Async/Await

Your hotel booking API confirms a reservation, then calls an email service to send the confirmation. To hand back the finished response in one step, the controller action calls .Result on that email call.

During a weekend sale, thousands of bookings land within minutes. Each request's thread just sits there, blocked on .Result, doing nothing but waiting for the email service to answer.

ASP.NET Core only hands out so many worker threads. Once they're all stuck waiting, new requests queue up behind them, even though the CPU itself is barely doing any work.

Blocking a thread on an async call wastes it instead of freeing it, and the thread pool runs out. That's exactly what async/await, used properly, avoids.

Swap in async/await and the threads stop piling up: use await instead of .Result or .Wait() when calling an async method, and make every method in the call chain async, from the controller action down to the HTTP call, instead of stopping partway and blocking. Do that, and the thread gets released back to the pool the moment the awaited call starts, picking the work back up once the response is ready, so your worker threads can start serving other requests while the email service is still working, instead of standing idle and blocked.

Await gives the thread back while it waits; .Result keeps it hostage.

A waiter takes your order and hands it to the kitchen, then goes straight to serving other tables while your food cooks.

A waiter who instead stood at the kitchen window watching until your plate came out could serve only one table for the whole meal, no matter how many other tables were waiting.

Walking away to serve someone else while the kitchen works, instead of standing frozen at the window.