#future #stream #async #networking #buffer-unordered


Stream::buffer_unordered where each future can have a different weight

3 releases

0.1.2 Nov 1, 2022
0.1.1 Oct 29, 2022
0.1.0 Oct 28, 2022

#1124 in Asynchronous

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buffer-unordered-weighted on crates.io Documentation (latest release) Documentation (main) Changelog License License

buffer_unordered_weighted is a variant of buffer_unordered, where each future can be assigned a different weight.

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


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. For nextest in particular, some tests can be much heavier than others, and fewer of those tests should be run simultaneously.

[^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 an adaptor on streams called buffer_unordered_weighted, which 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 the maximum weight is exceeded. Once that happens, 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.

Note that in some cases, the current weight may exceed the maximum weight. For example:

  • Let's say the maximum weight is 24, and the current weight is 20.
  • If the next future has weight 6, then it will be scheduled and the current weight will become 26.
  • No new futures will be scheduled until the current weight falls to 23 or below.

It is possible to have a variant of this adaptor which always stays below the limit and holds the next future in abeyance; however, the implementation for that variant is a bit more complicated, and is also not the behavior desired by nextest. This variant may be provided in the future.

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 buffer_unordered_weighted is exactly the same as buffer_unordered.


use futures::{channel::oneshot, stream, StreamExt as _};
use buffer_unordered_weighted::{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 buffered = stream_of_futures.buffer_unordered_weighted(10);

assert_eq!(buffered.next().await, Some(Ok("hello")));

assert_eq!(buffered.next().await, Some(Ok("world")));

assert_eq!(buffered.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.


See the CONTRIBUTING file for how to help out.


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.


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