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use crate::{
Error, Result,
observability::{Observability, QueueOverflowAction, RuntimeEvent, TelemetryEvent},
queue::{QueueReservation, QueueTracker, QueuedMessage},
};
use futures::channel::mpsc::UnboundedSender;
use std::fmt;
use std::sync::Arc;
/// A type alias for the internal dispatch function.
type DispatchFn<Msg> = Arc<dyn Fn(Msg) -> Result<()> + Send + Sync>;
/// Send messages back into a mounted [`crate::Program`].
///
/// Dispatchers are cheap to clone and can be moved into event handlers, subscriptions, and async
/// effects. A dispatcher remains valid only while the owning program is mounted.
pub struct Dispatcher<Msg> {
dispatch: DispatchFn<Msg>,
}
impl<Msg: Send + 'static> Dispatcher<Msg> {
/// Create a new dispatcher.
pub(crate) fn new(
sender: UnboundedSender<QueuedMessage<Msg>>,
queue_tracker: Arc<QueueTracker>,
observability: Observability<Msg>,
) -> Self {
Self {
dispatch: Arc::new(move |msg: Msg| {
let message_description = observability.describe_message_value(&msg);
let reservation = queue_tracker.reserve_enqueue();
match reservation {
QueueReservation::Rejected => {
observability.observe_telemetry(TelemetryEvent::QueueOverflow {
policy: observability.queue_policy(),
action: QueueOverflowAction::RejectedNew,
message_description,
});
Err(Error::QueueFull {
policy: observability.queue_policy(),
})
}
QueueReservation::DroppedNewest => {
observability.observe_telemetry(TelemetryEvent::QueueOverflow {
policy: observability.queue_policy(),
action: QueueOverflowAction::DroppedNewest,
message_description,
});
Ok(())
}
accepted @ QueueReservation::Accepted {
id,
overflow_action,
..
} => {
let result = sender
.unbounded_send(QueuedMessage { id, message: msg })
.map_err(|_| Error::ProgramUnavailable);
if let Ok(()) = &result {
if let Some(action) = overflow_action {
observability.observe_telemetry(TelemetryEvent::QueueOverflow {
policy: observability.queue_policy(),
action,
message_description: message_description.clone(),
});
}
observability.observe_runtime(RuntimeEvent::DispatchAccepted {
message_description: message_description.clone(),
});
observability.observe_telemetry(TelemetryEvent::DispatchAccepted {
message_description,
});
} else {
queue_tracker.rollback_enqueue(accepted);
observability.observe_runtime(RuntimeEvent::DispatchRejected {
message_description: message_description.clone(),
});
observability.observe_telemetry(TelemetryEvent::DispatchRejected {
message_description,
});
}
result
}
}
}),
}
}
/// Enqueue `msg` for processing by the mounted program.
///
/// # Arguments
///
/// * `msg` - The message to dispatch into the program.
///
/// # Errors
///
/// Returns [`Error::QueueFull`] when the active [`crate::QueuePolicy`] rejects new messages.
/// Returns [`Error::ProgramUnavailable`] when the program has already been dropped.
///
/// # Returns
///
/// A [`Result`] indicating success or failure of dispatching the message. Under
/// [`crate::QueuePolicy::DropNewest`] this may still return `Ok(())` even though the new
/// message was discarded, and under [`crate::QueuePolicy::DropOldest`] it may succeed after
/// displacing the oldest queued message.
pub fn dispatch(&self, msg: Msg) -> Result<()> {
(self.dispatch)(msg)
}
/// Adapt this dispatcher to accept `NewMsg` values.
///
/// The mapper runs synchronously at dispatch time before the message enters the runtime queue.
///
/// # Arguments
///
/// * `f` - A mapping closure that converts a `NewMsg` into a `Msg`.
