“Unfortunately, cashiers are seldom asynchronous.”

Have you ever been standing in the express lane of a grocery store, buying a single bottle of water, only to have the customer in front of you challenge the price of an item, causing you and everyone behind you to wait five minutes for the price to be verified? Plenty of explanations of asynchronous programming exist, but I think the best way to understand its benefits is to wait in line with an idle cashier. If the cashier were asynchronous, he or she would put the person in front of you on hold and conduct your transaction while waiting for the price check. Unfortunately, cashiers are seldom asynchronous. In the world of software, however, event-driven servers make the best use of available resources, because there are no threads holding up valuable memory waiting for traffic on a socket. Following the grocery store metaphor, a threaded server solves the problem of long lines by adding more cashiers, while an asynchronous model lets each cashier help more than one customer at a time.