Feature: Add RaftTypeConfig::Responder to customize returning clien…

…t write response

This commit introduces the `Responder` trait that defines the mechanism
by which `RaftCore` sends responses back to the client after processing
write requests.  Applications can now customize response handling by
implementing their own version of the `RaftTypeConfig::Responder` trait.

The `Responder::from_app_data(RaftTypeConfig::D)` method is invoked to
create a new `Responder` instance when a client write request is
received.
Once the write operation is completed within `RaftCore`,
`Responder::send(WriteResult)` is called to dispatch the result
back to the client.

By default, `RaftTypeConfig::Responder` retains the existing
functionality using a oneshot channel, ensuring backward compatibility.

This change is non-breaking, requiring no modifications to existing
applications.

- Fix: #1068

openraft/src/core/raft_core.rs

@@ -29,7 +29,6 @@ use crate::core::notify::Notify;
 use crate::core::raft_msg::external_command::ExternalCommand;
 use crate::core::raft_msg::AppendEntriesTx;
 use crate::core::raft_msg::ClientReadTx;
-use crate::core::raft_msg::ClientWriteTx;
 use crate::core::raft_msg::RaftMsg;
 use crate::core::raft_msg::ResultSender;
 use crate::core::raft_msg::VoteTx;
@@ -66,6 +65,7 @@ use crate::progress::entry::ProgressEntry;
 use crate::progress::Inflight;
 use crate::progress::Progress;
 use crate::quorum::QuorumSet;
+use crate::raft::responder::Responder;
 use crate::raft::AppendEntriesRequest;
 use crate::raft::AppendEntriesResponse;
 use crate::raft::ClientWriteResponse;
@@ -85,6 +85,7 @@ use crate::storage::RaftLogStorage;
 use crate::storage::RaftStateMachine;
 use crate::type_config::alias::AsyncRuntimeOf;
 use crate::type_config::alias::InstantOf;
+use crate::type_config::alias::ResponderOf;
 use crate::AsyncRuntime;
 use crate::ChangeMembers;
 use crate::Instant;
@@ -182,7 +183,7 @@ where
     pub(crate) engine: Engine<C>,
 
     /// Channels to send result back to client when logs are applied.
-    pub(crate) client_resp_channels: BTreeMap<u64, ClientWriteTx<C>>,
+    pub(crate) client_resp_channels: BTreeMap<u64, ResponderOf<C>>,
 
     pub(crate) leader_data: Option<LeaderData<C>>,
 
@@ -448,13 +449,13 @@ where
         &mut self,
         changes: ChangeMembers<C::NodeId, C::Node>,
         retain: bool,
-        tx: ResultSender<C, ClientWriteResponse<C>, ClientWriteError<C::NodeId, C::Node>>,
+        tx: ResponderOf<C>,
     ) {
         let res = self.engine.state.membership_state.change_handler().apply(changes, retain);
         let new_membership = match res {
             Ok(x) => x,
             Err(e) => {
-                let _ = tx.send(Err(ClientWriteError::ChangeMembershipError(e)));
+                tx.send(Err(ClientWriteError::ChangeMembershipError(e)));
                 return;
             }
         };
@@ -471,7 +472,7 @@ where
     /// The result of applying it to state machine is sent to `resp_tx`, if it is not `None`.
     /// The calling side may not receive a result from `resp_tx`, if raft is shut down.
     #[tracing::instrument(level = "debug", skip_all, fields(id = display(self.id)))]
-    pub fn write_entry(&mut self, entry: C::Entry, resp_tx: Option<ClientWriteTx<C>>) -> bool {
+    pub fn write_entry(&mut self, entry: C::Entry, resp_tx: Option<ResponderOf<C>>) -> bool {
         tracing::debug!(payload = display(&entry), "write_entry");
 
         let (mut lh, tx) = if let Some((lh, tx)) = self.engine.get_leader_handler_or_reject(resp_tx) {
@@ -504,9 +505,7 @@ where
             "send_heartbeat"
         );
 
-        let mut lh = if let Some((lh, _)) =
-            self.engine.get_leader_handler_or_reject::<(), ClientWriteError<C::NodeId, C::Node>>(None)
-        {
+        let mut lh = if let Some((lh, _)) = self.engine.get_leader_handler_or_reject(None) {
             lh
         } else {
             tracing::debug!(
@@ -785,7 +784,7 @@ where
 
