API to disallow a node to candidate

api.go

@@ -217,6 +217,11 @@ type Raft struct {
 	// preVoteDisabled control if the pre-vote feature is activated,
 	// prevote feature is disabled if set to true.
 	preVoteDisabled bool
+
+	// allowCandidating is used in situation where the node must no become a
+	// leader in case of election.
+	allowCandidating     bool
+	allowCandidatingLock sync.Mutex
 }
 
 // BootstrapCluster initializes a server's storage with the given cluster
@@ -566,6 +571,7 @@ func NewRaft(conf *Config, fsm FSM, logs LogStore, stable StableStore, snaps Sna
 		followerNotifyCh:      make(chan struct{}, 1),
 		mainThreadSaturation:  newSaturationMetric([]string{"raft", "thread", "main", "saturation"}, 1*time.Second),
 		preVoteDisabled:       conf.PreVoteDisabled || !transportSupportPreVote,
+		allowCandidating:      true,
 	}
 	if !transportSupportPreVote && !conf.PreVoteDisabled {
 		r.logger.Warn("pre-vote is disabled because it is not supported by the Transport")
@@ -1270,3 +1276,15 @@ func (r *Raft) LeadershipTransferToServer(id ServerID, address ServerAddress) Fu
 
 	return r.initiateLeadershipTransfer(&id, &address)
 }
+
+func (r *Raft) SetAllowCandidating(v bool) {
+	r.allowCandidatingLock.Lock()
+	defer r.allowCandidatingLock.Unlock()
+	r.allowCandidating = v
+}
+
+func (r *Raft) GetAllowCandidating() bool {
+	r.allowCandidatingLock.Lock()
+	defer r.allowCandidatingLock.Unlock()
+	return r.allowCandidating
+}

raft.go

@@ -239,12 +239,12 @@ func (r *Raft) runFollower() {
 				}
 			} else {
 				metrics.IncrCounter([]string{"raft", "transition", "heartbeat_timeout"}, 1)
-				if hasVote(r.configurations.latest, r.localID) {
+				if hasVote(r.configurations.latest, r.localID) && r.GetAllowCandidating() {
 					r.logger.Warn("heartbeat timeout reached, starting election", "last-leader-addr", lastLeaderAddr, "last-leader-id", lastLeaderID)
 					r.setState(Candidate)
 					return
 				} else if !didWarn {
-					r.logger.Warn("heartbeat timeout reached, not part of a stable configuration or a non-voter, not triggering a leader election")
+					r.logger.Warn("heartbeat timeout reached, not part of a stable configuration or a non-voter or not allowed to candidate, not triggering a leader election")
 					didWarn = true
 				}
 			}
@@ -2208,10 +2208,14 @@ func (r *Raft) initiateLeadershipTransfer(id *ServerID, address *ServerAddress)
 
 // timeoutNow is what happens when a server receives a TimeoutNowRequest.
 func (r *Raft) timeoutNow(rpc RPC, req *TimeoutNowRequest) {
-	r.setLeader("", "")
-	r.setState(Candidate)
-	r.candidateFromLeadershipTransfer.Store(true)
-	rpc.Respond(&TimeoutNowResponse{}, nil)
+	if r.GetAllowCandidating() {
+		r.setLeader("", "")
+		r.setState(Candidate)
+		r.candidateFromLeadershipTransfer.Store(true)
+		rpc.Respond(&TimeoutNowResponse{}, nil)
+	} else {
+		rpc.Respond(nil, fmt.Errorf("not allowed to candidate"))
+	}
 }
 
 // setLatestConfiguration stores the latest configuration and updates a copy of it.