Example of concurrent processing with rate limit and cancellation with async-std

.gitignore

/target

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Cargo.toml

[package]
name = "rate_limit_example"
version = "0.1.0"
authors = []
edition = "2018"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
async-std = "1.9.0"
governor = "0.3.2"
futures = "0.3.15"
rand = "0.8.3"
nonzero_ext = "0.2.0"
futures-util = "0.3.15"

main.rs

use async_std::stream;
use async_std::task;
use futures::StreamExt;
use governor::state::direct::StreamRateLimitExt;
use governor::{Jitter, Quota, RateLimiter};
use nonzero_ext::*;
use rand::random;
use std::time::Duration;
use async_std::channel::bounded;

enum Cancellable<T> {
    Cancelled(T),
    Processed(T),
}

async fn main_async() {
    // Produce 100 ints
    let mut input = stream::from_iter(0..100);

    // a channel of 1 item is used to implement cancellation with fallback for ignored items
    let (process_sender, process_stream) = bounded(1);
    let (result_sender, mut result_stream) = bounded(1);

    let result_task_handle = task::spawn(async move {
        while let Some(item) = result_stream.next().await {
            match item {
                Cancellable::Cancelled(i) => {
                    println!("cancelled item: {}", i)
                }
                Cancellable::Processed(i) => {
                    println!("processed item: {}", i)
                }
            }
        }
    });

    {
        let process_receiver = process_stream.clone();
        task::spawn(async move {
            task::sleep(Duration::from_secs(10)).await;
            println!("boom");
            // closing the channel effectively cancels the stream
            process_receiver.close();
        });
    }

    let process_until_cancellation = {
        let result_sender = result_sender.clone();
        task::spawn(async move {
            while let Some(item) = input.next().await {
                // will wait until the next item is pulled, which acts as backpressure
                if process_sender.send(item).await.is_err() {
                    result_sender.send(Cancellable::Cancelled(item)).await.expect("error while sending the cancelled item");
                    break; // short circuit from there to not test on the error again
                }
            }

            while let Some(item) = input.next().await {
                result_sender.send(Cancellable::Cancelled(item)).await.expect("error while sending the cancelled item");
            }
        })
    };

    // rate limit
    let limiter =
        RateLimiter::direct(Quota::per_minute(nonzero!(150u32)).allow_burst(nonzero!(1u32)));
    let process_stream = process_stream.ratelimit_stream_with_jitter(&limiter, Jitter::up_to(Duration::from_secs(1)));

    // do the work. the rate at which requests is limited, but we could even limit the amount of
    // parallel requests
    process_stream
        .for_each_concurrent(None, |item| {
            let result_sender = result_sender.clone();
            async move {
                println!("doing stuff with {}", item);
                task::sleep(Duration::from_millis((random::<f64>() * 500.0) as u64)).await;
                println!("done stuff with {}", item);
                result_sender.send(Cancellable::Processed(item)).await.expect("error while sending the processed item");
            }
        })
        .await;  // for_each_concurrent returns a future that resolves when all futures are resolved

    // channel closes as soon as the last clone is dropped ; they will be once
    // process_until_cancellation is done. at that point result_task loop ends after the last item
    // is consumed.
    drop(result_sender);

    process_until_cancellation.await;
    result_task_handle.await;
}

fn main() {
    task::block_on(main_async())
}