feat: add stream-oriented AppendEntries API for pipelined replication

Add `Raft::stream_append()` method that allows processing multiple AppendEntries
requests through a pipelined stream interface. This enables efficient batched
log replication with backpressure support via a bounded channel.

Example usage:

```rust
use std::pin::pin;
use futures::StreamExt;

let input = futures::stream::iter(vec![request1, request2, request3]);
let mut output = pin!(raft.stream_append(input));

while let Some(result) = output.next().await {
    match result {
        Ok(log_id) => println!("Flushed: {:?}", log_id),
        Err(err) => {
            println!("Error: {}", err);
            break;
        }
    }
}
```

Changes:
- Add `Raft::stream_append()` for stream-based AppendEntries processing
- Add `StreamAppendError` with `Conflict` and `HigherVote` variants
- Add `StreamAppendResult` type alias for stream item results
- Add `AppendEntriesResponse::into_stream_result()` conversion method
- Add `ProtocolApi::stream_append()` internal implementation
- Add integration tests for success, conflict, and higher vote scenarios

Author: drmingdrmer

openraft/src/raft/api/protocol.rs

@@ -1,9 +1,12 @@
+use std::sync::Arc;
 use std::time::Duration;
 
+use futures::Stream;
 use openraft_macros::since;
 
 use crate::LogIdOptionExt;
 use crate::LogIndexOptionExt;
+use crate::OptionalSend;
 use crate::RaftMetrics;
 use crate::RaftTypeConfig;
 use crate::Snapshot;
@@ -20,6 +23,8 @@ use crate::raft::TransferLeaderRequest;
 use crate::raft::VoteRequest;
 use crate::raft::VoteResponse;
 use crate::raft::raft_inner::RaftInner;
+use crate::raft::stream_append;
+use crate::raft::stream_append::StreamAppendResult;
 use crate::type_config::TypeConfigExt;
 use crate::type_config::alias::SnapshotDataOf;
 use crate::type_config::alias::VoteOf;
@@ -57,16 +62,16 @@ use crate::vote::raft_vote::RaftVoteExt;
 /// Remote Leader Node                                           Local Follower Node
 /// ```
 #[since(version = "0.10.0")]
-pub(crate) struct ProtocolApi<'a, C>
+pub(crate) struct ProtocolApi<C>
 where C: RaftTypeConfig
 {
-    inner: &'a RaftInner<C>,
+    inner: Arc<RaftInner<C>>,
 }
 
-impl<'a, C> ProtocolApi<'a, C>
+impl<C> ProtocolApi<C>
 where C: RaftTypeConfig
 {
-    pub(in crate::raft) fn new(inner: &'a RaftInner<C>) -> Self {
+    pub(in crate::raft) fn new(inner: Arc<RaftInner<C>>) -> Self {
         Self { inner }
     }
 
@@ -91,6 +96,17 @@ where C: RaftTypeConfig
         self.inner.call_core(RaftMsg::AppendEntries { rpc, tx }, rx).await
     }
 
+    #[since(version = "0.10.0")]
+    pub(crate) fn stream_append<S>(
+        self,
+        stream: S,
+    ) -> impl Stream<Item = StreamAppendResult<C>> + OptionalSend + 'static
+    where
+        S: Stream<Item = AppendEntriesRequest<C>> + OptionalSend + 'static,
+    {
+        stream_append::stream_append(self.inner, stream)
+    }
+
     #[since(version = "0.10.0")]
     #[tracing::instrument(level = "debug", skip_all)]
     pub(crate) async fn get_snapshot(&self) -> Result<Option<Snapshot<C>>, Fatal<C>> {

openraft/src/raft/message/append_entries.rs

@@ -3,6 +3,8 @@ use std::fmt;
 use crate::RaftTypeConfig;
 use crate::display_ext::DisplayOptionExt;
 use crate::display_ext::DisplaySlice;
+use crate::raft::StreamAppendError;
+use crate::raft::stream_append::StreamAppendResult;
 use crate::type_config::alias::LogIdOf;
 use crate::type_config::alias::VoteOf;
 
@@ -115,6 +117,24 @@ where C: RaftTypeConfig
     pub fn is_conflict(&self) -> bool {
         matches!(*self, AppendEntriesResponse::Conflict)
     }
+
+    /// Convert this response to a stream append result.
+    ///
+    /// Arguments:
+    /// - `prev_log_id`: The prev_log_id from the request, used for Conflict errors.
+    /// - `last_log_id`: The last_log_id of the sent entries, used for Success.
+    pub fn into_stream_result(
+        self,
+        prev_log_id: Option<LogIdOf<C>>,
+        last_log_id: Option<LogIdOf<C>>,
+    ) -> StreamAppendResult<C> {
+        match self {
+            AppendEntriesResponse::Success => Ok(last_log_id),
+            AppendEntriesResponse::PartialSuccess(log_id) => Ok(log_id),
+            AppendEntriesResponse::Conflict => Err(StreamAppendError::Conflict(prev_log_id)),
+            AppendEntriesResponse::HigherVote(vote) => Err(StreamAppendError::HigherVote(vote)),
+        }
+    }
 }
 
 impl<C> fmt::Display for AppendEntriesResponse<C>

openraft/src/raft/message/mod.rs

@@ -5,6 +5,7 @@
 
 mod append_entries;
 mod install_snapshot;
+mod stream_append_error;
 mod transfer_leader;
 mod vote;
 
