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Add Endpoint::eager_connect_errors to surface lazy-channel connection failures immediately #2841

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Add Endpoint::eager_connect_errors to surface lazy-channel connection failures immediately #2841

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e48a4c2
eager connect errors for lazy channels
hermeGarcia Aug 28, 2026
c8be97c
happy clippy
hermeGarcia Aug 28, 2026
565d5bf
reinsert endpoints with connection errors
hermeGarcia Aug 29, 2026
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30 changes: 30 additions & 0 deletions tonic/src/transport/channel/endpoint.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -71,6 +71,7 @@ pub struct Endpoint {
pub(crate) tcp_keepalive_interval: Option<Duration>,
pub(crate) tcp_keepalive_retries: Option<u32>,
pub(crate) tcp_nodelay: bool,
pub(crate) eager_connect_errors: bool,
pub(crate) http2_keep_alive_interval: Option<Duration>,
pub(crate) http2_keep_alive_timeout: Option<Duration>,
pub(crate) http2_keep_alive_while_idle: Option<bool>,
Expand Down Expand Up @@ -121,6 +122,7 @@ impl Endpoint {
tcp_keepalive_interval: None,
tcp_keepalive_retries: None,
tcp_nodelay: true,
eager_connect_errors: false,
http2_keep_alive_interval: None,
http2_keep_alive_timeout: None,
http2_keep_alive_while_idle: None,
Expand Down Expand Up @@ -152,6 +154,7 @@ impl Endpoint {
tcp_keepalive_interval: None,
tcp_keepalive_retries: None,
tcp_nodelay: true,
eager_connect_errors: false,
http2_keep_alive_interval: None,
http2_keep_alive_timeout: None,
http2_keep_alive_while_idle: None,
Expand Down Expand Up @@ -457,6 +460,33 @@ impl Endpoint {
}
}

/// Sets whether connection errors should be surfaced immediately, rather than only
/// once an actual call is made.
///
/// When enabled, a failed connection attempt is reported right away instead of being
/// deferred and retried transparently on the next call.
///
/// This has no effect on channels that connect eagerly which already surface their
/// initial connection error immediately; it only matters for lazily-connecting channels
/// (see [`Endpoint::connect_lazy`] and [`Endpoint::connect_with_connector_lazy`]).
///
/// For endpoints used with
/// [`Channel::balance_channel`](crate::transport::Channel::balance_channel) or
/// [`Channel::balance_list`](crate::transport::Channel::balance_list), enabling this
/// lets the load balancer identify a broken backend immediately and skip it rather
/// than routing a call to it. Such a backend is removed from the balancer as soon
/// as its connection attempt fails; it automatically rejoins the balancer shortly
/// after (on a short, fixed backoff) rather than being lost until the caller
/// explicitly re-adds it.
///
/// Defaults to `false`.
pub fn eager_connect_errors(self, enabled: bool) -> Self {
Endpoint {
eager_connect_errors: enabled,
..self
}
}

/// Set http2 KEEP_ALIVE_INTERVAL. Uses `hyper`'s default otherwise.
pub fn http2_keep_alive_interval(self, interval: Duration) -> Self {
Endpoint {
Expand Down
2 changes: 1 addition & 1 deletion tonic/src/transport/channel/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -165,7 +165,7 @@ impl Channel {
E: Executor<Pin<Box<dyn Future<Output = ()> + Send>>> + Send + Sync + 'static,
{
let (tx, rx) = channel(capacity);
let list = DynamicServiceStream::new(rx);
let list = DynamicServiceStream::new(rx, tx.clone());
(Self::balance(list, DEFAULT_BUFFER_SIZE, executor), tx)
}

Expand Down
112 changes: 107 additions & 5 deletions tonic/src/transport/channel/service/connection.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -22,7 +22,7 @@
*
*/

use super::{AddOrigin, Reconnect, SharedExec, UserAgent};
use super::{AddOrigin, Change, Reconnect, SharedExec, UserAgent};
use crate::{
body::Body,
transport::{Endpoint, channel::BoxFuture, service::GrpcTimeout},
Expand All @@ -34,7 +34,9 @@ use hyper_util::rt::TokioTimer;
use std::{
fmt,
task::{Context, Poll},
time::Duration,
};
use tokio::sync::mpsc;
use tower::load::Load;
use tower::{
ServiceBuilder, ServiceExt,
Expand All @@ -44,12 +46,16 @@ use tower::{
};
use tower_service::Service;

