This is more a theoretical question than a practical one, but given I undestand the principles of SOA I am still a bit unsure about if this can be applied to any app.
The usual example is where a cliente wants to know something from a server thus we implement a service that can provide that information given a client request, it can be stateless or statefull, etc.
But what happens when we want to be notified when something happens on the server, maybe we call a service to register a search and want to be notified when a new item arrives to the server that matches or search. Of course that can be implemented using polling and leveraging that using long timeouts, but I can see a way in the usual protocols to receive events from the server without making a call to ask.
If you can point me to an example, or tell me an architecture that could support then you have made my day.