async-implementation-details.md

Async Implementation Details

Main components of Async/Await

• Tasks
• Wakers
• Executors
• Pin

Tasks

• Every async function that awaits creates tasks
• Effectively "subtasks" of the function
• Tasks describe dependencies

Tasks example

async fn learn_and_sing() {
    let song = learn_song().await;  // 1
    sing_song(song).await;          // 2
    sing_song(song).await;          // 3
}

Tasks implementation

• Functions become a state machine
• State machines can restart once progress can be made
• Similar to generators

Wakers

• Polling all the tasks is a busy wait
• Wakers allow us to register a way to wake

Pin

• All types by default can move
• But what if you want to prevent it
• Pin: underlying memory can not move

Pin Example

#[tokio::main]
async fn main() {
    let mut stream = async_stream();
    let sleep = time::sleep(Duration::from_secs(10));
    tokio::pin!(sleep);

    loop {
        tokio::select! {
            maybe_v = stream.next() => {
                if maybe_v.is_none() { break }
                println!("got = {:?}", maybe_v);
            }
            _ = &mut sleep => {
                println!("timeout: 10 secs elapsed");
                break;
            }
        }
    }
}

Executors

• Start with the top-level Futures and drive to completions
• Calls wake() when a task can make progress