@@ -18,6 +19,7 @@ pub use client_write::ClientWriteResult;
 pub use install_snapshot::InstallSnapshotRequest;
 pub use install_snapshot::InstallSnapshotResponse;
 pub use install_snapshot::SnapshotResponse;
+pub use stream_append_error::StreamAppendError;
 pub use transfer_leader::TransferLeaderRequest;
 pub use vote::VoteRequest;
 pub use vote::VoteResponse;

openraft/src/raft/message/stream_append_error.rs

@@ -0,0 +1,36 @@
+use std::fmt;
+
+use crate::RaftTypeConfig;
+use crate::display_ext::DisplayOptionExt;
+use crate::type_config::alias::LogIdOf;
+use crate::type_config::alias::VoteOf;
+
+/// Error type for stream append entries.
+///
+/// When this error is returned, the stream is terminated.
+#[derive(Debug, Clone, PartialEq, Eq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize), serde(bound = ""))]
+pub enum StreamAppendError<C: RaftTypeConfig> {
+    /// Log conflict at the given prev_log_id.
+    ///
+    /// The follower's log at this position does not match the leader's.
+    Conflict(Option<LogIdOf<C>>),
+
+    /// Seen a higher vote from another leader.
+    HigherVote(VoteOf<C>),
+}
+
+impl<C> fmt::Display for StreamAppendError<C>
+where C: RaftTypeConfig
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            StreamAppendError::Conflict(log_id) => {
+                write!(f, "Conflict({})", log_id.display())
+            }
+            StreamAppendError::HigherVote(vote) => {
+                write!(f, "HigherVote({})", vote)
+            }
+        }
+    }
+}

openraft/src/raft/mod.rs

@@ -17,6 +17,7 @@ pub(crate) mod message;
 mod raft_inner;
 pub mod responder;
 mod runtime_config_handle;
+mod stream_append;
 pub mod trigger;
 
 use std::any::Any;
@@ -42,11 +43,13 @@ pub use message::ClientWriteResult;
 pub use message::InstallSnapshotRequest;
 pub use message::InstallSnapshotResponse;
 pub use message::SnapshotResponse;
+pub use message::StreamAppendError;
 pub use message::TransferLeaderRequest;
 pub use message::VoteRequest;
 pub use message::VoteResponse;
 pub use message::WriteRequest;
 use openraft_macros::since;
+pub use stream_append::StreamAppendResult;
 use tracing::Instrument;
 use tracing::Level;
 use tracing::trace_span;
@@ -655,8 +658,8 @@ where C: RaftTypeConfig
     /// - [`ProtocolApi::begin_receiving_snapshot`]
     /// - [`ProtocolApi::install_full_snapshot`]
     /// - [`ProtocolApi::handle_transfer_leader`]
-    pub(crate) fn protocol_api(&self) -> ProtocolApi<'_, C> {
-        ProtocolApi::new(self.inner.as_ref())
+    pub(crate) fn protocol_api(&self) -> ProtocolApi<C> {
+        ProtocolApi::new(self.inner.clone())
     }
 
     pub(crate) fn app_api(&self) -> AppApi<'_, C> {
@@ -688,6 +691,72 @@ where C: RaftTypeConfig
         self.protocol_api().append_entries(rpc).await.into_raft_result()
     }
 
+    /// Submit a stream of AppendEntries RPCs to this Raft node.
+    ///
+    /// This is a stream-oriented version of [`Self::append_entries`] with pipelining support.
+    /// It spawns a background task that reads from the input stream, sends requests to RaftCore,
+    /// and forwards response receivers to the output stream. Responses are yielded in order.
+    ///
+    /// ## Pipelining Behavior
+    ///
+    /// - A background task reads from the input stream and sends to RaftCore
+    /// - Uses a bounded channel (64 slots) for backpressure
+    /// - Responses are yielded in order (FIFO) as they complete
+    ///
+    /// ## Output
+    ///
+    /// The output stream emits:
+    /// - `Ok(log_id)` when logs are successfully flushed
+    /// - `Err(e)` when an error occurs, which terminates the stream
+    ///
+    /// ## Pinning
+    ///
+    /// The returned stream is `!Unpin` because it uses async closures internally.
+    /// You must pin the stream before calling `.next()`:
+    ///
+    /// ```ignore
+    /// use std::pin::pin;
+    ///
+    /// let mut output = pin!(raft.stream_append(input));
+    /// while let Some(result) = output.next().await { /* ... */ }
+    /// ```
+    ///
+    /// Alternatively, use `Box::pin` for heap pinning if the stream needs to be stored or returned:
+    ///
+    /// ```ignore
+    /// let mut output = Box::pin(raft.stream_append(input));
+    /// ```
+    ///
+    /// # Example
+    ///
+    /// ```ignore
+    /// use std::pin::pin;
+    /// use futures::StreamExt;
+    ///
+    /// let input_stream = futures::stream::iter(vec![request1, request2, request3]);
+    /// let mut output_stream = pin!(raft.stream_append(input_stream));
+    ///
+    /// while let Some(result) = output_stream.next().await {
+    ///     match result {
+    ///         Ok(log_id) => println!("Flushed: {:?}", log_id),
+    ///         Err(err) => {
+    ///             println!("Error: {}", err);
+    ///             break;
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    #[since(version = "0.10.0")]
+    pub fn stream_append<S>(
+        &self,
+        stream: S,
+    ) -> impl futures::Stream<Item = StreamAppendResult<C>> + OptionalSend + 'static
+    where
+        S: futures::Stream<Item = AppendEntriesRequest<C>> + OptionalSend + 'static,
+    {
+        self.protocol_api().stream_append(stream)
+    }
+
     /// Submit a VoteRequest (RequestVote in the spec) RPC to this Raft node.
     ///
     /// These RPCs are sent by cluster peers which are in candidate state attempting to gather votes

