how to design asynchronous web services

How to design asynchronous web services? That me开发者_如何学Cans each web service wait for the reply from another one asynchronously?

Can anyone give me any clue?

provide the service with a callback ... so that:

service1 => service2.DoSomething();
// at a later time
service2 => service1.DoSomethingCallback(result);