Message Broker

This sounds very much like the use of mocking, which iam familiar with injava or grove. You've abstracted that concept and moved it out to the h t t p server rather than the programming language. And then you can do the same thing with messaging as well. So if you've got services that are communicating, fiare a message broker, it's pretty straightforward to have a test double that when it sees when it receives message x, it sends back message y.