realized by taking advantage of the Http persistent connection. There are many solutions available, both open and proprietary. HTML 5 specification even attempts to standardize the Comet transport by adding a new HTML element, event-source and a new data format, called the DOM event stream. However, several issues have to be considered when one tries to adopt the technology.

First, most web servers assign a thread for handling a connection request. Since Comet keeps its http connection with a web server open, a web server can only serve limited amount of Comet connection and will run out of its threads pretty soon. A dedicated comet server should be deployed for pushing data.

Second, the traditional approach of scaling web applications by adding web servers might not be applicable to a Comet server. If a user connect to multiple event sources, it's difficult to distribute users among multiple Comet servers.

Third, proxy servers and firewalls could drop connections that have been open for too long. Though some Comet frameworks would tear down and recreate connection constantly, some proxies could send back buffered data and deceive clients into believing a connection is established.