I have exactly the same scenario.

What I understand, if I use SoapDocumentMethod(OneWay:=True) in webmethod, then the client need not wait for the response from the webservice. If this is true, then do we require a callback method in the client? If not, then can I pass nothing,nothing as a parameter inplace of the callback method and async object.

Since no callback is mentioned and the client doesn't expect any response from the webservice, then webservice will keep running even if the aspx is fully rendered or aspx session is closed. right? (assuming webservice is running on a different webserver)

In my case, the webservice will be called from an aspx page running in a webfarm, even then this scenario will run? Because the webservice will not know which callback method to call in webfarm.

Actually, I don't want callback because, I assume that this will not work in a webfarm scenario.