Observability

30 minutes read Keywords: rust, async, future, stream Edit this page

Imagine a collection of values T. This collection can be updated by inserting new values, removing existing ones, or the collection can truncated, cleared… This collection acts as the standard Vec. However, there is a subtlety: This collection is observable. It is possible for someone to subscribe to this collection and to receive its updates.

This observability pattern is the basis of reactive programming. It applies to any kind of type. Actually, it can be generalized as a single Observable<T> type. For collections though, we will see that an ObservableVector<T> type is more efficient.

I’ve recently played a lot with this pattern as part of my work inside the Matrix Rust SDK, a set of Rust libraries that aim at developing robust Matrix clients or bridges. It is notoriously used by the next generation Matrix client developed by Element, namely Element X. The Matrix Rust SDK is cross-platform. Element X has two implementations: on iOS, iPadOS and macOS with Swift, and on Android with Kotlin. Both languages are using our Rust bindings to Swift and Kotlin. This is the story for another series (how we have automated this, how we support asynchronous flows from Rust to foreign languages etc.), but for the moment, let’s keep focus on reactive programming.

Taking the Element X use case, the room list –which is the central piece of the app– is fully dynamic:

The rooms exposed by the room list are stored in a unique observable type. Why is it dynamic? Because the app continuously sync new data that update the internal state: when a room gets an update from the network, the room list is automatically updated. The beauty of it: we have nothing to do. Sorters and filters are run automatically. Why? Spoiler: because everything is a Stream.

Thanks to the Rust async model, every part is lazy. The app never needs to ask for Rust if a new update is present. It literally just waits for them.

I believe this reactive programming approach is pretty interesting to explore. And this is precisely the goal of this series. We are going to play with Stream a lot, with higher-order Stream a lot more, and w…

Le Comte

Hold on a second! I believe this first step is a bit steep for someone who's not familiar with asynchronous code in Rust, don't you think?

Before digging in the implementation details you are obviously eager to share, maybe we can start with examples.

Alrighty. Fair. Before digging into the really fun bits, we need some basis.

Baby steps with reactive programming

Everything we are going to share with you has been implemented in a library called eyeball. To give you a good idea of what reactive programming in Rust can look like, let's create a Rust program:

$ cargo new --bin playground
    Creating binary (application) `playground` package
$ cd playground
$ cargo add eyeball
    Updating crates.io index
      Adding eyeball v0.8.8 to dependencies
             Features:
             - __bench
             - async-lock
             - tracing
    Updating crates.io index
     Locking 3 packages to latest compatible versions

// in `src/main.rs`

use eyeball::Observable;

fn main() {
    let mut observable = Observable::new(7);

    let subscriber = Observable::subscribe(&observable);

    dbg!(Observable::get(&observable));
    dbg!(subscriber.get());

    Observable::set(&mut observable, 13);

    dbg!(Observable::get(&observable));
    dbg!(subscriber.get());
}

What do we see here? First off, observable is an observable value. Proof is: It is possible to subscribe to it, see subscriber. Both observable and subscriber are seeing the same initial value: 7. When observable receives a new value, 13, both observable and subscriber are seeing the updated value. Let's take it for a spin:

$ cargo run --quiet
[src/main.rs:8:5] Observable::get(&observable) = 7
[src/main.rs:9:5] subscriber.get() = 7
[src/main.rs:13:5] Observable::get(&observable) = 13
[src/main.rs:14:5] subscriber.get() = 13

Tadaa. Fantastic, isn't it?

Le Comte

I… I am… speechless? Is it really reactive programming? Where is the reactivity here? It seems like you've only shared a value between an owner and a watcher. You're calling them observable and subscriber, alright, but how is this thing reactive? I only see synchronous code for the moment.

