"Synchronous" or "Asynchronous" is the behavior of the client that is requesting the resource. It has nothing to do with the REST web services, its structure, or the supporting server.
- The client constructs an HTTP structure, sends over the socket connection.
- Waits for the response HTTP.
- The client constructs the HTTP structure, sends the request, and moves on.
- There's another thread that is waiting on the socket for the response. Once the response arrives, the original sender is notified (usually, using a callback like structure).