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[dbsp] Retransmit unacknowledged multihost exchange messages #6784

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[dbsp] Retransmit unacknowledged multihost exchange messages #6784

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46 changes: 45 additions & 1 deletion crates/dbsp/src/operator/communication/exchange.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -55,7 +55,7 @@ use tokio::{
},
sync::{Notify, OnceCell, futures::OwnedNotified},
task::JoinHandle,
time::sleep,
time::{sleep, timeout},
};
use tokio_util::sync::{CancellationToken, DropGuard};
use tracing::{error, info, warn};
Expand Down Expand Up @@ -331,6 +331,12 @@ impl ExchangeChannel {
fn drain_waiter(&self) -> Option<OwnedNotified> {
self.inner().drain_waiter()
}

/// True if the channel holds messages that the receiver has not
/// acknowledged yet.
fn has_unacknowledged(&self) -> bool {
!self.inner().messages.is_empty()
}
}

pub struct ExchangeClient {
Expand Down Expand Up @@ -373,6 +379,26 @@ impl ExchangeClient {
// Get the next message to send.
let (message, sequence) = match channel.get(min_sequence) {
Ok(result) => result,
Err(notified) if channel.has_unacknowledged() => {
// Nothing new to send, but the receiver has not
// acknowledged everything we sent. Retransmit from the
// oldest unacknowledged message if that does not change
// before the timeout.
//
// Without this, a message lost while the connection stays
// up wedges both ends forever: the receiver waits for the
// message, we wait for its acknowledgement, and only a new
// message or a reconnection would make us send anything.
// In a synchronous exchange no new message can arrive,
// because the workers on both ends are waiting for this
// one. Retransmitting costs nothing when it is not
// needed: the receiver drops messages whose sequence
// number it has already seen.
if timeout(retransmit_timeout(), notified).await.is_err() {
min_sequence = 0;
}
continue;
}
Err(notified) => {
notified.await;
continue;
Expand Down Expand Up @@ -2132,6 +2158,24 @@ fn inject_fault(_kind: impl Display) -> bool {
false
}

/// How long a connection waits for an acknowledgement before retransmitting
/// the messages it has already sent. See [ExchangeClient::run_connection_tx].
///
/// A receiver acknowledges a message only after its workers have consumed it,
/// which can legitimately take a while under load, and retransmitting a large
/// message needlessly is not free. The timeout is therefore generous: it
/// recovers from a wedged connection, which would otherwise last forever, and
/// is not meant to recover promptly.
#[cfg(test)]
fn retransmit_timeout() -> Duration {
Duration::from_millis(100)
}

#[cfg(not(test))]
fn retransmit_timeout() -> Duration {
Duration::from_secs(10)
}

#[cfg(test)]
fn backoff_time() -> Duration {
use rand::Rng;
Expand Down
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