Hold on. You told me to start slow. You're right though: the Observable owns the value. The Subscriber is able to read the value from the Observable. However, Subscriber::next returns a Future! Let's add this:

// in `src/main.rs`

// …

fn main() {
    // …

    dbg!(subscriber.next().await);
}

$ cargo run --quiet
error[E0728]: `await` is only allowed inside `async` functions and blocks
  --> src/main.rs:16:28
   |
3  | fn main() {
   | --------- this is not `async`
...
16 |     dbg!(subscriber.next().await);
   |                            ^^^^^ only allowed inside `async` functions and blocks

Indeed. Almighty rustc is correct. The main function is not async. We need an asynchronous runtime. Let's use the smol project, I enjoy it a lot: it's a small, fast and well-written async runtime:

$ cargo add smol
    Updating crates.io index
      Adding smol v2.0.2 to dependencies
      [ … snip … ]

Now let's modify our main function a little bit:

// in `src/main.rs`

use eyeball::Observable;

fn main() {
    smol::block_on(async {
        let mut observable = Observable::new(7);

        let mut subscriber = Observable::subscribe(&observable);

        dbg!(Observable::get(&observable));
        dbg!(subscriber.get());

        Observable::set(&mut observable, 13);

        dbg!(Observable::get(&observable));
        dbg!(subscriber.get());

        dbg!(subscriber.next().await);
    })
}

Please rustc, be nice…

[src/main.rs:9:9] Observable::get(&observable) = 7
[src/main.rs:10:9] subscriber.get() = 7
[src/main.rs:14:9] Observable::get(&observable) = 13
[src/main.rs:15:9] subscriber.get() = 13
[src/main.rs:17:9] subscriber.next().await = Some(
    13,
)

Hurray!

We can even have a bit more ergonomics by using the smol-macros crate which sets up a default async runtime Executor for us. It's useful in our case as we want to play with something else (reactive programming), and don't want to focus on the async runtime itself:

$ cargo add smol-macros macro_rules_attribute
    Updating crates.io index
      Adding smol-macros v0.1.1 to dependencies
      Adding macro_rules_attribute v0.2.0 to dependencies
             Features:
             - better-docs
             - verbose-expansions
    Updating crates.io index
     Locking 4 packages to latest compatible versions
      Adding macro_rules_attribute v0.2.0
      Adding macro_rules_attribute-proc_macro v0.2.0
      Adding paste v1.0.15
      Adding smol-macros v0.1.1

We will take the opportunity to improve our program a little bit. Let's spawn a Future that will continuously read new updates from the subscriber.

use std::time::Duration;

use eyeball::Observable;
use macro_rules_attribute::apply;
use smol::{Executor, Timer};
use smol_macros::main;

#[apply(main!)]
async fn main(executor: &Executor) {
    let mut observable = Observable::new(7);
    let mut subscriber = Observable::subscribe(&observable);

    // Task that reads new updates from `observable`.
    let task = executor.spawn(async move {
        while let Some(new_value) = subscriber.next().await {
            dbg!(new_value);
        }
    });

    // Now, let's update `observable`.
    Observable::set(&mut observable, 13);
    Timer::after(Duration::from_secs(1)).await;

    Observable::set(&mut observable, 17);
    Timer::after(Duration::from_secs(1)).await;

    Observable::set(&mut observable, 23);

    // Wait on the task.
    task.await;
}

The little Timer::after calls are here to pretend the values are coming from random events, for the moment. Let's run it again to see if we get the same result:

$ cargo run --quiet
[src/main.rs:16:13] new_value = 13
[src/main.rs:16:13] new_value = 17
[src/main.rs:16:13] new_value = 23
^C

Here we go, perfect! See, ah ha! It's async and nice now.

Le Comte

I believe I start to appreciate it. However, I foresee you might hide something behind these Time::after. Am I right?

And this task.await at the end makes the program to never finish. It explains the need to send a SIGINT signal to the program to interrupt it, right?

You're slick. Indeed, I wanted to focus on the observable and the subscriber. Because there is a subtlety here. If the Timer::after are removed, only the last update will be displayed on the output by dbg!. And that's perfectly normal. The async runtime will execute all the Observable::set(&mut observable, new_value) in a row, and then, once there is an await point, another task will have room to run. In this case, that's subscriber.next().await.

