Change: change-membership does not return error when replication lags

If `blocking` is `true`, `Raft::change_membership(..., blocking)` will
block until repliication to new nodes become upto date.
But it won't return an error when proposing change-membership log.

- Change: remove two errors: `LearnerIsLagging` and `LearnerNotFound`.

- Fix: #581

openraft/src/core/admin.rs

@@ -11,8 +11,6 @@ use crate::error::ClientWriteError;
 use crate::error::EmptyMembership;
 use crate::error::InProgress;
 use crate::error::InitializeError;
-use crate::error::LearnerIsLagging;
-use crate::error::LearnerNotFound;
 use crate::error::RemoveLearnerError;
 use crate::raft::AddLearnerResponse;
 use crate::raft::ClientWriteResponse;
@@ -144,7 +142,6 @@ impl<'a, D: AppData, R: AppDataResponse, N: RaftNetwork<D>, S: RaftStorage<D, R>
     pub(super) async fn change_membership(
         &mut self,
         members: BTreeSet<NodeId>,
-        blocking: bool,
         tx: RaftRespTx<ClientWriteResponse<R>, ClientWriteError>,
     ) -> Result<(), StorageError> {
         tracing::info!("change_membership: members: {:?}", members);
@@ -174,48 +171,6 @@ impl<'a, D: AppData, R: AppDataResponse, N: RaftNetwork<D>, S: RaftStorage<D, R>
 
         tracing::info!("change_membership: new_config: {:?}", new_config);
 
-        // Check the proposed config for any new nodes. If ALL new nodes already have replication
-        // streams AND are ready to join, then we can immediately proceed with entering joint
-        // consensus. Else, new nodes need to first be brought up-to-speed.
-        //
-        // Here, all we do is check to see which nodes still need to be synced, which determines
-        // if we can proceed.
-
-        // TODO(xp): test change membership without adding as learner.
-
-        // TODO(xp): 111 test adding a node that is not learner.
-        // TODO(xp): 111 test adding a node that is lagging.
-        for new_node in members.difference(curr.all_nodes()) {
-            match self.nodes.get(new_node) {
-                Some(node) => {
-                    if node.is_line_rate(&self.core.last_log_id, &self.core.config) {
-                        // Node is ready to join.
-                        continue;
-                    }
-
-                    if !blocking {
-                        // Node has repl stream, but is not yet ready to join.
-                        let _ = tx.send(Err(ClientWriteError::ChangeMembershipError(
-                            ChangeMembershipError::LearnerIsLagging(LearnerIsLagging {
-                                node_id: *new_node,
-                                matched: node.matched,
-                                distance: self.core.last_log_id.next_index().saturating_sub(node.matched.next_index()),
-                            }),
-                        )));
-                        return Ok(());
-                    }
-                }
-
-                // Node does not yet have a repl stream, spawn one.
-                None => {
-                    let _ = tx.send(Err(ClientWriteError::ChangeMembershipError(
-                        ChangeMembershipError::LearnerNotFound(LearnerNotFound { node_id: *new_node }),
-                    )));
-                    return Ok(());
-                }
-            }
-        }
-
         self.append_membership_log(new_config, Some(tx)).await?;
         Ok(())
     }

openraft/src/core/mod.rs

@@ -809,8 +809,8 @@ impl<'a, D: AppData, R: AppDataResponse, N: RaftNetwork<D>, S: RaftStorage<D, R>
             RaftMsg::RemoveLearner { id, tx } => {
                 self.remove_learner(id, tx);
             }
-            RaftMsg::ChangeMembership { members, blocking, tx } => {
-                self.change_membership(members, blocking, tx).await?;
+            RaftMsg::ChangeMembership { members, tx } => {
+                self.change_membership(members, tx).await?;
             }
             RaftMsg::ForceSnapshotting { tx } => {
                 for node in self.nodes.values() {

openraft/src/error.rs

@@ -102,12 +102,6 @@ pub enum ChangeMembershipError {
 
     #[error(transparent)]
     EmptyMembership(#[from] EmptyMembership),
-
-    #[error(transparent)]
-    LearnerNotFound(#[from] LearnerNotFound),
-
-    #[error(transparent)]
-    LearnerIsLagging(#[from] LearnerIsLagging),
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize, thiserror::Error)]
@@ -261,20 +255,6 @@ pub struct InProgress {
     pub membership_log_id: LogId,
 }
 
-#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize, thiserror::Error)]
-#[error("to add a member {node_id} first need to add it as learner")]
-pub struct LearnerNotFound {
-    pub node_id: NodeId,
-}
-
-#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize, thiserror::Error)]
-#[error("replication to learner {node_id} is lagging {distance}, matched: {matched:?}, can not add as member")]
-pub struct LearnerIsLagging {
-    pub node_id: NodeId,
-    pub matched: Option<LogId>,
-    pub distance: u64,
-}
-
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize, thiserror::Error)]
 #[error("new membership can not be empty")]
 pub struct EmptyMembership {}