openraft/src/raft/stream_append.rs

@@ -0,0 +1,83 @@
+//! Stream-based AppendEntries API implementation with pipelining.
+
+use std::sync::Arc;
+
+use futures::Stream;
+use futures::StreamExt;
+
+use crate::AsyncRuntime;
+use crate::OptionalSend;
+use crate::RaftTypeConfig;
+use crate::core::raft_msg::RaftMsg;
+use crate::entry::RaftEntry;
+use crate::raft::AppendEntriesRequest;
+use crate::raft::AppendEntriesResponse;
+use crate::raft::StreamAppendError;
+use crate::raft::raft_inner::RaftInner;
+use crate::type_config::alias::LogIdOf;
+use crate::type_config::alias::OneshotReceiverOf;
+use crate::type_config::async_runtime::MpscReceiver;
+use crate::type_config::async_runtime::MpscSender;
+use crate::type_config::util::TypeConfigExt;
+
+/// Result type for stream append operations.
+pub type StreamAppendResult<C> = Result<Option<LogIdOf<C>>, StreamAppendError<C>>;
+
+const PIPELINE_BUFFER_SIZE: usize = 64;
+
+struct Pending<C: RaftTypeConfig> {
+    response_rx: OneshotReceiverOf<C, AppendEntriesResponse<C>>,
+    prev_log_id: Option<LogIdOf<C>>,
+    last_log_id: Option<LogIdOf<C>>,
+}
+
+/// Create a pipelined stream that processes AppendEntries requests.
+///
+/// Spawns a background task that reads from input, sends to RaftCore,
+/// and forwards response receivers. The returned stream awaits responses in order.
+pub(crate) fn stream_append<C, S>(
+    inner: Arc<RaftInner<C>>,
+    input: S,
+) -> impl Stream<Item = StreamAppendResult<C>> + OptionalSend + 'static
+where
+    C: RaftTypeConfig,
+    S: Stream<Item = AppendEntriesRequest<C>> + OptionalSend + 'static,
+{
+    let (tx, rx) = C::mpsc::<Pending<C>>(PIPELINE_BUFFER_SIZE);
+
+    let inner2 = inner.clone();
+    let _handle = C::AsyncRuntime::spawn(async move {
+        let inner = inner2;
+        futures::pin_mut!(input);
+
+        while let Some(req) = input.next().await {
+            let prev = req.prev_log_id.clone();
+            let last = req.entries.last().map(|e| e.log_id()).or(prev.clone());
+            let (resp_tx, resp_rx) = C::oneshot();
+
+            if inner.send_msg(RaftMsg::AppendEntries { rpc: req, tx: resp_tx }).await.is_err() {
+                break;
+            }
+            let pending = Pending {
+                response_rx: resp_rx,
+                prev_log_id: prev,
+                last_log_id: last,
+            };
+            if MpscSender::send(&tx, pending).await.is_err() {
+                break;
+            }
+        }
+    });
+
+    futures::stream::unfold(Some((rx, inner)), |state| async move {
+        let (mut rx, inner) = state?;
+        let p: Pending<C> = MpscReceiver::recv(&mut rx).await?;
+
+        let resp = inner.recv_msg(p.response_rx).await.ok()?;
+
+        let result = resp.into_stream_result(p.prev_log_id, p.last_log_id);
+        let cont = result.is_ok();
+
+        Some((result, if cont { Some((rx, inner)) } else { None }))
+    })
+}

tests/tests/append_entries/main.rs

@@ -9,6 +9,7 @@ mod fixtures;
 
 mod t10_conflict_with_empty_entries;
 mod t10_see_higher_vote;
+mod t10_stream_append;
 mod t11_append_conflicts;
 mod t11_append_entries_with_bigger_term;
 mod t11_append_inconsistent_log;