Leader Election基本设计

• 按照rank表示优先级解决冲突问题，为每个monitor预先分配了一个rank
• 只会接受优先级(rank)比自己高、epoch比上次已接受的epoch大的选举请求
• 当选的leader，不一定有最新的数据。所以在phase 1中，会根据已经commit的数据，进行leader和peon之间的同步
• 用奇数的epoch表示选举状态，偶数表示稳定状态
• 一旦选举成功，会形成一个quorum，在该leader当选期间，所有提议，必须quorum中全部成员同意。

Leader Election主要过程和函数

• Elector::init()
  初始化处理，从kv读取之前持久化的信息
• Elector::shutdown()
  退出处理
• Elector::start()
  推选自己(自荐)

• Elector::defer()

  接受别人选举，延迟推选自己

• Elector::handle_propose()

  处理别人的自荐消息

• Elector::handle_ack()
  处理别人的ack
• Elector::victory()
  宣布自己当选
• Elector::handle_victory()
  处理别人当选消息
• Elector::expire()
  选举超时处理
• Elector::reset_timer()
  设置选举timer
• Elector::cancel_timer()
  取消timer
• Elector::dispatch()
  选举消息的分发函数

代码

初始化

/*Ceph的monitor使用leveldb作为持久化存储，下面的mon->store
就是leveldb操作的封装，由于很多数据共用同一leveldb，所以对
key的空间做了分级，Monitor::MONITOR_NAME可以认为是第一级key*/
void Elector::init()
{//选举的epoch，递增分配。每次修改都做了持久化，这是从kv db读取。epoch = mon->store->get(Monitor::MONITOR_NAME, "election_epoch"); if (!epoch)//首次使用 epoch = 1; }

bump_epoch

/*将自己的epoch修改为参数 e，需要持久化到 kv 存储。*/
void Elector::bump_epoch(epoch_t e)
{dout(10) << "bump_epoch " << epoch << " to " << e << dendl; assert(epoch <= e); epoch = e; //使用一个事务，写kv存储 MonitorDBStore::TransactionRef t(new MonitorDBStore::Transaction); t->put(Monitor::MONITOR_NAME, "election_epoch", epoch); mon->store->apply_transaction(t);//持久化收到的epoch。 //join election会使 monitor 进入 STATE_ELECTING 状态 mon->join_election(); // clear up some state electing_me = false; //因为别人的epoch比自己大，放弃选自己 acked_me.clear(); //这个acked_m，是选自己才有意义，在此清空。 classic_mons.clear(); }

推选自己为leader的函数(自荐)

//在启动后或者leader超时等场合，会发起自荐
void Elector::start()//推选自己
{if (!participating) {return;}//清空，表示还没人响应自己acked_me.clear();classic_mons.clear();//从store获取持久化的epoch. init(); if (epoch % 2 == 0) //epoch是偶数表明是稳定态 bump_epoch(epoch+1); //odd == election cycle 选举都是用的奇数epoch start_stamp = ceph_clock_now(g_ceph_context); electing_me = true; //设置为true，前面的bump_epoch可能设置为false了 //填写map的key是自己的rank，表明自己先同意了自己 acked_me[mon->rank] = CEPH_FEATURES_ALL; leader_acked = -1;//无效值，表明我没有ack别人 //给monmap中每个成员发送消息。可以认为monmap成员是预先配置的，且配置了rank for (unsigned i=0; i<mon->monmap->size(); ++i) { if ((int)i == mon->rank) continue; //消息中带有自己的epoch Message *m = new MMonElection(MMonElection::OP_PROPOSE, epoch, mon->monmap); mon->messenger->send_message(m, mon->monmap->get_inst(i)); } reset_timer();//设置选举用的timer，参见expire()函数 }