/// How long to wait before re-inserting a balanced endpoint that was evicted after a
/// failed connection attempt. See [`ReinsertOnError`].
const REINSERT_BACKOFF: Duration = Duration::from_secs(1);

pub(crate) struct Connection {
inner: BoxService<Request<Body>, Response<Body>, crate::BoxError>,
}

impl Connection {
fn new<C>(connector: C, endpoint: Endpoint, is_lazy: bool) -> Self
fn new<C>(connector: C, endpoint: Endpoint, is_lazy: bool, fail_early: bool) -> Self
where
C: Service<Uri> + Send + 'static,
C::Error: Into<crate::BoxError> + Send,
Expand Down Expand Up @@ -102,7 +108,7 @@ impl Connection {
let make_service =
MakeSendRequestService::new(connector, endpoint.executor.clone(), settings);

let conn = Reconnect::new(make_service, endpoint.uri().clone(), is_lazy);
let conn = Reconnect::new(make_service, endpoint.uri().clone(), is_lazy, fail_early);

Self {
inner: BoxService::new(stack.layer(conn)),
Expand All @@ -119,7 +125,10 @@ impl Connection {
C::Future: Unpin + Send,
C::Response: rt::Read + rt::Write + Unpin + Send + 'static,
{
Self::new(connector, endpoint, false).ready_oneshot().await
let fail_early = endpoint.eager_connect_errors;
Self::new(connector, endpoint, false, fail_early)
.ready_oneshot()
.await
}

pub(crate) fn lazy<C>(connector: C, endpoint: Endpoint) -> Self
Expand All @@ -129,7 +138,100 @@ impl Connection {
C::Future: Send,
C::Response: rt::Read + rt::Write + Unpin + Send + 'static,
{
Self::new(connector, endpoint, true)
let fail_early = endpoint.eager_connect_errors;
Self::new(connector, endpoint, true, fail_early)
}

/// Like [`Connection::lazy`], but for connections managed by a discovery-driven
/// [`tower::balance::p2c::Balance`].
///
/// `Balance` polls its services through a `tower::ready_cache::ReadyCache`, which
/// permanently evicts any service whose `poll_ready` returns `Err` and never
/// retries it on its own. Since [`Endpoint::eager_connect_errors`] intentionally
/// causes connect failures to surface from `poll_ready` (so `Balance` can skip a
/// broken endpoint instead of routing a call to it), a connection built here is
/// wrapped in [`ReinsertOnError`] so that an eviction like that is followed by an
/// automatic re-insert of the same key after a short backoff, instead of the
/// endpoint being lost until the caller notices and re-inserts it manually.
pub(crate) fn lazy_for_discovery<C, K>(
connector: C,
endpoint: Endpoint,
key: K,
reinsert: mpsc::Sender<Change<K, Endpoint>>,
) -> Self
where
C: Service<Uri> + Send + 'static,
C::Error: Into<crate::BoxError> + Send,
C::Future: Send,
C::Response: rt::Read + rt::Write + Unpin + Send + 'static,
K: Clone + Send + 'static,
{
let fail_early = endpoint.eager_connect_errors;
let executor = endpoint.executor.clone();
let retry_endpoint = endpoint.clone();
let inner = Self::new(connector, endpoint, true, fail_early);

Self {
inner: BoxService::new(ReinsertOnError {
inner,
key,
endpoint: retry_endpoint,
executor,
reinsert,
}),
}
}
}

/// Wraps a discovery-managed [`Connection`] so that a `poll_ready` error (which causes
/// `tower`'s `Balance`/`ReadyCache` to evict it permanently, see [`Connection::lazy_for_discovery`])
/// is followed by scheduling a fresh [`Change::Insert`] for the same key after
/// [`REINSERT_BACKOFF`], so the endpoint can rejoin the balancer on its own.
struct ReinsertOnError<K> {
inner: Connection,
key: K,
endpoint: Endpoint,
executor: SharedExec,
reinsert: mpsc::Sender<Change<K, Endpoint>>,
}

impl<K> ReinsertOnError<K>
where
K: Clone + Send + 'static,
{
fn schedule_reinsert(&self) {
let key = self.key.clone();
let endpoint = self.endpoint.clone();
let reinsert = self.reinsert.clone();