///
/// # Returns
///
/// A new dispatcher that accepts `NewMsg` values and maps them before dispatching.
pub fn map<F, NewMsg>(&self, f: F) -> Dispatcher<NewMsg>
where
F: Fn(NewMsg) -> Msg + Clone + Send + Sync + 'static,
NewMsg: Send + 'static,
{
let inner = self.dispatch.clone();
Dispatcher {
dispatch: Arc::new(move |msg| inner(f(msg))),
}
}
}
impl<Msg> Clone for Dispatcher<Msg> {
fn clone(&self) -> Self {
Self {
dispatch: self.dispatch.clone(),
}
}
}
impl<Msg> fmt::Debug for Dispatcher<Msg> {
fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
formatter.debug_tuple("Dispatcher").field(&"..").finish()
}
}
#[cfg(test)]
mod tests {
use super::*;
use crate::{
observability::{ProgramConfig, QueuePolicy},
queue::QueueTracker,
};
use futures::channel::mpsc::unbounded;
use std::sync::{Arc, Mutex};
#[derive(Clone, Debug, PartialEq, Eq)]
enum Msg {
Set(i32),
}
#[derive(Clone, Debug, PartialEq, Eq)]
enum DispatchEvent {
Accepted(Option<String>),
Rejected(Option<String>),
Overflow(String),
}
#[test]
fn dispatch_reports_acceptance_and_rejection_with_message_descriptions() {
let dispatch_events = Arc::new(Mutex::new(Vec::new()));
let config = ProgramConfig::default()
.describe_message(|msg: &Msg| match msg {
Msg::Set(value) => format!("set:{value}"),
})
.observer({
let dispatch_events = dispatch_events.clone();
move |event| match event {
RuntimeEvent::DispatchAccepted {
message_description,
} => dispatch_events
.lock()
.unwrap()
.push(DispatchEvent::Accepted(
message_description.map(|value| value.to_string()),
)),
RuntimeEvent::DispatchRejected {
message_description,
} => dispatch_events
.lock()
.unwrap()
.push(DispatchEvent::Rejected(
message_description.map(|value| value.to_string()),
)),
_ => {}
}
})
.telemetry_observer({
let dispatch_events = dispatch_events.clone();
move |event| {
if let TelemetryEvent::QueueOverflow {
message_description,
..
} = event.event
{
dispatch_events
.lock()
.unwrap()
.push(DispatchEvent::Overflow(message_description.map_or_else(
|| String::from("<none>"),
|value| value.to_string(),
)));
}
}
});
let queue_tracker = Arc::new(QueueTracker::new(QueuePolicy::Unbounded));
let observability = Observability::new(
config,
Arc::new({
let queue_tracker = queue_tracker.clone();
move || queue_tracker.depth()
}),
);
let (sender, receiver) = unbounded();
let dispatcher = Dispatcher::new(sender, queue_tracker, observability);
assert_eq!(dispatcher.dispatch(Msg::Set(1)), Ok(()));
drop(receiver);
let error = dispatcher.dispatch(Msg::Set(2)).unwrap_err();
assert_eq!(error, Error::ProgramUnavailable);
assert_eq!(error.to_string(), "program is no longer available");
assert_eq!(
dispatch_events.lock().unwrap().as_slice(),
&[
DispatchEvent::Accepted(Some(String::from("set:1"))),
DispatchEvent::Rejected(Some(String::from("set:2"))),
]
);
}
#[test]
fn dispatch_returns_queue_full_when_policy_rejects_new_messages() {
let config =
ProgramConfig::<Msg>::default().queue_policy(QueuePolicy::RejectNew { capacity: 0 });
let queue_tracker = Arc::new(QueueTracker::new(QueuePolicy::RejectNew { capacity: 0 }));
let observability = Observability::new(
config,
Arc::new({
let queue_tracker = queue_tracker.clone();
move || queue_tracker.depth()
}),
);
let (sender, _receiver) = unbounded();
let dispatcher = Dispatcher::new(sender, queue_tracker, observability);
let error = dispatcher.dispatch(Msg::Set(7)).unwrap_err();
assert_eq!(
error,
Error::QueueFull {
policy: QueuePolicy::RejectNew { capacity: 0 },
}
);
}
}