It is important to provide asynchronous messaging support for web services or a web service based system is likely to run into a nightmare of bottlenecks and hung services.

What type of asynchronous web service support do people currently like for their web services?

I'm using JAX-WS asynchronous client callbacks.

The reference implementation of JAX-WS also supports server-side asynchronous processing using the AsyncProvider interface though it hasn't made it into the standard yet:

http://weblogs.java.net/blog/kohsuke/archive/2006/08/jaxws_ri_goes_t.html

The use of continuation passing style in JAX-WS is actually a workflow technique adapted from compilers for functional programming languages.

Apache ODE uses a variation of this technique to implement BPEL. The Dalma project at Sun used this technique to create a system that allows workflows to be programmed in Java directly.

Unfortunately, Dalma uses runtime bytecode manipulation to implement continuations instead of the lightweight technique used in JAX-WS.

Writing an entire workflow process in continuation passing style would be difficult without support from the compiler and the VM.

Microsoft is supporting this technique in C# and F#, however, and Windows Workflow Foundation uses it.

Joe