Executor::<BoxFuture<'static, ()>>::execute(
&self.executor,
Box::pin(async move {
tokio::time::sleep(REINSERT_BACKOFF).await;
let _ = reinsert.send(Change::Insert(key, endpoint)).await;
}) as _,
);
}
}

impl<K> Service<Request<Body>> for ReinsertOnError<K>
where
K: Clone + Send + 'static,
{
type Response = Response<Body>;
type Error = crate::BoxError;
type Future = BoxFuture<'static, Result<Self::Response, Self::Error>>;

fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
let poll = Service::poll_ready(&mut self.inner, cx);
if let Poll::Ready(Err(_)) = &poll {
self.schedule_reinsert();
}
poll
}

fn call(&mut self, req: Request<Body>) -> Self::Future {
self.inner.call(req)
}
}

Expand Down
22 changes: 17 additions & 5 deletions tonic/src/transport/channel/service/discover.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -29,7 +29,7 @@ use std::{
pin::Pin,
task::{Context, Poll},
};
use tokio::sync::mpsc::Receiver;
use tokio::sync::mpsc::{Receiver, Sender};
use tokio_stream::Stream;
use tower::discover::Change as TowerChange;

Expand All @@ -44,23 +44,35 @@ pub enum Change<K, V> {

pub(crate) struct DynamicServiceStream<K: Hash + Eq + Clone> {
changes: Receiver<Change<K, Endpoint>>,
/// A sender back into the same channel `changes` reads from, handed to each
/// discovered [`Connection`] so it can re-insert itself after being evicted by
/// `Balance` for a connect failure. See [`Connection::lazy_for_discovery`].
reinsert: Sender<Change<K, Endpoint>>,
}

impl<K: Hash + Eq + Clone> DynamicServiceStream<K> {
pub(crate) fn new(changes: Receiver<Change<K, Endpoint>>) -> Self {
Self { changes }
pub(crate) fn new(
changes: Receiver<Change<K, Endpoint>>,
reinsert: Sender<Change<K, Endpoint>>,
) -> Self {
Self { changes, reinsert }
}
}

impl<K: Hash + Eq + Clone> Stream for DynamicServiceStream<K> {
impl<K: Hash + Eq + Clone + Send + 'static> Stream for DynamicServiceStream<K> {
type Item = Result<TowerChange<K, Connection>, crate::BoxError>;

fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
match Pin::new(&mut self.changes).poll_recv(cx) {
Poll::Pending | Poll::Ready(None) => Poll::Pending,
Poll::Ready(Some(change)) => match change {
Change::Insert(k, endpoint) => {
let connection = Connection::lazy(endpoint.http_connector(), endpoint);
let connection = Connection::lazy_for_discovery(
endpoint.http_connector(),
endpoint,
k.clone(),
self.reinsert.clone(),
);
Poll::Ready(Some(Ok(TowerChange::Insert(k, connection))))
}
Change::Remove(k) => Poll::Ready(Some(Ok(TowerChange::Remove(k)))),
Expand Down
10 changes: 8 additions & 2 deletions tonic/src/transport/channel/service/reconnect.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -44,6 +44,7 @@ where
error: Option<crate::BoxError>,
has_been_connected: bool,
is_lazy: bool,
fail_early: bool,
}

#[derive(Debug)]
Expand All @@ -58,16 +59,21 @@ where
M: Service<Target>,
M::Error: Into<crate::BoxError>,
{
pub(crate) fn new(mk_service: M, target: Target, is_lazy: bool) -> Self {
pub(crate) fn new(mk_service: M, target: Target, is_lazy: bool, fail_early: bool) -> Self {
Reconnect {
mk_service,
state: State::Idle,
target,
error: None,
has_been_connected: false,
is_lazy,
fail_early,
}
}

fn return_connection_errors_on_poll(&self) -> bool {
self.fail_early || !(self.has_been_connected || self.is_lazy)
}
}

impl<M, Target, S, Request> Service<Request> for Reconnect<M, Target>
Expand Down Expand Up @@ -121,7 +127,7 @@ where

state = State::Idle;

if !(self.has_been_connected || self.is_lazy) {
if self.return_connection_errors_on_poll() {
return Poll::Ready(Err(e.into()));
Comment on lines +130 to 131

@LucioFranco LucioFranco Aug 28, 2026

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I think this might cause problems, such as iirc tower will remove this connection from the readycache and make it never available again. I don't think that is the behavior you want right?

@hermeGarcia hermeGarcia Aug 28, 2026 •
edited
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Yes, we definitely don't want that.
Thanks for pointing it out!

} else {
let error = e.into();
Expand Down
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