7.2. Asynchronous Web Methods

By default, all web methods declared on a web service are synchronous on the server side. However, the call from the browser via XML HTTP is asynchronous, but the actual execution of the web method at the server is synchronous. This means that from the moment a request comes in to the moment the response is generated from that web method call, it occupies a thread from the ASP.NET worker pool. If it takes a relatively long period of time for a request to complete, then the thread that is processing the request will be in use until the method call is done. Unfortunately, most lengthy calls are due to something like a long database query or perhaps a call to another web service. For instance, if you make a database call, the current thread waits for the database call to complete. The thread simply has to wait around doing nothing until it hears back from its query. Similar issues arise when a thread waits for a call to a TCP socket or a backend web service to complete.

When you write a typical ASP.NET web service using web methods, the compiler compiles your code to create the assembly that will be called when requests for its web methods are received. When your application is first launched, the ASMX handler reflects over the assembly to determine which web methods are exposed.