actix-raft/async-raft/src/core/vote.rs

use tokio::time::Instant;
use tokio::sync::mpsc;
use tracing_futures::Instrument;

use crate::{AppData, AppDataResponse, AppError, NodeId, RaftNetwork, RaftStorage};
use crate::error::RaftResult;
use crate::core::{CandidateState, RaftCore, State, UpdateCurrentLeader};
use crate::raft::{VoteRequest, VoteResponse};

impl<D: AppData, R: AppDataResponse, E: AppError, N: RaftNetwork<D, E>, S: RaftStorage<D, R, E>> RaftCore<D, R, E, N, S> {
    /// An RPC invoked by candidates to gather votes (§5.2).
    ///
    /// See `receiver implementation: RequestVote RPC` in raft-essentials.md in this repo.
    #[tracing::instrument(level="trace", skip(self, msg))]
    pub(super) async fn handle_vote_request(&mut self, msg: VoteRequest) -> RaftResult<VoteResponse, E> {
        // If candidate's current term is less than this nodes current term, reject.
        if &msg.term < &self.current_term {
            tracing::trace!({candidate=msg.candidate_id, self.current_term, rpc_term=msg.term}, "RequestVote RPC term is less than current term");
            return Ok(VoteResponse{term: self.current_term, vote_granted: false});
        }

        // Do not respond to the request if we've received a heartbeat within the election timeout minimum.
        if let Some(inst) = &self.last_heartbeat {
            let now = Instant::now();
            let delta = now.duration_since(*inst);
            if self.config.election_timeout_min >= (delta.as_millis() as u64) {
                tracing::trace!({candidate=msg.candidate_id}, "rejecting vote request received within election timeout minimum");
                return Ok(VoteResponse{term: self.current_term, vote_granted: false});
            }
        }

        // Per spec, if we observe a term greater than our own, we must update
        // term & immediately become follower, we still need to do vote checking after this.
        if &msg.term > &self.current_term {
            self.update_current_term(msg.term, None);
            self.update_next_election_timeout();
            self.set_target_state(State::Follower);
            self.save_hard_state().await?;
        }

        // Check if candidate's log is at least as up-to-date as this node's.
        // If candidate's log is not at least as up-to-date as this node, then reject.
        let client_is_uptodate = (&msg.last_log_term >= &self.last_log_term) && (&msg.last_log_index >= &self.last_log_index);
        if !client_is_uptodate {
            tracing::trace!({candidate=msg.candidate_id}, "rejecting vote request as candidate's log is not up-to-date");
            return Ok(VoteResponse{term: self.current_term, vote_granted: false});
        }

        // Candidate's log is up-to-date so handle voting conditions.
        match &self.voted_for {
            // This node has already voted for the candidate.
            Some(candidate_id) if candidate_id == &msg.candidate_id => {
                Ok(VoteResponse{term: self.current_term, vote_granted: true})
            }
            // This node has already voted for a different candidate.
            Some(_) => Ok(VoteResponse{term: self.current_term, vote_granted: false}),
            // This node has not yet voted for the current term, so vote for the candidate.
            None => {
                self.voted_for = Some(msg.candidate_id);
                self.set_target_state(State::Follower);
                self.update_next_election_timeout();
                self.save_hard_state().await?;
                tracing::trace!({candidate=msg.candidate_id, msg.term}, "voted for candidate");
                Ok(VoteResponse{term: self.current_term, vote_granted: true})
            },
        }
    }
}

impl<'a, D: AppData, R: AppDataResponse, E: AppError, N: RaftNetwork<D, E>, S: RaftStorage<D, R, E>> CandidateState<'a, D, R, E, N, S> {
    /// Handle response from a vote request sent to a peer.
    #[tracing::instrument(level="trace", skip(self, res, target))]
    pub(super) async fn handle_vote_response(&mut self, res: VoteResponse, target: NodeId) -> RaftResult<(), E> {
        // If peer's term is greater than current term, revert to follower state.
        if res.term > self.core.current_term {
            self.core.update_current_term(res.term, None);
            self.core.update_current_leader(UpdateCurrentLeader::Unknown);
            self.core.set_target_state(State::Follower);
            self.core.save_hard_state().await?;
            tracing::trace!("reverting to follower state due to greater term observed in RequestVote RPC response");
            return Ok(());
        }

        // If peer granted vote, then update campaign state.
        if res.vote_granted {
            // Handle vote responses from the C0 config group.
            if self.core.membership.members.contains(&target) {
                self.votes_granted_old += 1;
            }
            // Handle vote responses from members of C1 config group.
            if self.core.membership.members_after_consensus.as_ref().map(|members| members.contains(&target)).unwrap_or(false) {
                self.votes_granted_new += 1;
            }
            // If we've received enough votes from both config groups, then transition to leader state`.
            if &self.votes_granted_old >= &self.votes_needed_old && &self.votes_granted_new >= &self.votes_needed_new {
                tracing::trace!("transitioning to leader state as minimum number of votes have been received");
                self.core.set_target_state(State::Leader);
                return Ok(());
            }
        }

        // Otherwise, we just return and let the candidate loop wait for more votes to come in.
        Ok(())
    }

    /// Build a future of vote requests sent to all peers.
    #[tracing::instrument(level="trace", skip(self))]
    pub(super) fn spawn_parallel_vote_requests(&self) -> mpsc::Receiver<(VoteResponse, NodeId)> {
        let all_members = self.core.membership.all_nodes();
        let (tx, rx) = mpsc::channel(all_members.len());
        for member in all_members.into_iter().filter(|member| member != &self.core.id) {
            let rpc = VoteRequest::new(self.core.current_term, self.core.id, self.core.last_log_index, self.core.last_log_term);
            let (network, mut tx_inner) = (self.core.network.clone(), tx.clone());
            let _ = tokio::spawn(async move {
                match network.vote(member, rpc).await {
                    Ok(res) => {
                        let _ = tx_inner.send((res, member)).await;
                    }
                    Err(err) => tracing::error!({error=%err, peer=member}, "error while requesting vote from peer"),
                }
            }.instrument(tracing::trace_span!("requesting vote from peer", target=member)));
        }
        rx
    }
}