The subscriber only receives the last update, and that's pretty important to understand. There is no buffer of all the previous updates here, no memory, no trace, subscriber returns the last value when it is called. Note that this is not always the case as we will see with ObservableVector later, but for the moment, that's the case.

And yes, if we want the task to get a chance to consume more updates, we need to tell the executor we will wait while the current other tasks are waken up. To do that, we can use the smol::yield_now function:

    // Now, let's update `observable`.
    Observable::set(&mut observable, 13);
    // Eh `executor`: `task` can run now, we will wait!
    yield_now().await;

    // More updates.
    Observable::set(&mut observable, 17);
    Observable::set(&mut observable, 23);
    // Eh `executor`: _bis repetita placent_!
    yield_now().await;

    drop(observable)
    task.await;
}

Let's see what happens:

$ cargo run --quiet
[src/main.rs:14:13] new_value = 13
[src/main.rs:14:13] new_value = 23

Eh, see, new_value = 17 is not displayed, because the observable is updated but the subscriber is suspended by the executor. But the others are read, good good.

Note that we are dropping the observable. Once it's dropped, the subscriber won't be able to read any value from it, so it's going to close itself, and the task will end. That's why waiting on the task with task.await will terminate this time. And thus, the program will finish gracefully.

And that's it. That's the basis of reactive programming. Also note that Subscriber<T> implements Send and Sync if T implements Send and Sync, i.e. if the observed type implements these traits. That's pretty useful actually: it is possible to send the subscriber in a different thread, and keep waiting for new updates.

Attack of the Clones

However, at the beginning of this episode, we were talking about a collection. Let's focus on Vec.

Le Comte

Why do we focus on Vec only? Why not HashMap, HashSet, BTreeSet, BTreeMap, BinaryHeap, LinkedList or even VecDeque? It seems a bit non-inclusive if you ask me. Are you aware there isn't only Vec in life?

Well, the reason is simple: Vec is supported by eyeball. It's a matter of time and work to support other collections, it's definitely not impossible but you will see that it's not trivial neither to support all these collections for a simple reason: Did you notice that Subscriber produces an owned T? Not a &T, but a T. That's because Subscriber::next requires T: Clone. It means that the observed value will be cloned every time it is broadcasted to a subscriber.

Cloning a value may be expensive. Here we are manipulating usize, which is a primitive type, so it's all fine (it boils down to a memcpy). But imagine an Observable<Vec<BigType>> where BigType is 512 bytes: the memory impact is going to be quickly noticeable. So th…

Le Comte

… Excuse my interruption! You know how I love reading books. I like defining myself as a bibliophile. Anyway. During my perusal of the eyeball documentation, I have found Subscriber::next_ref. The documentation says:

Wait for an update and get a read lock for the updated value.

and later:

You can use this method to get updates of an Observable where the inner type does not implement Clone.

Can you stop cutting me off please? It's really unpleasant. And do not forget we are not alone… doing sideways head movement

You're right though. There is Subscriber::next_ref. However, if you are such an assiduous reader, you may have read the end of the documentation, aren't you?

However, the Observable will be locked (not updateable) while any read guards are alive.

Blocking the Observable might be tolerable in some cases, but it cannot be generalized to all use cases. A user is more likely to prefer next instead of next_ref by default.

Back to our Observable<Vec<BigType>> then. Imagine the collection contains a lot of items: cloning the entire Vec<_> for every update to every subscriber is a pretty inefficient way of programming. Remember that, as a programmer, we have the responsability to make our programs use as few resources as possible, so that hardwares can be used longer. The hardware is the most polluting segment of our digital world.

