【Solr原理】Leader Shard选举

曾经终败给现在 2022-05-04 03:30 203阅读 0赞

> Leader Shard的选举原理主要是依靠[LeaderElector.java][]类来实现的。

函数checkIfIamLeader开始真正的leader选举，根据和Zookeeper上创建的znode的nodeName对比，判断自己是否是leader。

1.  如果是leader，好了，更新的活以后该你去干了，执行注册leader的一系列更新操作。
2.  如果不是leader，而是replica的话，则在leader的znode上注册watcher，关注leader znode的状态。
3.  一旦发现leader znode消失，则会触发下一次的leader选举过程。

/**
       * 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 leader
          try {
            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 me
          String 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 again
            checkIfIamLeader(context, true);
          } catch (KeeperException e) {
            // we couldn't set our watch for some other reason, retry
            log.warn("Failed setting watch", e);
            checkIfIamLeader(context, true);
          }
        }
      }

## Leader重新选举 ##

1.  正常运行的SolrCloud已产生一个leader(Znode编号最小，比如XXX\_node1\_0000001)，后续的Replica后在leader节点上注册Watcher。当Leader下线时候，即短连接断开，那么Zookeeper上的Znode(比如XXX\_node1\_0000001)就会被删除。
2.  此时，所有Replica在Leader节点上的watcher就会监控到这一变化，所有的Replica就会进行leader选举，选举的原则依然是判断自己是不是目前注册在/collections/collectionTest/leader\_select/shard1/election下的Znode编号最小的那位，是的话就是Leader，否则就是Replica。
3.  如果判断自己是Replica，就会继续在leader的Znode上(这个时候的leader是XXX\_node1\_0000002)注册watcher，等待leader下线再次触发选举leader。
4.  如果这个时候原先下线的leader上线了会怎么样，它就会被当做新的一个Solr节点注册到Zookeeper上，并获取一个比现有Znode更大的编号，在Leader Znode节点上注册watcher，等待它的选举机会。

[这篇文章][Link 1]讲得很好，但它base的版本是比较老的Solr 4.x了。

**Reference**  
[http://quentinxxz.iteye.com/blog/2149891][http_quentinxxz.iteye.com_blog_2149891]  
[https://www.cnblogs.com/rcfeng/p/4082568.html][https_www.cnblogs.com_rcfeng_p_4082568.html]  
[https://www.cnblogs.com/saratearing/p/5690476.html][https_www.cnblogs.com_saratearing_p_5690476.html]  
[https://blog.csdn.net/u011026968/article/details/50336709][https_blog.csdn.net_u011026968_article_details_50336709]  
[https://blog.csdn.net/iteye\_16982/article/details/82574099][https_blog.csdn.net_iteye_16982_article_details_82574099]

Zookeeper简介  
[https://www.cnblogs.com/xinfang520/p/7717684.html][https_www.cnblogs.com_xinfang520_p_7717684.html]

SolrCloud Recovery原理及无法选举分片Leader  
[https://www.sohu.com/a/130752460\_505885][https_www.sohu.com_a_130752460_505885]

[LeaderElector.java]: https://github.com/apache/lucene-solr/blob/master/solr/core/src/java/org/apache/solr/cloud/LeaderElector.java
[Link 1]: https://www.cnblogs.com/rcfeng/p/4077663.html
[http_quentinxxz.iteye.com_blog_2149891]: http://quentinxxz.iteye.com/blog/2149891
[https_www.cnblogs.com_rcfeng_p_4082568.html]: https://www.cnblogs.com/rcfeng/p/4082568.html
[https_www.cnblogs.com_saratearing_p_5690476.html]: https://www.cnblogs.com/saratearing/p/5690476.html
[https_blog.csdn.net_u011026968_article_details_50336709]: https://blog.csdn.net/u011026968/article/details/50336709
[https_blog.csdn.net_iteye_16982_article_details_82574099]: https://blog.csdn.net/iteye_16982/article/details/82574099
[https_www.cnblogs.com_xinfang520_p_7717684.html]: https://www.cnblogs.com/xinfang520/p/7717684.html
[https_www.sohu.com_a_130752460_505885]: https://www.sohu.com/a/130752460_505885