There is a strange API I need to work with. 
I want to make a HTTP call to the API and the API will return success but I need to wait for request from this API before I respond to the client. 
What is the best way to accomplish that?
Is it an option to make your API REST-ful?
An example flow: Have the client POST to a url to create a new resource and GET/HEAD for the state of that resource, that way you don't need to block your client while you do any blocking stuff.
Listen for http request in the body of RequestHandler