# future-queue
[](https://crates.io/crates/future-queue)
[](https://docs.rs/future-queue/)
[](https://nextest-rs.github.io/future-queue/rustdoc/future_queue)
[](CHANGELOG.md)
[](LICENSE-APACHE)
[](LICENSE-MIT)
`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](https://github.com/nextest-rs) on GitHub, and is
designed to serve the needs of [cargo-nextest](https://nexte.st).
## Motivation
Async programming in Rust often uses an adaptor called
[`buffer_unordered`](https://docs.rs/futures/latest/futures/stream/trait.StreamExt.html#method.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](https://nexte.st).
`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.
## About this crate
This crate provides two adaptors on streams.
### 1. The `future_queue` adaptor
The [`future_queue`](StreamExt::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 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.
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
```rust
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`](StreamExt::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.
The current weight for groups may exceed the maximum weight, similar to `future_queue`.
#### Examples
```rust
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](CONTRIBUTING.md) file for how to help out.
## License
This project is available under the terms of either the [Apache 2.0 license](LICENSE-APACHE) or
the [MIT license](LICENSE-MIT).
The code is derived from [futures-rs](https://github.com/rust-lang/futures-rs), and is used under
the Apache 2.0 and MIT licenses.