raft: lower split vote rate

raft/raft.go

@@ -176,9 +176,14 @@ type raft struct {
 
 	heartbeatTimeout int
 	electionTimeout  int
-	rand             *rand.Rand
-	tick             func()
-	step             stepFunc
+	// randomizedElectionTimeout is a random number between
+	// [electiontimeout, 2 * electiontimeout - 1]. It gets reset
+	// when raft changes its state to follower or candidate.
+	randomizedElectionTimeout int
+
+	rand *rand.Rand
+	tick func()
+	step stepFunc
 
 	logger Logger
 }
@@ -392,6 +397,7 @@ func (r *raft) reset(term uint64) {
 
 	r.electionElapsed = 0
 	r.heartbeatElapsed = 0
+	r.resetRandomizedElectionTimeout()
 
 	r.votes = make(map[uint64]bool)
 	for id := range r.prs {
@@ -422,7 +428,7 @@ func (r *raft) tickElection() {
 		return
 	}
 	r.electionElapsed++
-	if r.isElectionTimeout() {
+	if r.pastElectionTimeout() {
 		r.electionElapsed = 0
 		r.Step(pb.Message{From: r.id, Type: pb.MsgHup})
 	}
@@ -863,15 +869,15 @@ func (r *raft) loadState(state pb.HardState) {
 	r.Vote = state.Vote
 }
 
-// isElectionTimeout returns true if r.electionElapsed is greater than the
-// randomized election timeout in (electiontimeout, 2 * electiontimeout - 1).
+// pastElectionTimeout returns true if r.electionElapsed is greater than the
+// randomized election timeout in [electiontimeout, 2 * electiontimeout - 1].
 // Otherwise, it returns false.
-func (r *raft) isElectionTimeout() bool {
-	d := r.electionElapsed - r.electionTimeout
-	if d < 0 {
-		return false
-	}
-	return d > r.rand.Int()%r.electionTimeout
+func (r *raft) pastElectionTimeout() bool {
+	return r.electionElapsed >= r.randomizedElectionTimeout
+}
+
+func (r *raft) resetRandomizedElectionTimeout() {
+	r.randomizedElectionTimeout = r.electionTimeout + r.rand.Int()%r.electionTimeout
 }
 
 // checkQuorumActive returns true if the quorum is active from

raft/raft_paper_test.go

@@ -381,8 +381,8 @@ func testNonleadersElectionTimeoutNonconflict(t *testing.T, state StateType) {
 		}
 	}
 
-	if g := float64(conflicts) / 1000; g > 0.4 {
-		t.Errorf("probability of conflicts = %v, want <= 0.4", g)
+	if g := float64(conflicts) / 1000; g > 0.3 {
+		t.Errorf("probability of conflicts = %v, want <= 0.3", g)
 	}
 }
 

raft/raft_test.go

@@ -754,9 +754,10 @@ func TestIsElectionTimeout(t *testing.T) {
 		round        bool
 	}{
 		{5, 0, false},
-		{13, 0.3, true},
-		{15, 0.5, true},
-		{18, 0.8, true},
+		{10, 0.1, true},
+		{13, 0.4, true},
+		{15, 0.6, true},
+		{18, 0.9, true},
 		{20, 1, false},
 	}
 
@@ -765,7 +766,8 @@ func TestIsElectionTimeout(t *testing.T) {
 		sm.electionElapsed = tt.elapse
 		c := 0
 		for j := 0; j < 10000; j++ {
-			if sm.isElectionTimeout() {
+			sm.resetRandomizedElectionTimeout()
+			if sm.pastElectionTimeout() {
 				c++
 			}
 		}