*: introduce judge_split_prevote

harness/tests/integration_cases/test_raft.rs

@@ -4333,6 +4333,58 @@ fn test_prevote_with_split_vote() {
     assert_eq!(network.peers[&3].state, StateRole::Follower, "peer 3 state",);
 }
 
+/// TestPreVoteWithJudgeSplitPreVote verifies that during split vote, cluster can finish campaign
+/// by letting raft node with larger hash value to finish campaign.
+#[test]
+fn test_prevote_with_judge_split_prevote() {
+    let l = default_logger();
+    for cnt in &[3, 5] {
+        let peers = (1..=*cnt).map(|id| {
+            let mut config = new_test_config(id, 10, 1);
+            config.pre_vote = true;
+            config.judge_split_prevote = true;
+            let storage = new_storage();
+            storage.initialize_with_conf_state(((1..=*cnt).collect::<Vec<_>>(), vec![]));
+            let mut raft = new_test_raft_with_config(&config, storage, &l);
+            raft.become_follower(1, INVALID_ID);
+            Some(raft)
+        });
+        let mut network = Network::new(peers.collect(), &l);
+        network.send(vec![new_message(1, 1, MessageType::MsgHup, 0)]);
+
+        // simulate leader down. followers start split vote.
+        network.isolate(1);
+        let msgs = (2..=*cnt)
+            .map(|id| new_message(id, id, MessageType::MsgHup, 0))
+            .collect();
+        network.send(msgs);
+
+        // check whether the term values are expected
+        for id in 2..=*cnt {
+            assert_eq!(network.peers[&id].term, 3, "[{}] peer {} term", cnt, id);
+        }
+
+        let mut hashes: Vec<_> = (2..=*cnt).map(|id| (fxhash::hash64(&id), id)).collect();
+        hashes.sort_unstable();
+        for (_, id) in &hashes[..hashes.len() - 1] {
+            assert_eq!(
+                network.peers[&id].state,
+                StateRole::Follower,
+                "[{}] peer {} state",
+                cnt,
+                id
+            );
+        }
+        assert_eq!(
+            network.peers[&hashes[hashes.len() - 1].1].state,
+            StateRole::Leader,
+            "[{}] peer {} state",
+            cnt,
+            cnt
+        );
+    }
+}
+
 // ensure that after a node become pre-candidate, it will checkQuorum correctly.
 #[test]
 fn test_prevote_with_check_quorum() {
@@ -5292,6 +5344,14 @@ fn test_group_commit_consistent() {
 /// of the election with both priority and log.
 #[test]
 fn test_election_with_priority_log() {
+    let entry_set = vec![
+        (
+            "large term",
+            vec![new_entry(2, 1, SOME_DATA)],
+            vec![new_entry(1, 1, SOME_DATA)],
+        ),
+        ("large index", vec![new_entry(1, 1, SOME_DATA)], vec![]),
+    ];
     let tests = vec![
         // log is up to date or not 1..3, priority 1..3, id, state
         (true, false, false, 3, 1, 1, 1, StateRole::Leader),
@@ -5307,30 +5367,37 @@ fn test_election_with_priority_log() {
         (false, false, true, 1, 1, 3, 1, StateRole::Leader),
     ];
 
-    for (_i, &(l1, l2, l3, p1, p2, p3, id, state)) in tests.iter().enumerate() {
-        let l = default_logger();
-        let mut n1 = new_test_raft(1, vec![1, 2, 3], 10, 1, new_storage(), &l);
-        let mut n2 = new_test_raft(2, vec![1, 2, 3], 10, 1, new_storage(), &l);
-        let mut n3 = new_test_raft(3, vec![1, 2, 3], 10, 1, new_storage(), &l);
-        n1.set_priority(p1);
-        n2.set_priority(p2);
-        n3.set_priority(p3);
-        let entries = vec![new_entry(1, 1, SOME_DATA), new_entry(1, 1, SOME_DATA)];
-        if l1 {
-            n1.raft_log.append(&entries);
-        }
-        if l2 {
-            n2.raft_log.append(&entries);
-        }
-        if l3 {
-            n3.raft_log.append(&entries);
-        }
+    for (tag, latest_entries, outdated_entries) in entry_set {
+        for (_i, &(l1, l2, l3, p1, p2, p3, id, state)) in tests.iter().enumerate() {
+            let l = default_logger();
+            let mut n1 = new_test_raft(1, vec![1, 2, 3], 10, 1, new_storage(), &l);
+            let mut n2 = new_test_raft(2, vec![1, 2, 3], 10, 1, new_storage(), &l);
+            let mut n3 = new_test_raft(3, vec![1, 2, 3], 10, 1, new_storage(), &l);
+            n1.set_priority(p1);
+            n2.set_priority(p2);
+            n3.set_priority(p3);
+            if l1 {
+                n1.raft_log.append(&latest_entries);
+            } else {
+                n1.raft_log.append(&outdated_entries);
+            }
+            if l2 {
+                n2.raft_log.append(&latest_entries);
+            } else {
+                n2.raft_log.append(&outdated_entries);
+            }
+            if l3 {
+                n3.raft_log.append(&latest_entries);
+            } else {
+                n3.raft_log.append(&outdated_entries);
+            }
 