响应别人自荐

//defer就是暂时放弃推选自己
void Elector::defer(int who)
{if (electing_me) {//放弃选自己acked_me.clear();classic_mons.clear();electing_me = false;}//表明我支持了"who"当leader. leader_acked不需要持久化，因为任何一个monitor在 //reboot后都会重新发起election。 leader_acked = who; ack_stamp = ceph_clock_now(g_ceph_context); //返回OP_ACK消息,即赞成对方当leader MMonElection *m = new MMonElection(MMonElection::OP_ACK, epoch, mon->monmap); m->sharing_bl = mon->get_supported_commands_bl(); mon->messenger->send_message(m, mon->monmap->get_inst(who)); // set a timer 对方在一定时间内，应该宣布自己当选才对 reset_timer(1.0); // give the leader some extra time to declare victory }

timer相关工具函数

设置timer的工具函数

void Elector::reset_timer(double plus)
{// set the timercancel_timer();expire_event = new C_ElectionExpire(this); mon->timer.add_event_after(g_conf->mon_lease + plus, expire_event); }

取消timer的工具函数

void Elector::cancel_timer()
{if (expire_event) {mon->timer.cancel_event(expire_event);expire_event = 0;}
}

超时处理

void Elector::expire()
{// 如果是自荐，只要超过半数同意，就认为成功if (electing_me &&acked_me.size() > (unsigned)(mon->monmap->size() / 2)) { //注意，expire判断的是 > monmap->size()/2，而handle_ack里面是等待全部ack。 // i win victory(); } else {//没有推选自己 // whoever i deferred to didn't declare victory quickly enough. if (mon->has_ever_joined) start();//之前我加入过quorum，直接重新发动选举。因为monmap中会包含我。 else mon->bootstrap();//否则，走bootstrap } }

成功当选leader处理

void Elector::victory()
{leader_acked = -1;electing_me = false;uint64_t features = CEPH_FEATURES_ALL;set<int> quorum;for (map<int, uint64_t>::iterator p = acked_me.begin(); p != acked_me.end(); ++p) {//如果是从expire()调用的victory()，则不是monmap的记录的所有node， //但是肯定是超过半数。 quorum.insert(p->first);//ack过我的，全部进入quorum。 features &= p->second; } // decide what command set we're supporting bool use_classic_commands = !classic_mons.empty(); // keep a copy to share with the monitor; we clear classic_mons in bump_epoch set<int> copy_classic_mons = classic_mons; cancel_timer(); assert(epoch % 2 == 1); // 选举期间用的奇数epoch bump_epoch(epoch+1); // 选举完成，epoch变成偶数 // decide my supported commands for peons to advertise const bufferlist *cmds_bl = NULL; const MonCommand *cmds; int cmdsize; if (use_classic_commands) { mon->get_classic_monitor_commands(&cmds, &cmdsize); cmds_bl = &mon->get_classic_commands_bl(); } else { mon->get_locally_supported_monitor_commands(&cmds, &cmdsize); cmds_bl = &mon->get_supported_commands_bl(); } //通知大家自己当选 for (set<int>::iterator p = quorum.begin(); p != quorum.end(); ++p) { if (*p == mon->rank) continue; MMonElection *m = new MMonElection(MMonElection::OP_VICTORY, epoch, mon->monmap); m->quorum = quorum; m->quorum_features = features; m->sharing_bl = *cmds_bl; mon->messenger->send_message(m, mon->monmap->get_inst(*p)); } //调用monitor的函数，它会发起paxos的propose mon->win_election(epoch, quorum, features, cmds, cmdsize, &copy_classic_mons); }