     /// Send result of applying a log entry to its client.
     #[tracing::instrument(level = "debug", skip_all)]
-    pub(super) fn send_response(entry: ApplyingEntry<C::NodeId, C::Node>, resp: C::R, tx: Option<ClientWriteTx<C>>) {
+    pub(super) fn send_response(entry: ApplyingEntry<C::NodeId, C::Node>, resp: C::R, tx: Option<ResponderOf<C>>) {
         tracing::debug!(entry = debug(&entry), "send_response");
 
         let tx = match tx {
@@ -801,11 +800,7 @@ where
             membership,
         });
 
-        let send_res = tx.send(res);
-        tracing::debug!(
-            "send client response through tx, send_res is error: {}",
-            send_res.is_err()
-        );
+        tx.send(res);
     }
 
     /// Spawn a new replication stream returning its replication state handle.
@@ -1644,16 +1639,12 @@ where
                     #[allow(clippy::let_underscore_future)]
                     let _ = AsyncRuntimeOf::<C>::spawn(async move {
                         for (log_index, tx) in removed.into_iter() {
-                            let res = tx.send(Err(ClientWriteError::ForwardToLeader(ForwardToLeader {
+                            tx.send(Err(ClientWriteError::ForwardToLeader(ForwardToLeader {
                                 leader_id,
                                 leader_node: leader_node.clone(),
                             })));
 
-                            tracing::debug!(
-                                "sent ForwardToLeader for log_index: {}, is_ok: {}",
-                                log_index,
-                                res.is_ok()
-                            );
+                            tracing::debug!("sent ForwardToLeader for log_index: {}", log_index,);
                         }
                     });
                 }

openraft/src/core/raft_msg/mod.rs

@@ -2,20 +2,19 @@ use std::collections::BTreeMap;
 
 use crate::core::raft_msg::external_command::ExternalCommand;
 use crate::error::CheckIsLeaderError;
-use crate::error::ClientWriteError;
 use crate::error::Infallible;
 use crate::error::InitializeError;
 use crate::raft::AppendEntriesRequest;
 use crate::raft::AppendEntriesResponse;
 use crate::raft::BoxCoreFn;
-use crate::raft::ClientWriteResponse;
 use crate::raft::SnapshotResponse;
 use crate::raft::VoteRequest;
 use crate::raft::VoteResponse;
 use crate::type_config::alias::LogIdOf;
 use crate::type_config::alias::NodeIdOf;
 use crate::type_config::alias::NodeOf;
 use crate::type_config::alias::OneshotSenderOf;
+use crate::type_config::alias::ResponderOf;
 use crate::type_config::alias::SnapshotDataOf;
 use crate::ChangeMembers;
 use crate::MessageSummary;
@@ -34,9 +33,6 @@ pub(crate) type VoteTx<C> = ResultSender<C, VoteResponse<NodeIdOf<C>>>;
 /// TX for Append Entries Response
 pub(crate) type AppendEntriesTx<C> = ResultSender<C, AppendEntriesResponse<NodeIdOf<C>>>;
 
-/// TX for Client Write Response
-pub(crate) type ClientWriteTx<C> = ResultSender<C, ClientWriteResponse<C>, ClientWriteError<NodeIdOf<C>, NodeOf<C>>>;
-
 /// TX for Linearizable Read Response
 pub(crate) type ClientReadTx<C> =
     ResultSender<C, (Option<LogIdOf<C>>, Option<LogIdOf<C>>), CheckIsLeaderError<NodeIdOf<C>, NodeOf<C>>>;
@@ -75,7 +71,7 @@ where C: RaftTypeConfig
 
     ClientWriteRequest {
         app_data: C::D,
-        tx: ClientWriteTx<C>,
+        tx: ResponderOf<C>,
     },
 
     CheckIsLeaderRequest {
@@ -94,7 +90,7 @@ where C: RaftTypeConfig
         /// config will be converted into learners, otherwise they will be removed.
         retain: bool,
 
-        tx: ResultSender<C, ClientWriteResponse<C>, ClientWriteError<C::NodeId, C::Node>>,
+        tx: ResponderOf<C>,
     },
 
