Asynchronous means that the requestor gets the response immediately, but the result is not there. Instead, the requestor waits until the result is sent to them, or saved in the database, or, for example, presented as an object that allows checking if the result is ready. If the latter is the case, the requestor calls a certain method to this object periodically and, when the result is ready, retrieves it using another method on the same object. The advantage of asynchronous processing is that the requestor can do other things while waiting.
In Chapter 8, Multithreading and Concurrent Processing, we have demonstrated how a child thread can be created. Such a child thread then sends a non-asynchronous (blocking) request and waits for its return doing nothing. The main thread, meanwhile, continues executing and periodically calls the child thread object to...