-        let mut network = Network::new(vec![Some(n1), Some(n2), Some(n3)], &l);
+            let mut network = Network::new(vec![Some(n1), Some(n2), Some(n3)], &l);
 
-        network.send(vec![new_message(id, id, MessageType::MsgHup, 0)]);
+            network.send(vec![new_message(id, id, MessageType::MsgHup, 0)]);
 
-        assert_eq!(network.peers[&id].state, state);
+            assert_eq!(network.peers[&id].state, state, "{}", tag);
+        }
     }
 }
 

src/config.rs

@@ -67,6 +67,10 @@ pub struct Config {
     /// rejoins the cluster.
     pub pre_vote: bool,
 
+    /// If split vote happens, only vote for the large id. This can save a round of campaign
+    /// when combined with `pre_vote`.
+    pub judge_split_prevote: bool,
+
     /// The range of election timeout. In some cases, we hope some nodes has less possibility
     /// to become leader. This configuration ensures that the randomized election_timeout
     /// will always be suit in [min_election_tick, max_election_tick).
@@ -112,6 +116,7 @@ impl Default for Config {
             max_inflight_msgs: 256,
             check_quorum: false,
             pre_vote: false,
+            judge_split_prevote: false,
             min_election_tick: 0,
             max_election_tick: 0,
             read_only_option: ReadOnlyOption::Safe,

src/raft.rs

@@ -14,7 +14,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use std::cmp;
+use std::cmp::{self, Ordering};
 use std::ops::{Deref, DerefMut};
 
 use crate::eraftpb::{
@@ -238,6 +238,7 @@ pub struct RaftCore<T: Storage> {
 
     skip_bcast_commit: bool,
     batch_append: bool,
+    judge_split_prevote: bool,
 
     heartbeat_timeout: usize,
     election_timeout: usize,
@@ -337,6 +338,7 @@ impl<T: Storage> Raft<T> {
                 promotable: false,
                 check_quorum: c.check_quorum,
                 pre_vote: c.pre_vote,
+                judge_split_prevote: c.judge_split_prevote,
                 read_only: ReadOnly::new(c.read_only_option),
                 heartbeat_timeout: c.heartbeat_tick,
                 election_timeout: c.election_tick,
@@ -1344,6 +1346,18 @@ impl<T: Storage> Raft<T> {
                         "msg type" => ?m.get_msg_type(),
                     );
 
+                    // When judge_split_prevote, reject explicitly to make candidate exit PreCandiate early
+                    // so it will vote for other peer later.
+                    if self.judge_split_prevote
+                        && m.get_msg_type() == MessageType::MsgRequestPreVote
+                    {
+                        let mut to_send =
+                            new_message(m.from, MessageType::MsgRequestPreVoteResponse, None);
+                        to_send.term = m.term;
+                        to_send.reject = true;
+                        self.r.send(to_send, &mut self.msgs);
+                    }
+
                     return Ok(());
                 }
             }
@@ -1455,10 +1469,7 @@ impl<T: Storage> Raft<T> {
                     // ...or this is a PreVote for a future term...
                     (m.get_msg_type() == MessageType::MsgRequestPreVote && m.term > self.term);
                 // ...and we believe the candidate is up to date.
-                if can_vote
-                    && self.raft_log.is_up_to_date(m.index, m.log_term)
-                    && (m.index > self.raft_log.last_index() || self.priority <= m.priority)
-                {
+                if can_vote && self.may_vote(&m) {
                     // When responding to Msg{Pre,}Vote messages we include the term
                     // from the message, not the local term. To see why consider the
                     // case where a single node was previously partitioned away and
@@ -1479,6 +1490,10 @@ impl<T: Storage> Raft<T> {
                         self.election_elapsed = 0;
                         self.vote = m.from;
                     }
+                    // This means it's in split vote, give up election.
+                    if self.judge_split_prevote && self.state == StateRole::PreCandidate {
+                        self.become_follower(self.term, INVALID_ID);
+                    }
                 } else {
                     self.log_vote_reject(&m);
                     let mut to_send =
@@ -1501,6 +1516,40 @@ impl<T: Storage> Raft<T> {
         Ok(())
     }
 