     ExternalCoreRequest {

openraft/src/core/tick.rs

@@ -175,6 +175,7 @@ mod tests {
         type Entry = crate::Entry<TickUTConfig>;
         type SnapshotData = Cursor<Vec<u8>>;
         type AsyncRuntime = TokioRuntime;
+        type Responder = crate::impls::OneshotResponder<Self>;
     }
 
     // AsyncRuntime::spawn is `spawn_local` with singlethreaded enabled.

openraft/src/engine/engine_impl.rs

@@ -2,7 +2,6 @@ use std::time::Duration;
 
 use validit::Valid;
 
-use crate::async_runtime::AsyncOneshotSendExt;
 use crate::core::raft_msg::AppendEntriesTx;
 use crate::core::raft_msg::ResultSender;
 use crate::core::sm;
@@ -31,20 +30,21 @@ use crate::error::NotInMembers;
 use crate::error::RejectAppendEntries;
 use crate::internal_server_state::InternalServerState;
 use crate::membership::EffectiveMembership;
+use crate::raft::responder::Responder;
 use crate::raft::AppendEntriesResponse;
 use crate::raft::SnapshotResponse;
 use crate::raft::VoteRequest;
 use crate::raft::VoteResponse;
 use crate::raft_state::LogStateReader;
 use crate::raft_state::RaftState;
 use crate::summary::MessageSummary;
+use crate::type_config::alias::ResponderOf;
 use crate::type_config::alias::SnapshotDataOf;
 use crate::AsyncRuntime;
 use crate::Instant;
 use crate::LogId;
 use crate::LogIdOptionExt;
 use crate::Membership;
-use crate::OptionalSend;
 use crate::RaftLogId;
 use crate::RaftTypeConfig;
 use crate::Snapshot;
@@ -221,15 +221,10 @@ where C: RaftTypeConfig
     ///
     /// If tx is None, no response will be sent.
     #[tracing::instrument(level = "debug", skip_all)]
-    pub(crate) fn get_leader_handler_or_reject<T, E>(
+    pub(crate) fn get_leader_handler_or_reject(
         &mut self,
-        tx: Option<ResultSender<C, T, E>>,
-    ) -> Option<(LeaderHandler<C>, Option<ResultSender<C, T, E>>)>
-    where
-        T: OptionalSend,
-        E: OptionalSend,
-        E: From<ForwardToLeader<C::NodeId, C::Node>>,
-    {
+        tx: Option<ResponderOf<C>>,
+    ) -> Option<(LeaderHandler<C>, Option<ResponderOf<C>>)> {
         let res = self.leader_handler();
         let forward_err = match res {
             Ok(lh) => {
@@ -240,7 +235,7 @@ where C: RaftTypeConfig
         };
 
         if let Some(tx) = tx {
-            let _ = tx.send(Err(forward_err.into()));
+            tx.send(Err(forward_err.into()));
         }
 
         None

openraft/src/engine/testing.rs

@@ -15,4 +15,5 @@ impl RaftTypeConfig for UTConfig {
     type Entry = crate::Entry<UTConfig>;
     type SnapshotData = Cursor<Vec<u8>>;
     type AsyncRuntime = TokioRuntime;
+    type Responder = crate::impls::OneshotResponder<Self>;
 }

openraft/src/impls/mod.rs

@@ -0,0 +1,7 @@
+//! Collection of implementations of usually used traits defined by Openraft
+
+pub use crate::async_runtime::TokioRuntime;
+pub use crate::entry::Entry;
+pub use crate::node::BasicNode;
+pub use crate::node::EmptyNode;
+pub use crate::raft::responder::impls::OneshotResponder;

openraft/src/lib.rs

@@ -41,6 +41,7 @@ mod vote;
 pub mod async_runtime;
 pub mod entry;
 pub mod error;
+pub mod impls;
 pub mod instant;
 pub mod log_id;
 pub mod metrics;

openraft/src/raft/declare_raft_types_test.rs

@@ -14,6 +14,7 @@ declare_raft_types!(
         Entry = crate::Entry<Self>,
         SnapshotData = Cursor<Vec<u8>>,
         AsyncRuntime = TokioRuntime,
+        Responder = crate::impls::OneshotResponder<Self>,
 );
 
 declare_raft_types!(