future-queue

future_queue provides ways to run several futures:

• concurrently
• in the order they're spawned
• with global limits
• and with an optional group specified for each future, with its own limits.

This crate is part of the nextest organization on GitHub, and is designed to serve the needs of cargo-nextest.

Motivation

Async programming in Rust often uses an adaptor called buffer_unordered: this adaptor takes a stream of futures[^1], and executes all the futures limited to a maximum amount of concurrency.

• Futures are started in the order the stream returns them in.
• Once started, futures are polled simultaneously, and completed future outputs are returned in arbitrary order (hence the unordered).

Common use cases for buffer_unordered include:

• Sending network requests concurrently, but limiting the amount of concurrency to avoid overwhelming the remote server.
• Running tests with a tool like cargo-nextest.

buffer_unordered works well for many use cases. However, one issue with it is that it treats all futures as equally taxing: there's no way to say that some futures consume more resources than others, or that some subsets of futures should be mutually excluded from others.

For nextest in particular, some tests can be much heavier than others, and fewer of those tests should be run simultaneously. Also, some tests need to be mutually excluded from others, or other concurrency limits placed on them.

[^1]: This adaptor takes a stream of futures for maximum generality. In practice this is often an iterator of futures, converted over using stream::iter.

About this crate

This crate provides two adaptors on streams.

1. The future_queue adaptor

The future_queue adaptor can run several futures simultaneously, limiting the concurrency to a maximum weight.

Rather than taking a stream of futures, this adaptor takes a stream of (usize, future) pairs, where the usize indicates the weight of each future. This adaptor will schedule and buffer futures to be run until queueing the next future will exceed the maximum weight.

• The maximum weight is never exceeded while futures are being run.
• If the weight of an individual future is greater than the maximum weight, its weight will be set to the maximum weight.

Once all possible futures are scheduled, this adaptor will wait until some of the currently executing futures complete, and the current weight of running futures drops below the maximum weight, before scheduling new futures.

The weight of a future can be zero, in which case it doesn't count towards the maximum weight.

If all weights are 1, then future_queue is exactly the same as buffer_unordered.

Examples

use futures::{channel::oneshot, stream, StreamExt as _};
use future_queue::{StreamExt as _};

let (send_one, recv_one) = oneshot::channel();
let (send_two, recv_two) = oneshot::channel();

let stream_of_futures = stream::iter(vec![(1, recv_one), (2, recv_two)]);
let mut queue = stream_of_futures.future_queue(10);

send_two.send("hello")?;
assert_eq!(queue.next().await, Some(Ok("hello")));

send_one.send("world")?;
assert_eq!(queue.next().await, Some(Ok("world")));

assert_eq!(queue.next().await, None);

2. The future_queue_grouped adaptor

The future_queue_grouped adaptor is like future_queue, except it is possible to specify an optional group for each future. Each group has a maximum weight, and a future will only be scheduled if both the maximum weight and the group weight aren't exceeded.

The adaptor is as fair as possible under the given constraints: it will schedule futures in the order they're returned by the stream, without doing any reordering based on weight. When a future from a group completes, queued up futures in this group will be preferentially scheduled before any other futures from the provided stream.

Like with future_queue:

• The maximum global and group weights is never exceeded while futures are being run.
• While accounting against global weights, if the weight of an individual future is greater than the maximum weight, its weight will be set to the maximum weight.
• If a future belongs to a group: While accounting against the group weight, if its weight is greater than the maximum group weight, its weight will be set to the maximum group weight.

Examples

use futures::{channel::oneshot, stream, StreamExt as _};
use future_queue::{StreamExt as _};

let (send_one, recv_one) = oneshot::channel();
let (send_two, recv_two) = oneshot::channel();

let stream_of_futures = stream::iter(
    vec![
        (1, Some("group1"), recv_one),
        (2, None, recv_two),
    ],
);
let mut queue = stream_of_futures.future_queue_grouped(10, [("group1", 5)]);

send_two.send("hello")?;
assert_eq!(queue.next().await, Some(Ok("hello")));

send_one.send("world")?;
assert_eq!(queue.next().await, Some(Ok("world")));

assert_eq!(queue.next().await, None);

Minimum supported Rust version (MSRV)

The minimum supported Rust version is Rust 1.56.

The MSRV will likely not change in the medium term, but while this crate is a pre-release (0.x.x) it may have its MSRV bumped in a patch release. Once this crate has reached 1.x, any MSRV bump will be accompanied with a new minor version.

Notes

This crate used to be called buffer-unordered-weighted. It was renamed to future-queue to be more descriptive about what the crate does rather than how it's implemented.

Contributing

See the CONTRIBUTING file for how to help out.

License

This project is available under the terms of either the Apache 2.0 license or the MIT license.

The code is derived from futures-rs, and is used under the Apache 2.0 and MIT licenses.