So. How a data structure like Vec can be cloned cheaply? We could put it inside an Arc right? Cloning an Atomically Reference Counted value is really cheap: it increases the counter by 1 atomically, the inner value is untouched. Nonetheless, we have a mutation problem now. If we have Observable<Arc<Vec<_>>>, it means that the subscribers will be Subscriber<Arc<Vec<_>>>. In this case, every time the observable wants to mutate the data, it is going to… be… impossible because an Arc is nothing less than a shared reference, and shared references in Rust disallow mutation by default. Using Observable::set will create a new Arc, but we cannot update the value inside the Arc, except if we use a lock… Well, we are adding more and more complexity.

Spes salutis1! Fortunately for us, immutable data structures exist in Rust.

An immutable data structure is a data structure which can be copied and modified efficiently without altering the original.

It can be modified. However, as soon as it is copied (or cloned), it is still possible to modify the copy but the original data is not modified. That's extremely powerful.

Such structures bring many advantages, but one of them is structural sharing:

If two data structures are mostly copies of each other, most of the memory they take up will be shared between them. This implies that making copies of an immutable data structure is cheap: it's really only a matter of copying a pointer and increasing a reference counter, where in the case of Vec you have to allocate the same amount of memory all over again and make a copy of every element it contains. For immutable data structures, extra memory isn't allocated until you modify either the copy or the original, and then only the memory needed to record the difference.

Well, taking a deep breath, it sounds exactly like what we need to solve our issue, isn't it? The Observable<Immutable<_>> and the Subscriber<Immutable<_>>s will share the same value, with the observable being able to mutate its inner value. The subscribers can modify the received value too, in an efficient way, without conflicting with the value from the observable. Both values will continue to live on their side, but cloning the value is cheap.

Le Comte

Dare I ask how immutable data structures are implemented? It sounds like complex beasts.

I mean… a naive implementation sounds relatively doable but I am guessing there is a lot of subtleties, possible conflicts, and many memory guarantees that I am not anticipating yet, right?

Oh… beati pauperes in spiritu2… it is actually really complex. It may be a topic for another series or articles. For the moment, if you interested, let me redirect you to one research paper that proposes an immutable Vec: RRR Vector: A Practical General Immutable Sequence3. Be cool though, understanding this part is not necessary at all for what we are talking now. It's a great tool we are going to use, no matter how it works internally.

Do you know the other good news? We don't have to implement it by ourselves, because some nice people already did it! Enter the imbl crate. This crate provides a Vector type. It can be use like a regular Vec. (Side note: it's even smarter than a Vec because it implements smart head and tail chunking4, and allocates in the stack or on the heap depending on the size of the collection, similarly to the smallvec crate. End of digression)

Observable (immutable) collection

The imbl crate then. It provides a Vector type. eyeball provides a crate for working with immutable data structures (how surprising huh?): this crate is eyeball-im.

Instead of providing an Observable<T> type, it provides an ObservableVector<T> type which is a Vector, but an observable one! Let's see… what do we have… scroll the documentation, hmm, interesting, scroll more…, okay, that's interesting:

Let's explore VectorSubscriber a bit more, would you? Scroll the document, contrary to Subscriber::next, there is no next method. How are we supposed to wait on an update?

Le Comte

Confer to the assiduous reader! If you read carefully the documentation of the Subscriber::next method, you will see:

This method is a convenience so you don't have to import a Stream extension trait such as futures::StreamExt or tokio_stream::StreamExt.

… fair enough. So Subscriber::next mimics StreamExt::next. Okay. Let's look at Stream first, it's from the futures crate. Stream defines itself as:

A stream of values produced asynchronously.

If Future<Output = T> is an asynchronous version of T, then Stream<Item = T> is an asynchronous version of Iterator<Item = T>. A stream represents a sequence of value-producing events that occur asynchronously to the caller.

The trait is modeled after Future, but allows poll_next to be called even after a value has been produced, yielding None once the stream has been fully exhausted.