openraft/src/raft.rs

@@ -284,14 +284,18 @@ impl<D: AppData, R: AppDataResponse, N: RaftNetwork<D>, S: RaftStorage<D, R>> Ra
     /// If a node in the proposed config but is not yet a voter or learner, it first calls `add_learner` to setup
     /// replication to the new node.
     ///
-    /// Internal:
-    /// - It proposes a **joint** config.
-    /// - When the **joint** config is committed, it proposes a uniform config.
-    ///
-    /// If blocking is true, it blocks until every learner becomes up to date.
-    /// Otherwise it returns error `ChangeMembershipError::LearnerIsLagging` if there is a lagging learner.
-    ///
-    /// If it lost leadership or crashed before committing the second **uniform** config log, the cluster is left in the
+    /// Internally it commits two logs to get this task done:
+    /// - Proposes a **joint** config, e.g. `[{1,2,3}, {4,5,6}]` if trying to change config from `{1,2,3}` to `{4,5,6}`.
+    /// - When the **joint** config is committed, then proposes a **uniform** config, e.g. `{4,5,6}`.
+    ///
+    /// If `blocking` is true, it does not propose the membership log until replication to every learner becomes up to
+    /// date.
+    /// Otherwise it just proposes membership log at once. In this case, the change-membership log may be
+    /// committed very quickly: if the **old** cluster constitutes a quorum.
+    /// E.g., when changing from `[1,2,3,4]` to `[1,2,3,4,5]`, `[1,2,3]` is a quorum in both the old and the new
+    /// cluster. Then it will return before the replication to node-5 becomes up to date.
+    ///
+    /// If this node crashes before committing the second **uniform** config log, the cluster may be left in the
     /// **joint** config.
     #[tracing::instrument(level = "info", skip_all)]
     pub async fn change_membership(
@@ -330,7 +334,6 @@ impl<D: AppData, R: AppDataResponse, N: RaftNetwork<D>, S: RaftStorage<D, R>> Ra
             .call_core(
                 RaftMsg::ChangeMembership {
                     members: members.clone(),
-                    blocking,
                     tx,
                 },
                 rx,
@@ -350,7 +353,7 @@ impl<D: AppData, R: AppDataResponse, N: RaftNetwork<D>, S: RaftStorage<D, R>> Ra
         tracing::info!("the second step is to change to uniform config: {:?}", members);
 
         let (tx, rx) = oneshot::channel();
-        let res = self.call_core(RaftMsg::ChangeMembership { members, blocking, tx }, rx).await?;
+        let res = self.call_core(RaftMsg::ChangeMembership { members, tx }, rx).await?;
 
         tracing::info!("res of second change_membership: {}", res.summary());
 
@@ -500,11 +503,6 @@ pub(crate) enum RaftMsg<D: AppData, R: AppDataResponse> {
     },
     ChangeMembership {
         members: BTreeSet<NodeId>,
-        /// with blocking==false, respond to client a ChangeMembershipError::LearnerIsLagging error at once if a
-        /// non-member is lagging.
-        ///
-        /// Otherwise, wait for commit of the member change log.
-        blocking: bool,
         tx: RaftRespTx<ClientWriteResponse<R>, ClientWriteError>,
     },
     /// Force to switch to snapshot replication to every target.
@@ -542,8 +540,8 @@ where
             RaftMsg::RemoveLearner { id, .. } => {
                 format!("RemoveLearner: id: {}", id)
             }
-            RaftMsg::ChangeMembership { members, blocking, .. } => {
-                format!("ChangeMembership: members: {:?}, blocking: {}", members, blocking)
+            RaftMsg::ChangeMembership { members, .. } => {
+                format!("ChangeMembership: members: {:?}", members)
             }
             RaftMsg::ForceSnapshotting { .. } => "ForceSnapshotting".to_string(),
         }

openraft/tests/membership/t20_change_membership.rs

@@ -1,9 +1,7 @@
-use std::convert::TryInto;
 use std::sync::Arc;
 use std::time::Duration;
 
 use maplit::btreeset;
-use openraft::error::ChangeMembershipError;
 use openraft::Config;
 use openraft::StorageHelper;
 
@@ -59,7 +57,7 @@ async fn change_with_new_learner_blocking() -> anyhow::Result<()> {
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 4)]
 async fn change_with_lagging_learner_non_blocking() -> anyhow::Result<()> {
-    // Add a learner into membership config, expect error NonVoterIsLagging.
+    // Add a learner into membership config.
 
     let (_log_guard, ut_span) = init_ut!();
     let _ent = ut_span.enter();
@@ -90,27 +88,36 @@ async fn change_with_lagging_learner_non_blocking() -> anyhow::Result<()> {
         router.wait(&0, timeout()).await?.log(Some(log_index), "received 500 logs").await?;
     }
 
-    tracing::info!("--- restore replication and change membership at once, expect NonVoterIsLagging");
+    tracing::info!(
+        "--- restore replication and change membership at once, it still blocks until logs are replicated to node-1"
+    );
     {
         router.restore_node(1).await;
         let res = router.change_membership_with_blocking(0, btreeset! {0,1}, false).await;
+        log_index += 2;
 
         tracing::info!("--- got res: {:?}", res);
+        assert!(res.is_ok());
+        router.wait(&1, timeout()).await?.log(Some(log_index), "received 500+2 logs").await?;
+    }
 
-        let err = res.unwrap_err();
-        let err: ChangeMembershipError = err.try_into().unwrap();
-
-        match err {
-            ChangeMembershipError::LearnerIsLagging(e) => {
-                tracing::info!(e.distance, "--- distance");
-                assert_eq!(1, e.node_id);
-                assert!(e.distance >= lag_threshold);
-                assert!(e.distance < 500);
-            }
-            _ => {
-                panic!("expect ChangeMembershipError::NonVoterNotFound");
-            }
-        }
+    tracing::info!("--- add node-3, with blocking=false, won't block, because [0,1] is a quorum");
+    {
+        router.new_raft_node(2).await;
+
+        router.change_membership_with_blocking(0, btreeset! {0,1,2}, false).await?;
+        log_index += 2;
+        let m = router.get_metrics(&2).await?;
+        assert!(m.last_log_index < Some(log_index));
+    }
+
+    tracing::info!("--- make sure replication to node-3 works as expected");
+    {
+        router
+            .wait(&2, timeout())
+            .await?
+            .log(Some(log_index), "received all logs, replication works")
+            .await?;
     }
 
     Ok(())