处理别人的自荐消息

根据情况决定是否支持，或者决定该推荐自己

void Elector::handle_propose(MonOpRequestRef op)
{MMonElection *m = static_cast<MMonElection*>(op->get_req());dout(5) << "handle_propose from " << m->get_source() << dendl;int from = m->get_source().num();//获取对方的rankassert(m->epoch % 2 == 1); // election uint64_t required_features = mon->get_required_features(); if ((required_features ^ m->get_connection()->get_features()) & required_features) {//要求对方的feature，覆盖required_features nak_old_peer(op); return; } else if (m->epoch > epoch) {//对方epoch比我大，放弃选自己，追随它。 bump_epoch(m->epoch); } else if (m->epoch < epoch) {//对方epoch太小，否决 // got an "old" propose, //发送消息的peer可能是刚刚加进来的，以前不在quorum里面。所以epoch比较小 if (epoch % 2 == 0 && // in a non-election cycle mon->quorum.count(from) == 0) { // from someone outside the quorum // a mon just started up, call a new election so they can rejoin! //为什么我要start_election? 因为其epoch太旧，不可能当选。 mon->start_election(); } else {//认为收到了旧消息，忽略 dout(5) << " ignoring old propose" << dendl; return; } } //我比发送方的rank高。如果我没有响应过其他比我rank高的，就推选自己 if (mon->rank < from) { // i would win over them. if (leader_acked >= 0) { // we already acked someone assert(leader_acked < from); // and they still win, of course 否则不可能ack它 } else {//如果没有acked过，比我优先级低的在推选自己，那么我应该选自己才对。 // wait, i should win! if (!electing_me) { mon->start_election(); } } } else {//发送方rank比我高 // they would win over me //之前我没有赞成谁，或者之前那个优先级没现在这个高，则赞成现在这个 if (leader_acked < 0 || // haven't acked anyone yet, or leader_acked > from || // they would win over who you did ack, or leader_acked == from) { // this is the guy we're already deferring to defer(from);//这个函数内部会发送 OP_ACK,支持发送方 } else {//我之前响应过别人，坚持之前的选择 // ignore them! dout(5) << "no, we already acked " << leader_acked << dendl; } } }

收到 ack之后的处理

//收到别人响应后的处理
void Elector::handle_ack(MonOpRequestRef op)
{op->mark_event("elector:handle_ack");MMonElection *m = static_cast<MMonElection*>(op->get_req());dout(5) << "handle_ack from " << m->get_source() << dendl;int from = m->get_source().num(); assert(m->epoch % 2 == 1); // election状态，必须是奇数 //下面dout解释了出现这个现象的原因，即我在自己重启，out了。 if (m->epoch > epoch) { dout(5) << "woah, that's a newer epoch, i must have rebooted. bumping and re-starting!" << dendl; bump_epoch(m->epoch);//必须用新的epoch才能引起有效选举，否则被忽略了 start(); return; } assert(m->epoch == epoch); uint64_t required_features = mon->get_required_features(); if ((required_features ^ m->get_connection()->get_features()) & required_features) { dout(5) << " ignoring ack from mon" << from << " without required features" << dendl; return; } //如果正在推选自己 if (electing_me) { // thanks //acked_me是个map acked_me[from] = m->get_connection()->get_features(); if (!m->sharing_bl.length()) classic_mons.insert(from); dout(5) << " so far i have " << acked_me << dendl; //所有人都赞成我 if (acked_me.size() == mon->monmap->size()) { // if yes, shortcut to election finish victory(); } } else {//以前我曾经推选过自己，但是现在我已经投别人了 // ignore, i'm deferring already. assert(leader_acked >= 0); } }