We aren't going to teach everything about Stream: why this design, its pros and cons… However, wave its hand to ask you to come closer, did you notice how Future::poll returns Poll<Self::Output>, whilst Stream::poll_next returns Poll<Option<Self::Item>>? It's really similar to Iterator::next which returns Option<Self::Item>.

Let's take a look at Poll<T> don't you mind? It's an enum with 2 variants:

Then, what Poll<Option<T>> represents for a Stream?

It makes perfect sense. A Future produces a single value, whilst a Stream produces multiple values, and Poll::Ready(None) represents the termination of the stream, similarly to None to represent the termination of an Iterator. Ahh, I love consistency.

We have the basis. Now let's see StreamExt. It's a trait extending Stream to add convenient combinator methods. Amongst other things, we find StreamExt::next! Ah ha! It returns a Next type which implements a Future, exactly what eyeball does actually. Remember our:

// from `main.rs`

while let Some(new_value) = subscriber.next().await {
    dbg!(new_value);
}

It is exactly the same pattern with StreamExt::next:

// from the documentation of `StreamExt::Next`

use futures::stream::{self, StreamExt};

let mut stream = stream::iter(1..=3);

assert_eq!(stream.next().await, Some(1));
assert_eq!(stream.next().await, Some(2));
assert_eq!(stream.next().await, Some(3));
assert_eq!(stream.next().await, None);

Pieces start to come together, don't they?

End of the detour. Back to eyeball_im::VectorSubscriber<T> . It is possible to transform this type into a Stream with its into_stream method. It returns a VectorSubscriberStream. Naming is hard, but if I would have to guess, I would say it implements… a… Stream?

// from `eyeball-im`

impl<T: Clone + Send + Sync + 'static> Stream for VectorSubscriberStream<T> {
    type Item = VectorDiff<T>;

Yes, it does!

Dust blown away, the puzzle starts to appear clearly. Let's back on coding!

$ cargo add eyeball-im futures
    Updating crates.io index
      Adding eyeball-im v0.5.0 to dependencies
             Features:
             - serde
             - tracing
      Adding futures v0.3.30 to dependencies
             Features:
             + alloc
             + async-await
             + executor
             + std
             - bilock
             - cfg-target-has-atomic
             - compat
             - futures-executor
             - io-compat
             - thread-pool
             - unstable
             - write-all-vectored
      [ … snip … ]

// in `src/main.rs`

use eyeball_im::ObservableVector;
use futures::stream::StreamExt;
use macro_rules_attribute::apply;
use smol::{future::yield_now, Executor};
use smol_macros::main;

#[apply(main!)]
async fn main(executor: &Executor) {
    let mut observable = ObservableVector::new();
    // Subscribe to `observable` and get a `Stream`.
    let mut subscriber = observable.subscribe().into_stream();

    // Push one value.
    observable.push_back('a');

    // Task that reads new updates from `observable`.
    let task = executor.spawn(async move {
        while let Some(new_value) = subscriber.next().await {
            dbg!(new_value);
        }
    });

    // Now, let's update `observable`.
    observable.push_back('b');
    // Eh `executor`: `task` can run now!
    yield_now().await;

    // More updates.
    observable.push_back('c');
    observable.push_back('d');
    // Eh `executor`, same.
    yield_now().await;

    drop(observable);
    task.await;
}

Time to show off:

$ cargo run --quiet
[src/main.rs:18:13] new_value = PushBack {
    value: 'a',
}
[src/main.rs:18:13] new_value = PushBack {
    value: 'b',
}
[src/main.rs:18:13] new_value = PushBack {
    value: 'c',
}
[src/main.rs:18:13] new_value = PushBack {
    value: 'd',
}

Do you see something new?

Le Comte

Hmm, indeed. With Observable, some values may “miss” because Observable and Subscriber have no buffer. The subscribers only return the current value when asked for. However, with ObservableVector, things are different: no missing values. There are all here. As if there… was a buffer!

And the values returned by the subscriber are not the raw T: we see PushBack. It comes from, check the documentation, VectorDiff::PushBack!

