I think you left the realm of REST and reached out to the application level in order to make an non-existing case. Your second example is only asynchronous in

I think you left the realm of REST and reached out to the application level in order to make an non-existing case. Your second example is only asynchronous in so far as you are oblivious to the completion of of the request, but that hardly makes it non-Restful. To me both of your examples are REST by definition.