在别人当选后的处理

//收到别人宣告选举胜利的消息后的处理
void Elector::handle_victory(MonOpRequestRef op)
{op->mark_event("elector:handle_victory");MMonElection *m = static_cast<MMonElection*>(op->get_req());int from = m->get_source().num();assert(from < mon->rank);assert(m->epoch % 2 == 0); leader_acked = -1; //之前我一定选举了它，所以epoch必须match，否则有问题。 // i should have seen this election if i'm getting the victory. if (m->epoch != epoch + 1) { //在victory()中，已经加1，所以是偶数，且比peon看到的大1 dout(5) << "woah, that's a funny epoch, i must have rebooted. bumping and re-starting!" << dendl; bump_epoch(m->epoch); start(); return; } bump_epoch(m->epoch);//我也变成偶数epoch // they win mon->lose_election(epoch, m->quorum, from, m->quorum_features); // cancel my timer cancel_timer();//选举timer没用了 // stash leader's commands if (m->sharing_bl.length()) { MonCommand *new_cmds; int cmdsize; bufferlist::iterator bi = m->sharing_bl.begin(); MonCommand::decode_array(&new_cmds, &cmdsize, bi); mon->set_leader_supported_commands(new_cmds, cmdsize); } else { // they are a legacy monitor; use known legacy command set const MonCommand *new_cmds; int cmdsize; mon->get_classic_monitor_commands(&new_cmds, &cmdsize); mon->set_leader_supported_commands(new_cmds, cmdsize); } }

消息分发函数

消息的dispatch，里面有些关于monmap的处理。

/*monmap，是集群当前配置的所有monitor的集合。*monmap在bootstrp过程中会在montior间同步，这里没仔细讨论。*monmap中的各个monitor，只有参与选举投票的，才会进入quorum。*/
void Elector::dispatch(MonOpRequestRef op)
{op->mark_event("elector:dispatch");assert(op->is_type_election());switch (op->get_req()->get_type()) {case MSG_MON_ELECTION://elector只收election这个类别的消息 { if (!participating) { return; } if (op->get_req()->get_source().num() >= mon->monmap->size()) { dout(5) << " ignoring bogus election message with bad mon rank " << op->get_req()->get_source() << dendl; return; } MMonElection *em = static_cast<MMonElection*>(op->get_req()); // assume an old message encoding would have matched if (em->fsid != mon->monmap->fsid) { dout(0) << " ignoring election msg fsid " << em->fsid << " != " << mon->monmap->fsid << dendl; return; } //选举是根据monmap干活的。monmap在bootstrap阶段大家已经同步了。 if (!mon->monmap->contains(em->get_source_addr())) { dout(1) << "discarding election message: " << em->get_source_addr() << " not in my monmap " << *mon->monmap << dendl; return; } MonMap *peermap = new MonMap; peermap->decode(em->monmap_bl); //比较二者的monmap的epoch，即二者看到的monitor配置应该相同。 if (peermap->epoch > mon->monmap->epoch) { dout(0) << em->get_source_inst() << " has newer monmap epoch " << peermap->epoch << " > my epoch " << mon->monmap->epoch << ", taking it" << dendl; mon->monmap->decode(em->monmap_bl); MonitorDBStore::TransactionRef t(new MonitorDBStore::Transaction); //更新了自己的monmap，并且写盘。实际上信任了对方的monmap。 t->put("monmap", mon->monmap->epoch, em->monmap_bl); t->put("monmap", "last_committed", mon->monmap->epoch); mon->store->apply_transaction(t); //mon->monmon()->paxos->stash_latest(mon->monmap->epoch, em->monmap_bl); cancel_timer(); mon->bootstrap();//重新做一次自举。自举后会重新选举 delete peermap; return; } if (peermap->epoch < mon->monmap->epoch) { //这种情况下，会用我的map去同步对方的。 dout(0) << em->get_source_inst() << " has older monmap epoch " << peermap->epoch << " < my epoch " << mon->monmap->epoch << dendl; } delete peermap; switch (em->op) { case MMonElection::OP_PROPOSE: handle_propose(op); return; } if (em->epoch < epoch) www.rcsx.org {//为什么又比较这个epoch? dout(5) << "old epoch, dropping" << dendl; break; } switch (em->op) { case MMonElection::OP_ACK: handle_ack(op); return; case MMonElection::OP_VICTORY: handle_victory(op); return; case MMonElection::OP_NAK: handle_nak(op); return; default: assert(0); } } break; default: assert(0); } }

//admin command处理，触发选举
void Elector::start_participating()
{if (!participating) {participating = true; call_election(); } }