Good eyes, well done.

First off, that's correct that PushBack comes from VectorDiff. Let's come back to this piece in a second: it is the cornerstone of the entire series, it deserves a bit of explanations.

Second, yes, VectorSubscriber returns all values! There is actually a buffer. It's a bit annoying to continue with a task as we did so far, let's use assert_eq! instead.

// in `src/main.rs`

use eyeball_im::{ObservableVector, VectorDiff};
//                                 ^^^^^^^^^^ new!
// …

#[apply(main!)]
async fn main(_executor: &Executor) {
    let mut observable = ObservableVector::new();
    let mut subscriber = observable.subscribe().into_stream();

    // Push one value.
    observable.push_back('a');

    assert_eq!(
        dbg!(subscriber.next().await),
        Some(VectorDiff::PushBack { value: 'a' }),
    );

    // Push another value.
    observable.push_back('b');
    observable.push_back('c');
    observable.push_back('d');

    assert_eq!(
        dbg!(subscriber.next().await),
        Some(VectorDiff::PushBack { value: 'b' }),
    );
    assert_eq!(
        dbg!(subscriber.next().await),
        Some(VectorDiff::PushBack { value: 'c' }),
    );
    assert_eq!(
        dbg!(subscriber.next().await),
        Some(VectorDiff::PushBack { value: 'd' }),
    );
}

$ cargo run --quiet
[src/main.rs:16:9] subscriber.next().await = Some(
    PushBack {
        value: 'a',
    },
)
[src/main.rs:26:9] subscriber.next().await = Some(
    PushBack {
        value: 'b',
    },
)
[src/main.rs:30:9] subscriber.next().await = Some(
    PushBack {
        value: 'c',
    },
)
[src/main.rs:34:9] subscriber.next().await = Some(
    PushBack {
        value: 'd',
    },
)

Beautiful! However… the code is a bit verbose, isn't it? Desperately waiting for an affirmative answer, okay, okay, something you may not know about me: I love macros. There. I said it. Let's quickly craft one:

// in `src/main.rs`
// before the `main` function

macro_rules! assert_next_eq {
    ( $stream:ident, $expr:expr $(,)? ) => {
        assert_eq!(dbg!( $stream .next().await), Some( $expr ));
    };
}

This macro does exactly what our assert_eq! was doing, except now it's shorter to use, and thus more pleasant. Don't believe me? See by yourself:

// in `src/main.rs`
// at the end of the `main` function

// Push one value.
observable.push_back('a');

assert_next_eq!(subscriber, VectorDiff::PushBack { value: 'a' });

// Push another value.
observable.push_back('b');
observable.push_back('c');
observable.push_back('d');

assert_next_eq!(subscriber, VectorDiff::PushBack { value: 'b' });
assert_next_eq!(subscriber, VectorDiff::PushBack { value: 'c' });
assert_next_eq!(subscriber, VectorDiff::PushBack { value: 'd' });

There we go.

Having a scientific and rigorous approach is important in our domain. We said ObservableVector seems to contain a buffer, and VectorSubscriber seems to pop values from this buffer. Let's play with that. I see two things to test:

  1. Modify the ObservableVector, and subscribe to it after: Does the subscriber receive the update before it was created?
  2. How many values the buffer can hold?
let mut observable = ObservableVector::new();

// Push a value before the subscriber exists.
observable.push_back('a');

let mut subscriber = observable.subscribe().into_stream();

// Push another value.
observable.push_back('b');

assert_next_eq!(subscriber, VectorDiff::PushBack { value: 'b' });

If the subscriber receives a, it must fail, otherwise no error:

$ cargo run --quiet
[src/main.rs:25:5] subscriber.next().await = Some(
    PushBack {
        value: 'b',
    },
)

Look Ma', no error!

Le Comte

We have learned that a VectorSubscriber is aware of the new updates that are made once it exists. A VectorSubscriber is not aware of updates that happened before its creation.

