Experiment: Async Reactive Streams

[aschaeffer]

Problem

• Reactive stream subscribers are executed serially
• If a subscriber is blocking, the following subscribers are not executed
• Behaviours cannot create green threads without a runtime

Idea

The GraphQL API is async already. If the reactive streams would be async, it would be possible to avoid blocking completely and further increase performance.

Research tasks

• [x] Annotate the main function with tokio main
• [ ] Make reactive streams async
  • [ ] Make subscriber "callback" async FnOnce
  • [ ] Observer: Run all subscribers asynchronously (await all)
• [ ] Make property setter calls async
• [ ] Make business layer async
• [ ] Is it possible to simplify the GraphQL subscriptions implementation?

This may work or not - the goal is to get an idea if it would be possible.

[aschaeffer]

This may take a while until support for async closures land in rust:

• [ ] https://blog.rust-lang.org/inside-rust/2022/11/17/async-fn-in-trait-nightly.html
• [ ] Rust async fn fundamentals initiative
  • [ ] async closures

Current (sync)

type Subscriber<'a, Sig> = (u128, dyn FnMut(&Sig) + Send + 'a);

subscribers_mut.par_iter_mut().for_each(|sub| sub.1(signal));

Goal (async)

type Subscriber<'a, Sig> = (u128, async dyn FnMut(&Sig) + Send + 'a);

let mut tasks = Vec::new();
for subscriber in subscribers_mut.iter_mut() {
  tokio::spawn(async {
    tasks.push(subscriber(signal).await);
  }
}
tasks.join_all();