If client is not connected to the server, it means that the client is unknown to the server, virtually does not exist. In this case, requiring HTTP server to "call" the client (what is that, by the way? :-)) would mean to contradict yourself.
HTTP itself is one the asymmetric (client-server) stateless protocol. It's based on pure
pull technology. The clients connect to the HTTP server (not visa versa), because they "know" the IP address and port to connect to. There is nothing like that to allow the HTTP server to connect to client (even if the server was designed to do so). And what you need looks like
server push. Please see:
http://en.wikipedia.org/wiki/Pull_technology[
^],
http://en.wikipedia.org/wiki/Server_push[
^],
http://msdn.microsoft.com/en-us/library/ms751522.aspx[
^].
[EDIT]
Pay attention for the SignalR technology:
http://www.asp.net/signalr/overview/getting-started/introduction-to-signalr[
^].
Real-time web functionality is the ability to have server code push content to connected clients instantly as it becomes available, rather than having the server wait for a client to request new data.
Pay special attention:
to connected clients.
—SA
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