I am writing web services with VB.NET (2.0). The client side will use different technologies such as Java and Siebel. So, if I write asynchronous web services, is it possible for different platforms to consume them? I know it is quite easy with a consumer having .NET platform, but not sure for different platforms. Is it enough for them to have asynchronous ability, or should it be a similar platform?
I don't know that you are ever writing an asynchronous web service. Your call to a web service is asynchronous. So in that case it's more of what can the caller consume. If you create a web service that returns a type that your caller cannot consume except a certain platform, then you have created a web service that is platform dependent in it's payload response, while not in it's mechanism. Yes, you can call a web service with anything, say, Java, but can you consume everything being send back? No you cannot.
So again, you're not creating async web services. You're making async calls to those services and hopefully you're sending something that everyone that calls that service can use (xml, json, etc.)