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Let's review: Comet basically means keeping an HTTP connection open between the server and the browser so the former can stream messages to the latter without polling (see the Ajax Patterns page on HTTP Streaming for info on how it's done). This poses a problem for commonly used web infrastructure, like app servers, which usually associate one thread per connection. If you have lots of open connections, let's say 10,000, then you will need that many threads that are doing very little but sitting around waiting for the next message to be sent. Regardless of OS, 10,000 threads puts a pretty heavy load on a system. As a solution, Greg Wilkins proposed an extension of the Servlet standard to allow "continuations." What are they?