In the example, VectorDiff::PushBack { value: 'a' } is not received before subscriber was created. However, VectorDiff::PushBack { value: 'b' } is received because it happened after subscriber was created. It makes perfect sense.

It suggests that the buffer lives inside VectorSubscriber, and not inside ObservableVector. Or maybe the buffer is shared between the observable and the subscribers, with the buffer having some specific semantics, like a channel. We would need to look at the implementation to be sure.

Agree. This is left as an exercise for the reader, wink to you.

We have an answer to question 1. What about question 2? The size of the buffer.

// in `src/main.rs`

let mut observable = ObservableVector::new();
let mut subscriber = observable.subscribe().into_stream();

// Push ALL THE VALUES!
observable.push_back('a');
observable.push_back('b');
observable.push_back('c');
observable.push_back('d');
observable.push_back('e');
observable.push_back('f');
observable.push_back('g');
observable.push_back('h');
observable.push_back('i');
observable.push_back('j');
observable.push_back('k');
observable.push_back('l');
observable.push_back('m');
observable.push_back('n');
observable.push_back('o');
observable.push_back('p');

assert_next_eq!(subscriber, VectorDiff::PushBack { value: 'a' });
// no need to assert the others

$ cargo run --quiet
[src/main.rs:36:5] subscriber.next().await = Some(
    PushBack {
        value: 'a',
    },
)

Hmm, the buffer doesn't seem to be full with 16 values. Let's add a couple more:

// in `src/main.rs`

// [ … snip … ]
observable.push_back('n');
observable.push_back('o');
observable.push_back('p');
observable.push_back('q');
//                    ^ new!
observable.push_back('r');
//                    ^ new!

assert_next_eq!(subscriber, VectorDiff::PushBack { value: 'a' });

$ cargo run --quiet
[src/main.rs:38:5] subscriber.next().await = Some(
    Reset {
        values: [
            'a',
            'b',
            'c',
            'd',
            'e',
            'f',
            'g',
            'h',
            'i',
            'j',
            'k',
            'l',
            'm',
            'n',
            'o',
            'p',
            'q',
            'r',
        ],
    },
)
thread 'main' panicked at src/main.rs:38:5:
assertion `left == right` failed
  left: Some(Reset { values: ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r'] })
 right: Some(PushBack { value: 'a' })
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Oh! An error, great! Our assert_next_eq! has failed. subscriber does not receive a VectorDiff::PopBack but a VectorDiff::Reset. Let's play with ObservableVector::with_capacity a moment, maybe it's related to the buffer capacity? Let's change a single line:

let mut observable = ObservableVector::with_capacity(32);
//                                     ^^^^^^^^^^^^^^^^^ new!

$ cargo run --quiet
[src/main.rs:38:5] subscriber.next().await = Some(
    PushBack {
        value: 'a',
    },
)
Le Comte

We have learned that ObservableVector::with_capacity controls the size of the buffer.

The name could suggest that it controls the capacity of the observed Vector, à la Vec::with_capacity, but it must not be confused.

For a reason we ignore so far, when the buffer is full, we receive a VectorDiff::Reset. We need to learn more about this type.

Observable differences

The previous section was explaining how immutable data structures could save us by cheaply and efficiently cloning the data between the observable and its subscribers. However, we see that eyeball-im, despite using imbl, does not share a imbl::Vector but a eyeball_im::VectorDiff. Why such design? It looks like a drama. A betrayal. An act of treachery!

Well. Firstly, eyeball-im is relying on some immutable properties of Vector. And secondly, the reason for which VectorDiff exists is simple. If a subscriber receives Vectors, how is the user able to see what has changed? The user (!) would be responsible to calculate the differences between 2 Vectors every time! Not only this is costly, but it is utterly error-prone.

Le Comte

Are you suggesting that VectorSubscriber (or VectorSubscriberStream) calculates the differences between the Vectors itself so that the user doesn't have to?