+    /// Checks if this node may vote for the request. It may vote when from node
+    /// contains the same logs or more logs. When they contains same logs, priority
+    /// is considered. If priorities are still the same, and this node is also
+    /// starting campaign, then split vote happens. In this case, if `judge_split_prevote`
+    /// and `pre_vote` are enabled, it will vote only when from node has greater ID.
+    fn may_vote(&self, m: &Message) -> bool {
+        match self.raft_log.is_up_to_date(m.index, m.log_term) {
+            Ordering::Greater => true,
+            Ordering::Equal => {
+                match self.priority.cmp(&m.priority) {
+                    Ordering::Greater => false,
+                    Ordering::Equal => {
+                        // judge split vote can break symmetry of campaign, but as
+                        // it only happens during split vote, the impact should not
+                        // be significant.
+                        // Transfering leader skips prevote, so they won't have impact
+                        // on the other.
+                        !self.judge_split_prevote
+                            || self.state != StateRole::PreCandidate
+                            || m.get_msg_type() != MessageType::MsgRequestPreVote
+                            || {
+                                let from_id = m.from;
+                                let (my_h, from_h) =
+                                    (fxhash::hash64(&self.id), fxhash::hash64(&from_id));
+                                my_h < from_h || (my_h == from_h && self.id < from_id)
+                            }
+                    }
+                    Ordering::Less => true,
+                }
+            }
+            Ordering::Less => false,
+        }
+    }
+
     fn hup(&mut self, transfer_leader: bool) {
         if self.state == StateRole::Leader {
             debug!(

src/raft_log.rs

@@ -14,7 +14,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use std::cmp;
+use std::cmp::{self, Ordering};
 
 use slog::warn;
 use slog::Logger;
@@ -415,8 +415,11 @@ impl<T: Storage> RaftLog<T> {
     /// later term is more up-to-date. If the logs end with the same term, then
     /// whichever log has the larger last_index is more up-to-date. If the logs are
     /// the same, the given log is up-to-date.
-    pub fn is_up_to_date(&self, last_index: u64, term: u64) -> bool {
-        term > self.last_term() || (term == self.last_term() && last_index >= self.last_index())
+    /// Returns `Ordering::Greater` means given log is more up-to-date, `Ordering::Equal`
+    /// means they have the same logs, `Ordering::Less` means existing logs is more
+    /// up-to-date.
+    pub fn is_up_to_date(&self, last_index: u64, term: u64) -> Ordering {
+        (term, last_index).cmp(&(self.last_term(), self.last_index()))
     }
 
     /// Returns committed and persisted entries since max(`since_idx` + 1, first_index).
@@ -663,7 +666,7 @@ impl<T: Storage> RaftLog<T> {
 #[cfg(test)]
 mod test {
     use std::{
-        cmp,
+        cmp::{self, Ordering},
         panic::{self, AssertUnwindSafe},
     };
 
@@ -756,22 +759,25 @@ mod test {
         raft_log.append(&previous_ents);
         let tests = vec![
             // greater term, ignore lastIndex
-            (raft_log.last_index() - 1, 4, true),
-            (raft_log.last_index(), 4, true),
-            (raft_log.last_index() + 1, 4, true),
+            (raft_log.last_index() - 1, 4, Ordering::Greater),
+            (raft_log.last_index(), 4, Ordering::Greater),
+            (raft_log.last_index() + 1, 4, Ordering::Greater),
             // smaller term, ignore lastIndex
-            (raft_log.last_index() - 1, 2, false),
-            (raft_log.last_index(), 2, false),
-            (raft_log.last_index() + 1, 2, false),
+            (raft_log.last_index() - 1, 2, Ordering::Less),
+            (raft_log.last_index(), 2, Ordering::Less),
+            (raft_log.last_index() + 1, 2, Ordering::Less),
             // equal term, lager lastIndex wins
-            (raft_log.last_index() - 1, 3, false),
-            (raft_log.last_index(), 3, true),
-            (raft_log.last_index() + 1, 3, true),
+            (raft_log.last_index() - 1, 3, Ordering::Less),
+            (raft_log.last_index(), 3, Ordering::Equal),
+            (raft_log.last_index() + 1, 3, Ordering::Greater),
         ];
         for (i, &(last_index, term, up_to_date)) in tests.iter().enumerate() {
             let g_up_to_date = raft_log.is_up_to_date(last_index, term);
             if g_up_to_date != up_to_date {
-                panic!("#{}: uptodate = {}, want {}", i, g_up_to_date, up_to_date);
+                panic!(
+                    "#{}: uptodate = {:?}, want {:?}",
+                    i, g_up_to_date, up_to_date
+                );
             }
         }
     }