presentations/futures-async-await/async.md

---
title:
- Getting comfy with async await
author:
- Sanchayan Maity
theme:
- default
classoption:
- aspectratio=169
---

# Who

- Who am I?
  * Embedded Systems background
  * Prefer C, Haskell and Rust
  * Organize and speak at Rust and Haskell meet-ups in Bangalore
- Work?
  * Software Engineer @ [asymptotic](https://asymptotic.io/)
  * Open source consulting firm based out of Bangalore and Toronto
  * Work on low level systems software centred around multimedia
  * GStreamer, PipeWire, PulseAudio
  * Language Polyglots

# Agenda

- `Future` trait
- `async`/`await`
- Using futures/Runtime
- Working with multiple futures (`select`, `join`, `FuturesOrdered`)
- Streams
- Pitfalls
- `Pin`/`Unpin`/`pin_project`

# Future[^1]

```rust
use std::future::Future;
use std::pin::Pin;
use std::task::Context;

pub trait Future {
    type Output;
    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>)
        -> Poll<Self::Output>;
}

pub enum Poll<T> {
    Ready(T),
    Pending,
}
```

[^1]: [Associated types](https://doc.rust-lang.org/reference/items/associated-items.html)

# Example

```rust
async fn hello() {
    println!("Hello from async");
}

fn main() {
    hello();
    println!("Hello from main");
}
```

# Where's the future

```rust
async fn give_number() -> u32 {
    100
}
```

# Sugar town[^2]

```rust
fn give_number() -> impl Future<Output = u32> {
    GiveNumberFuture
}

struct GiveNumberFuture {}

impl Future for GiveNumberFuture {
    type Output = u32;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>)
                -> Poll<Self::Output> {
        Poll::Ready(100)
    }
}
```

[^2]: [Syntactic sugar for Future](https://ruspiro.github.io/ruspiro-async-book/02-03-async.html)


# Runtimes

![*_Run Futures_*](future-meme.jpg){width=80%}


# Runtimes[^3]

- `futures::executor`
- `tokio`
- `smol-rs`
- `embassy`
- `glommio`
- `async-std`

[^3]: [The state of Async Rust: Runtimes](https://corrode.dev/blog/async/)


# Example

```rust
use futures::executor::block_on;

async fn hello() {
    println!("hello, world!");
}

fn main() {
    block_on(hello());
    println!("Hello from main");
}
```

# Example

```rust
async fn hello() {
    println!("Hello from async");
}

#[tokio::main]
async fn main() {
    hello().await;
    println!("Hello from main");
}
```

# Multiple futures

- `join`
- `join_all`
- `select`
- `select!`
- `select_all`
- `FuturesOrdered`
- `FuturesUnordered`
- `JoinSet`

# `join`

```rust
use futures::future;

#[tokio::main]
async fn main() {
    let a = async { "Future 1" };
    let b = async { "Future 2" };
    let pair = future::join(a, b);

    println!("{:?}", pair.await);
}
```

# `join_all`

```rust
use futures::future::join_all;
async fn hello(msg: String) -> String {
    msg
}

#[tokio::main]
async fn main() {
    let futures = vec![
        hello("Future 1".to_string()),
        hello("Future 2".to_string()),
        hello("Future 3".to_string()),
        hello("Future 4".to_string()),
    ];

    println!("{:?}", join_all(futures).await);
}
```

# `JoinSet`

```rust
use tokio::task::JoinSet;

#[tokio::main]
async fn main() {
    let mut set = JoinSet::new();
    for i in 0..10 {
        set.spawn(async move { i });
    }

    while let Some(res) = set.join_next().await {
        println!("{}", res.unwrap());
    }
}
```

# `future::select`

```rust
pub fn select<A, B>(future1: A, future2: B) -> Select<A, B>
where
    A: Future + Unpin,
    B: Future + Unpin,
```


# `future::select`


```rust
use futures::{future, future::Either, future::FutureExt, select};
use tokio::time::{sleep, Duration};

async fn task1(delay: u64) -> u64 {
    sleep(Duration::from_millis(delay)).await;
    delay
}

async fn task2(delay: u64) -> String {
    sleep(Duration::from_millis(delay)).await;
    "Hello".to_string()
}
```

# `future::select`

```rust
#[tokio::main]
async fn main() {
    let t1 = task1(200u64).fuse();
    let t2 = task2(300u64).fuse();

    tokio::pin!(t1, t2);

    match future::select(t1, t2).await {
        Either::Left((value1, _)) => println!("{}", value1),
        Either::Right((value2, _)) => println!("{}", value2),
    };
}
```


# `futures::select!`[^4]

```rust
use futures::{future::FutureExt, pin_mut, select};
use tokio::time::{sleep, Duration};
async fn task(delay: u64) {
    sleep(Duration::from_millis(delay)).await;
}

#[tokio::main]
async fn main() {
    let t1 = task(300u64).fuse();
    let t2 = task(200u64).fuse();
    pin_mut!(t1, t2);
    select! {
        () = t1 => println!("task one completed first"),
        () = t2 => println!("task two completed first"),
    }
}
```

[^4]: [futures::select!](https://docs.rs/futures/latest/futures/macro.select.html)


# `tokio::select!`[^5]

```rust
use tokio::time::{sleep, Duration};
async fn task(delay: u64) {
    sleep(Duration::from_millis(delay)).await;
}

#[tokio::main]
async fn main() {
    let t1 = task(300u64);
    let t2 = task(200u64);
    tokio::pin!(t1, t2);
    tokio::select! {
        () = t1 => println!("task one completed first"),
        () = t2 => println!("task two completed first"),
    }
}
```

[^5]: [tokio::select!](https://docs.rs/tokio/latest/tokio/macro.select.html)

