Comet is an umbrella term for technologies that attempt to eliminate both the limitations of the page-by-page web model and traditional polling. Comet-like applications offer real-time interaction by relying on a persistent HTTP connection (or where not possible a long lasting HTTP connection) to provide the browser with updates as designated by the web application. Since browsers and proxies are not designed with server events in mind, web application developers have tried to work around several unintended side-effects to implement Comet-like behavior, each with different benefits and drawbacks. In particular, the HTTP 1.1 specification states that a browser should not have more than 2 simultaneous connections with a web server. However, holding one connection open for real-time events has a negative impact on browser usability. The browser may be blocked from sending a new request while it still loads, for example, a series of images. This can be worked around by creating a distinct hostname for real-time information, which is an alias for the same physical server.
Specific methods of implementing Comet fall into two major categories: streaming and long polling.
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