# 触发选举
REPL_HB  Heartbeat to xx.xx.xx.xx:xx failed after 2 retries, response status: HostUnreachable: Error connecting to xx.xx.xx.xx:xx :: caused by :: Connection refused
REPL     Starting an election, since we've seen no PRIMARY in the past 10000ms
# 预选举
REPL     conducting a dry run election to see if we could be elected. current term: 3
REPL     VoteRequester(term 3 dry run) failed to receive response from xx.xx.xx.xx:xx: HostUnreachable: Error connecting to xx.xx.xx.xx:xx :: caused by :: Connection refused
REPL     VoteRequester(term 3 dry run) received a yes vote from xx.xx.xx.xx:xx; response message: { term: 3, voteGranted: true, reason: "", ok: 1.0, operationTime: Timestamp(1706176453, 1), $clusterTime: { clusterTime: Timestamp(1706176453, 1)
REPL     VoteRequester(term 4) failed to receive response from xx.xx.xx.xx:xx: HostUnreachable: Error connecting to xx.xx.xx.xx:xx :: causedby :: Connection refused
REPL     dry election run succeeded, running for election in term 4
# 选举
REPL     VoteRequester(term 4) received a yes vote from xx.xx.xx.xx:xx; response message: { term: 4, voteGranted: true, reason: "", ok: 1.0, operationTime: Timestamp(1706176453, 1), 
REPL     election succeeded, assuming primary role in term 4
# 状态变更
REPL     transition to PRIMARY from SECONDARY
REPL     Resetting sync source to empty, which was :xx
# 追赶阶段
REPL     Entering primary catch-up mode.
REPL     Caught up to the latest optime known via heartbeats after becoming primary. Target optime: { ts: Timestamp(1706176453, 1), t: 3 }. My Last Applied: { ts: Timestamp(1706176453, 1), t: 3 }
REPL     Exited primary catch-up mode.
REPL     Stopping replication producer
# 允许写入
REPL     transition to primary complete; database writes are now permitted

switch (reason) {
    case StartElectionReasonEnum::kElectionTimeout:
        # 无主，副本集内探活10s内无primary
        LOG_FOR_ELECTION(0) << 
          "Starting an election, since we've seen no PRIMARY in the past "
                            << _rsConfig.getElectionTimeoutPeriod();
        break;
    case StartElectionReasonEnum::kPriorityTakeover:
        # 优先级抢占，有节点设置更高优先级
        LOG_FOR_ELECTION(0) << 
          "Starting an election for a priority takeover";
        break;
    case StartElectionReasonEnum::kStepUpRequest:
    case StartElectionReasonEnum::kStepUpRequestSkipDryRun:
        # 主动提升为primary
        # 跳过预选举
        LOG_FOR_ELECTION(0) << 
          "Starting an election due to step up request";
        break;
    case StartElectionReasonEnum::kCatchupTakeover:
        # 追赶抢占，primary处于catchup阶段时发生了takeover
        LOG_FOR_ELECTION(0) << 
          "Starting an election for a catchup takeover";
        break;
    case StartElectionReasonEnum::kSingleNodePromptElection:
        # 单节点选举
        LOG_FOR_ELECTION(0) << 
          "Starting an election due to single node replica set prompt election";
        break;
    default:
        MONGO_UNREACHABLE;
}

_startElectSelfV1_inlock(reason)

// 获取当前term
long long term = _topCoord->getTerm();
int primaryIndex = -1;
// 跳过预选举
if (reason == TopologyCoordinator::StartElectionReason::kStepUpRequestSkipDryRun) {
    // term自增
    long long newTerm = term + 1;
    log() << "skipping dry run and running for election in term " << newTerm;
    // 真正选举
    _startRealElection_inlock(newTerm, reason);
    lossGuard.dismiss();
    return;
}

// 选举准备
_voteRequester.reset(new VoteRequester);

// Only set primaryIndex if the primary's vote is required during the dry run.
if (reason == TopologyCoordinator::StartElectionReason::kCatchupTakeover) {
    primaryIndex = _topCoord->getCurrentPrimaryIndex();
}
// 预选举
_processDryRunResult(term, reason);
lossGuard.dismiss();

const VoteRequester::Result endResult = _voteRequester->getResult();
// 预选举失败
if (endResult == VoteRequester::Result::kInsufficientVotes) {
    log() << "not running for primary, we received insufficient votes";
    return;
} else if (endResult == VoteRequester::Result::kStaleTerm) {
    log() << "not running for primary, we have been superseded already";
    return;
} else if (endResult == VoteRequester::Result::kPrimaryRespondedNo) {
    log() << "not running for primary, the current primary responded no in the dry run";
    return;
} else if (endResult != VoteRequester::Result::kSuccessfullyElected) {
    log() << "not running for primary, we received an unexpected problem";
    return;
}

// 预选举成功
long long newTerm = originalTerm + 1;
log() << "dry election run succeeded, running for election in term " << newTerm;
// 选举
_startRealElection_inlock(newTerm, reason);

// 给自己投票
_topCoord->voteForMyselfV1()
..
  // 保存副本集投票情况
  _writeLastVoteForMyElection()
....
    // 向其他节点发送投票请求
    _startVoteRequester_inlock()
......
      // 收到投票结果并处理
      _onVoteRequestComplete()
........
        // 副本集状态机更改
        _postWonElectionUpdateMemberState_inlock()

// 修改role等配置
_topCoord->processWinElection(_electionId, ts);
const PostMemberStateUpdateAction nextAction =
    _updateMemberStateFromTopologyCoordinator_inlock(nullptr);
// 设置同步端为空
_onFollowerModeStateChange();
// 通知其他节点选举成功
_restartHeartbeats_inlock();
// 进入追赶阶段
_catchupState->start_inlock();

// 通过心跳获得最新oplog来追赶
_handleHeartbeatResponse()
// 判断自身和最新oplog的时间
CatchupState::signalHeartbeatUpdate_inlock()
// 结束追赶阶段
CatchupState::abort_inlock()