# `loop tokio::select!`

```rust
#[tokio::main]
async fn main() {
    let mut count = 0;
    let t1 = task(300u64);
    let t2 = task(200u64);
    tokio::pin!(t1, t2);
    loop {
        if count > 5 {
            break;
        }
        tokio::select! {
            () = &mut t1 => println!("task one completed first"),
            () = &mut t2 => println!("task two completed first"),
        }
        count += 1;
    }
}
```


# `loop futures::select!`

```rust
#[tokio::main]
async fn main() {
    let mut count = 0;
    let t1 = task(300u64).fuse();
    let t2 = task(200u64).fuse();
    tokio::pin!(t1, t2);
    loop {
        if count > 5 {
            break;
        }
        futures::select! {
            () = &mut t1 => println!("task one completed first"),
            () = &mut t2 => println!("task two completed first"),
        }
        count += 1;
    }
}
```

# `Stream`[^6]

```rust
pub trait Stream {
    type Item;

    // Required method
    fn poll_next(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>
    ) -> Poll<Option<Self::Item>>;
}
```

[^6]: [Guided tour of Streams](https://www.qovery.com/blog/a-guided-tour-of-streams-in-rust/)


# `async-stream`

```rust
fn zero_to_three() -> impl Stream<Item = u32> {
    stream! {
        for i in 0..3 {
            yield i;
        }
    }
}

#[tokio::main]
async fn main() {
    let s = zero_to_three();
    pin_mut!(s); // needed for iteration

    while let Some(value) = s.next().await {
        println!("got {}", value);
    }
}
```


# `futures::select!` vs `tokio::select!`

- [futures::select!](https://docs.rs/futures/latest/futures/macro.select.html)
- [tokio::select!](https://docs.rs/tokio/latest/tokio/macro.select.html)
- [SO - What's the difference between futures::select and tokio::select?](https://stackoverflow.com/questions/60811657/what-is-the-difference-between-futuresselect-and-tokioselect)
- [Provide `select!` macro](https://stackoverflow.com/questions/60811657/what-is-the-difference-between-futuresselect-and-tokioselect)

# Multiple futures

- [FuturesUnordered](https://docs.rs/futures/latest/futures/stream/struct.FuturesUnordered.html)
- [FuturesOrdered](https://docs.rs/futures/latest/futures/stream/struct.FuturesOrdered.html)
- Must read
    - [FuturesUnordered and the order of futures](https://without.boats/blog/futures-unordered/)

# Cancellation

- [futures::future::Abortable](https://docs.rs/futures/latest/futures/future/struct.Abortable.html)

# Pitfalls

- Blocking in `async`
    - [Async: What's blocking](https://ryhl.io/blog/async-what-is-blocking/)
    - TLDR: Async code should never spend a long time without reaching an `.await`
- Cancellation safety
- Holding a `Mutex` across an `await`
- Must read
    - [Async cancellation: a case study of pub-sub in mini-redis](https://smallcultfollowing.com/babysteps/blog/2022/06/13/async-cancellation-a-case-study-of-pub-sub-in-mini-redis/)
    - [Yoshua Wuyts - Async Cancellation](https://blog.yoshuawuyts.com/async-cancellation-1/)
    - [Common mistakes with Rust Async](https://www.qovery.com/blog/common-mistakes-with-rust-async/)
    - [Rust tokio task cancellation patterns](https://cybernetist.com/2024/04/19/rust-tokio-task-cancellation-patterns/)
    - [`for await` and the battle of buffered streams](https://tmandry.gitlab.io/blog/posts/for-await-buffered-streams/)
    - [Mutex without lock, Queue without push: cancel safety in lilos](https://cliffle.com/blog/lilos-cancel-safety/)


# Cancellation safety with `select!`

So the TLDR

- futures in `select!` other than the future that yields `Poll::Ready` get dropped
- futures which own some form of state aren't cancellation safe, since the
  owned state gets dropped when another future returns `Poll::Ready`


# Pinning

```rust
use std::pin::Pin;
use pin_project::pin_project;

#[pin_project]
struct Struct<T, U> {
    #[pin]
    pinned: T,
    unpinned: U,
}

impl<T, U> Struct<T, U> {
    fn method(self: Pin<&mut Self>) {
        let this = self.project();
        let _: Pin<&mut T> = this.pinned; // Pinned reference to the field
        let _: &mut U = this.unpinned; // Normal reference to the field
    }
}
```

# Pinning

- Must read
    - [std::pin](https://doc.rust-lang.org/std/pin/index.html#projections-and-structural-pinning)
    - [Pin and suffering](https://fasterthanli.me/articles/pin-and-suffering)
    - [Pin, Unpin, and why Rust needs them](https://blog.cloudflare.com/pin-and-unpin-in-rust/)
    - [Async Book - Pinning](https://rust-lang.github.io/async-book/04_pinning/01_chapter.html)
    - [pin_project](https://docs.rs/pin-project/latest/pin_project/)

# More references

- [Meetup code samples](https://git.sanchayanmaity.net/sanchayanmaity/async-await-rust-meetup-examples)
- [Tokio tutorial](https://tokio.rs/tokio/tutorial)
- [Tokio internals](https://cafbit.com/post/tokio_internals/)
- [How Rust optimizes async/await - I](https://tmandry.gitlab.io/blog/posts/optimizing-await-1/)
- [How Rust optimizes async/await - II](https://tmandry.gitlab.io/blog/posts/optimizing-await-2/)

# Questions

- Reach out on
  * Email: me@sanchayanmaity.net
  * Mastodon: [sanchayanmaity.com](https://sanchayanmaity.com/@sanchayan)
  * Telegram: https://t.me/SanchayanMaity
  * Blog: [sanchayanmaity.net](https://sanchayanmaity.net/)