Leader Shard的選舉原理主要是依靠LeaderElector.java類來實現的。

函數checkIfIamLeader開始真正的leader選舉，根據和Zookeeper上創建的znode的nodeName對比，判斷自己是否是leader。

如果是leader，好了，更新的活以后該你去干了，執行注冊leader的一系列更新操作。

如果不是leader，而是replica的話，則在leader的znode上注冊watcher，關注leader znode的狀態。

一旦發現leader znode消失，則會觸發下一次的leader選舉過程。

/*** Check if the candidate with the given n_* sequence number is the leader.* If it is, set the leaderId on the leader zk node. If it is not, start* watching the candidate that is in line before this one - if it goes down, check* if this candidate is the leader again.** @param replacement has someone else been the leader already?*/private void checkIfIamLeader(final ElectionContext context, boolean replacement) throws KeeperException,InterruptedException, IOException {context.checkIfIamLeaderFired();// get all other numbers...final String holdElectionPath = context.electionPath + ELECTION_NODE;List<String> seqs = zkClient.getChildren(holdElectionPath, null, true);sortSeqs(seqs);String leaderSeqNodeName = context.leaderSeqPath.substring(context.leaderSeqPath.lastIndexOf('/') + 1);if (!seqs.contains(leaderSeqNodeName)) {log.warn("Our node is no longer in line to be leader");return;}// If any double-registrations exist for me, remove all but this latest one!// TODO: can we even get into this state?String prefix = zkClient.getSolrZooKeeper().getSessionId() + "-" + context.id + "-";Iterator<String> it = seqs.iterator();while (it.hasNext()) {String elec = it.next();if (!elec.equals(leaderSeqNodeName) && elec.startsWith(prefix)) {try {String toDelete = holdElectionPath + "/" + elec;log.warn("Deleting duplicate registration: {}", toDelete);zkClient.delete(toDelete, -1, true);} catch (KeeperException.NoNodeException e) {// ignore}it.remove();}}if (leaderSeqNodeName.equals(seqs.get(0))) {// I am the leadertry {runIamLeaderProcess(context, replacement);} catch (KeeperException.NodeExistsException e) {log.error("node exists",e);retryElection(context, false);return;}} else {// I am not the leader - watch the node below meString toWatch = seqs.get(0);for (String node : seqs) {if (leaderSeqNodeName.equals(node)) {break;}toWatch = node;}try {String watchedNode = holdElectionPath + "/" + toWatch;zkClient.getData(watchedNode, watcher = new ElectionWatcher(context.leaderSeqPath, watchedNode, getSeq(context.leaderSeqPath), context), null, true);log.debug("Watching path {} to know if I could be the leader", watchedNode);} catch (KeeperException.SessionExpiredException e) {throw e;} catch (KeeperException.NoNodeException e) {// the previous node disappeared, check if we are the leader againcheckIfIamLeader(context, true);} catch (KeeperException e) {// we couldn't set our watch for some other reason, retrylog.warn("Failed setting watch", e);checkIfIamLeader(context, true);}}}

Leader重新選舉

正常運行的SolrCloud已產生一個leader(Znode編號最小，比如XXX_node1_0000001)，后續的Replica后在leader節點上注冊Watcher。當Leader下線時候，即短連接斷開，那么Zookeeper上的Znode(比如XXX_node1_0000001)就會被刪除。

此時，所有Replica在Leader節點上的watcher就會監控到這一變化，所有的Replica就會進行leader選舉，選舉的原則依然是判斷自己是不是目前注冊在/collections/collectionTest/leader_select/shard1/election下的Znode編號最小的那位，是的話就是Leader，否則就是Replica。

如果判斷自己是Replica，就會繼續在leader的Znode上(這個時候的leader是XXX_node1_0000002)注冊watcher，等待leader下線再次觸發選舉leader。

如果這個時候原先下線的leader上線了會怎么樣，它就會被當做新的一個Solr節點注冊到Zookeeper上，并獲取一個比現有Znode更大的編號，在Leader Znode節點上注冊watcher，等待它的選舉機會。

這篇文章講得很好，但它base的版本是比較老的Solr 4.x了。

Reference
http://quentinxxz.iteye.com/blog/2149891
https://www.cnblogs.com/rcfeng/p/4082568.html
https://www.cnblogs.com/saratearing/p/5690476.html
https://blog.csdn.net/u011026968/article/details/50336709
https://blog.csdn.net/iteye_16982/article/details/82574099

Zookeeper簡介
https://www.cnblogs.com/xinfang520/p/7717684.html

SolrCloud Recovery原理及無法選舉分片Leader
https://www.sohu.com/a/130752460_505885

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