Hello,
I found this very old post
http://tech.groups.yahoo.com/group/xml-rpc/message/2306, asking a similar
question as I will do in a moment. The only reply mentions that there *is* a
solution, but not how it looks like.

I have a situation where clients (behind a firewall, blocking incoming
connections) need to register with a server for the purpose of receiving
notification of certain events.

The asynchronous client documentation
http://xmlrpc-c.sourceforge.net/doc/libxmlrpc_client++.html#asynch together with
the example code xmlrpc_asynch_client.c show, how I can register a one-time
callback. But my understanding is that this only waits for the server side
procedure to return, upon the return the callback is executed, and then the
callback can't be executed again from the server side.

How can I setup a client-server pair, where the server can execute the same
client callback repeatedly? A workaround is of course to again register the
client callback immediately whenever the client is calledback from the server.
But this causes a large overhead.

Is it possible to use xmlrpc for bidirectional communication, where both the
client and server can initiate a RPC at any time? Preferably using only a single
connection that is initially established from the client to the server
(firewall!) without keeping a second connection open for RPCs initiated by the
server? Theoretically I could come up with such a solution using the basic HTTP
protocol after *lots* of programming efforts - it is definitely possible. But
I'd prefer an xmlrpc solution, as I find that protocol much easier to apply.

Kind regards,
A. Kalaghan