How to keep a HTTP connection alive?

Question

Is there a way to keep a HTTP connection alive with JavaScript?

Answer 1

In short, I think the concept of long lived http connections in javascript really revolve around a style of communication called COMET. This can be achieved in several different ways, but essentially involves the client (using XmlHttp powers) requesting data from the server immediately, and the server withholding the response until some event triggers it. Upon receipt of this response, the client immediately makes another request (which will once again hang at the server end until something needs sending). This simulates server push, but is effectively nothing more than a delayed response used in a clever way. In the worst case, there can be fairly high latency (i.e. 2 messages need sending, so the cycle must be twice repeated, with all the costs involved) but generally, if the messaging rate is low, this gives a reasonable impression of real-time push.

Implementing the server-side for this kind of communication is far from trivial, and requires a good deal of asynchronous communications, concurrency issues and the like. It's quite easy to write an implementation that can support a few hundred users each on their own thread, but to scale to the thousands requires a much more considered approach.