async and actors are not concepts commonly familiar to embedded developers, and have usually been associated with programming languages not suitable for embedded. With Rust, this
is about to change, as Rust allows you to use these techniques without the traditional overhead associated with it.
Software written without async may block on I/O operations. In an std environment, such as a PC, software can handle this either by using threads or non-blocking operations.
With threads, one thread blocks on an I/O operation, another is able to take its place. However, even on a PC, threads are relatively heavy, and therefore some programming languages, such as Go, have implemented a concept called coroutines or 'goroutines' that are much lighter and less-intensive than threads.
The other way to handle blocking I/O operations is to support polling the state of the underlying peripherals to check whether it is available to perform the requested operation. In programming languages without builtin async support, this requires building a complex loop checking for events.
In Rust, non-blocking operations can be implemented using async-await. Async-await works by transforming each async function into an object called a future. When a future blocks on I/O the future yields, and the scheduler, called an executor, can select a different future to execute. Compared to alternatives such as an RTOS, async can yield better performance and lower power consumption because the executor doesn’t have to guess when a future is ready to execute. However, program size may be higher than other alternatives, which may be a problem for certain space-constrained devices with very low memory. On the devices Embassy supports, such as stm32 and nrf, memory is generally large enough to accommodate the modestly-increased program size.