Async Channels

Several crates have support for asynchronous channels. For instance tokio:

use tokio::sync::mpsc::{self, Receiver};

async fn ping_handler(mut input: Receiver<()>) {
    let mut count: usize = 0;

    while let Some(_) = input.recv().await {
        count += 1;
        println!("Received {count} pings so far.");

    println!("ping_handler complete");

async fn main() {
    let (sender, receiver) = mpsc::channel(32);
    let ping_handler_task = tokio::spawn(ping_handler(receiver));
    for i in 0..10 {
        sender.send(()).await.expect("Failed to send ping.");
        println!("Sent {} pings so far.", i + 1);

    ping_handler_task.await.expect("Something went wrong in ping handler task.");
This slide should take about 8 minutes.
  • Change the channel size to 3 and see how it affects the execution.

  • Overall, the interface is similar to the sync channels as seen in the morning class.

  • Try removing the std::mem::drop call. What happens? Why?

  • The Flume crate has channels that implement both sync and async send and recv. This can be convenient for complex applications with both IO and heavy CPU processing tasks.

  • What makes working with async channels preferable is the ability to combine them with other futures to combine them and create complex control flow.