Introduction

When your program asks for something that takes time, e.g reading a file, fetching data from a server, waiting for a timer, it has two choices: sit and wait doing nothing, or go do something else while it waits.

Blocking means sitting and waiting. Your program freezes until the result comes back. Nothing else can happen in the meantime.

Non-blocking (async) means your program starts the task, steps away, and comes back to collect the result when it is ready. While it waits, it can work on other things.

In Rust, async code is written using two keywords:

• async marks a function as non-blocking. Calling it does not run it immediately. It gives you back a task that is ready to be run.
• .await is where you hand control back and say "run this task, and come back to me when it is done."