I still see many problems though. I believe the order of the VectorDiffs matters a lot for some use cases. For example, let's consider two consecutive Vectors:

  1. ['a', 'b', 'c'] and
  2. ['a', 'c', 'b'].

Has 'b' been removed and pushed back, or 'c' been popped back and inserted? How can you decide between the twos?

We can't —it would be implementation specifics anyway— and we don't want to. The user is manipulating the ObservableVector in a special way, and we should ideally not change that.

These VectorDiff actually comes from ObservableVector itself! Let's look at the implementation of ObservableVector::push_back:

pub fn push_back(&mut self, value: T) {
    // [ … snip … ]

    self.values.push_back(value.clone());
    //   ^^^^^^ this is a `Vector`!
    self.broadcast_diff(VectorDiff::PushBack { value });
    //                  ^^^^^^^^^^ here you are…
}

Each method adding or removing values on the ObservableVector emits its own VectorDiff variant. No calculation, it's purely a mapping:

ObservableVector::…VectorDiff::…Meaning
append(values)Append { values }Append many values
clear()ClearClear out all the values
insert(index, value)Insert { index, value }Insert a value at index 
pop_back()PopBackRemove the value at the back
pop_front()PopFrontRemove the value at the front
push_back(value)PushBack { value }Add value at the back
push_front(value)PushFront { value }Add value at the front
remove(index)Remove { index } Remove value at index
set(index, value)Set { index, value }Replace value at index by value
truncate(length)Truncate { length }Truncate to length values

Mappings of ObservableVector methods to VectorDiff variants.

See, for each VectorDiff variant, there is an ObservableVector method triggering it.

Le Comte

And what about VectorDiff::Reset?

We were receiving it when the buffer was full apparently. You are not mentioning it, and if I take a close look at ObservableVector's documentation, I don't see any reset method. Is it only an internal thing?

You are correct. When the buffer is full, the subscriber will provide a VectorDiff::Reset { values } where values is the full list of values. The documentation says:

The subscriber lagged too far behind, and the next update that should have been received has already been discarded from the internal buffer.

If the subscriber didn't catch all the updates, the best thing it can do is to say: Okay, I am late at the party, I've missed several things, so here is the current state!. This is not ideal, but the subscriber is responsible to not lag, and this design avoids having missing values. If a subscriber receives too much VectorDiff::Resets, the user may consider increasing the capacity of the ObservableVector.

Filtering and sorting with higher-order Streams

We are reaching the end of this episode. And you know what? We have set all the parts to talk about higher-order Stream, chante victory and dance at the same time!

At the beginning of this episode, we were saying that the Matrix Rust SDK is able to filter and to sort an ObservableVector representing all the rooms. How? VectorSubscriberStream is a Stream. More specifically, it is a Stream<Item = VectorDiff<T>>. Now questions:

All of them are strictly Stream<Item = VectorDiff<T>>! However, the VectorDiffs aren't the same. A simple example. Let's say we build a vector by inserting 1, 2, 3 and 4. We subscribe to it, and we want to filter out all the even numbers. Instead of receiving:

… we want to receive:

We will see how all that works in the next episodes and how powerful this design is, especially when it comes to cross-platform UI (user interface). We are going to learn so much about Stream and Future, it's going to be fun!

1

Latine expression meaning salavation hope.

2

Latine expression meaning bless are the poor in spirit.

3

Relaxed-Radix-Balanced (RRR) Vector: A Practical General Purpose Immutable Sequence by Sticki N., Rompf T., Ureche V. and Bagwell P. (2015, August), in Proceedings of the 20th ACM SIGPLAN International Conference on Functional Programming (pp. 342-354).

4

Theory and Practise of Chunked Sequences by Acar U. A., Charguéraud A., and Rainey M. (2014), in Algorithms-ESA 2014: 22th Annual European Symposium, Wroclaw, Poland, September 8-10, 2014. Proceedings 21 (pp. 25-36)., Springer Berlin Heidelberg.