Async: "function color bleed" problems and other issues...

[atulkhare4096]

A recent discussion with a co-worker got me thinking about how most of use async. Most tutorials start off with a top-down approach, starting with the framework. Yes, it's absolutely wonderful to use say axum to create a REST API in minutes and the prototype works amazing well out of the box.

Flash forward, and it's time to create a production solution, and now the "function color bleed" problem starts to creep in. Now, async tends to bleed its way through function signatures, starting from the top. Now, almost everything from the top to bottom requires async; that harmless sleep(10); that we had used with abandon in other code bases can have all sort of unexpected side-effects. The mutexes that worked superbly, are now a potential booby trap if we did an .await when holding it. Even simple IO operations require a special mock library while testing to account for async.

Some of us reach for spawn_blocking() in desperation but never question the basic design. These questions inspired me to create a new chapter on it: https://github.com/microsoft/RustTraining/blob/main/async-book/src/ch14-async-is-an-optimization-not-an-architecture.md.

The above are my opinions, and it will be interesting to know what others think about it.