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Solr4.8.0源码分析(20)之SolrCloud的Recovery策略(一)
Solr4.8.0源码分析(20)之SolrCloud的Recovery策略(一)
题记:
我们在使用SolrCloud中会经常发现会有备份的shard出现状态Recoverying,这就表明SolrCloud的数据存在着不一致性,需要进行Recovery,这个时候的SolrCloud建索引是不会写入索引文件中的(每个shard接受到update后写入自己的ulog中)。关于Recovery的内容包含三篇,本文是第一篇介绍Recovery的原因以及总体流程。
1. Recovery的起因
Recovery一般发生在以下三个时候:
- SolrCloud启动的时候,主要由于在建索引的时候发生意外关闭,导致一些shard的数据与leader不一致,那么在启动的时候刚起的shard就会从leader那里同步数据。
- SolrCloud在进行leader选举中出现错误,一般出现在leader宕机引起replica进行选举成leader过程中。
- SolrCloud在进行update时候,由于某种原因leader转发update至replica没有成功,会迫使replica进行recoverying进行数据同步。
前面两种情况暂时不介绍,本文先介绍下第三种情况。大致原理如下图所示:
之前在<Solr4.8.0源码分析(15) 之 SolrCloud索引深入(2)>中讲到,不管update请求发送到哪个shard 分片中,最后在solrcloud里面进行分发的顺序都是从Leader发往Replica。Leader接受到update请求后先将document放入自己的索引文件以及update写入ulog中,然后将update同时转发给各个Replica分片。这就流程在就是之前讲到的add的索引链过程。
那么在索引链的add过程完毕后,SolrCloud会再依次调用finish()函数用来接受每一个Replica的响应,检查Replica的update操作是否成功。如果一旦有一个Replica没有成功,就会向update失败的Replica发送RequestRecovering命令强迫该分片进行Recoverying。
1 private void doFinish() { 2 // TODO: if not a forward and replication req is not specified, we could 3 // send in a background thread 4 5 cmdDistrib.finish(); 6 List<Error> errors = cmdDistrib.getErrors(); 7 // TODO - we may need to tell about more than one error... 8 9 // if its a forward, any fail is a problem - 10 // otherwise we assume things are fine if we got it locally11 // until we start allowing min replication param12 if (errors.size() > 0) {13 // if one node is a RetryNode, this was a forward request14 if (errors.get(0).req.node instanceof RetryNode) {15 rsp.setException(errors.get(0).e);16 } else {17 if (log.isWarnEnabled()) {18 for (Error error : errors) {19 log.warn("Error sending update", error.e);20 }21 }22 }23 // else24 // for now we don‘t error - we assume if it was added locally, we25 // succeeded 26 }27 28 29 // if it is not a forward request, for each fail, try to tell them to30 // recover - the doc was already added locally, so it should have been31 // legit32 33 for (final SolrCmdDistributor.Error error : errors) {34 if (error.req.node instanceof RetryNode) {35 // we don‘t try to force a leader to recover36 // when we cannot forward to it37 continue;38 }39 // TODO: we should force their state to recovering ??40 // TODO: do retries??41 // TODO: what if its is already recovering? Right now recoveries queue up -42 // should they?43 final String recoveryUrl = error.req.node.getBaseUrl();44 45 Thread thread = new Thread() {46 {47 setDaemon(true);48 }49 @Override50 public void run() {51 log.info("try and ask " + recoveryUrl + " to recover");52 HttpSolrServer server = new HttpSolrServer(recoveryUrl);53 try {54 server.setSoTimeout(60000);55 server.setConnectionTimeout(15000);56 57 RequestRecovery recoverRequestCmd = new RequestRecovery();58 recoverRequestCmd.setAction(CoreAdminAction.REQUESTRECOVERY);59 recoverRequestCmd.setCoreName(error.req.node.getCoreName());60 try {61 server.request(recoverRequestCmd);62 } catch (Throwable t) {63 SolrException.log(log, recoveryUrl64 + ": Could not tell a replica to recover", t);65 }66 } finally {67 server.shutdown();68 }69 }70 };71 ExecutorService executor = req.getCore().getCoreDescriptor().getCoreContainer().getUpdateShardHandler().getUpdateExecutor();72 executor.execute(thread);73 74 }75 }
2. Recovery的总体流程
Replica接收到来自Leader的RequestRecovery命令后就会开始进行RecoveryStrategy线程,然后进行Recovery。总体流程如下图索引:
- 在RequestRecovery请求判断中,我例举了一部分(不是全部)请求命令,这是正常的索引链过程。
- 如果接受到的是RequestRecovery命令,那么本分片就会启动RecoveryStrategy线程来进行Recovery。
1 // if true, we are recovering after startup and shouldn‘t have (or be receiving) additional updates (except for local tlog recovery)2 boolean recoveringAfterStartup = recoveryStrat == null;3 4 recoveryStrat = new RecoveryStrategy(cc, cd, this);5 recoveryStrat.setRecoveringAfterStartup(recoveringAfterStartup);6 recoveryStrat.start();7 recoveryRunning = true;
- 分片会设置分片的状态recoverying。需要指出的是如果一旦检测到本分片成为了leader,那么Recovery过程就会退出。因为Recovery是从leader中同步数据的。
1 zkController.publish(core.getCoreDescriptor(), ZkStateReader.RECOVERING);
- 这里要判断下firsttime是否为true(在重启分片的时候会检查之前是否进行replication且没做完就被关闭了),firsttime是控制是否先进入PeerSync Recovery策略的,如果为false则跳过PeerSync进入Replicate。
1 if (recoveringAfterStartup) { 2 // if we‘re recovering after startup (i.e. we have been down), then we need to know what the last versions were 3 // when we went down. We may have received updates since then. 4 recentVersions = startingVersions; 5 try { 6 if ((ulog.getStartingOperation() & UpdateLog.FLAG_GAP) != 0) { 7 // last operation at the time of startup had the GAP flag set... 8 // this means we were previously doing a full index replication 9 // that probably didn‘t complete and buffering updates in the10 // meantime.11 log.info("Looks like a previous replication recovery did not complete - skipping peer sync. core="12 + coreName);13 firstTime = false; // skip peersync14 }15 } catch (Exception e) {16 SolrException.log(log, "Error trying to get ulog starting operation. core="17 + coreName, e);18 firstTime = false; // skip peersync19 }20 }
- 最后进行选择进入是PeerSync策略和Replicate策略,在<Solr In Action 笔记(4) 之 SolrCloud分布式索引基础>中简单提到过两者的区别。关于具体的不同将在后面两节详细介绍。
- Peer sync, 如果中断的时间较短,recovering node只是丢失少量update请求,那么它可以从leader的update log中获取。这个临界值是100个update请求,如果大于100,就会从leader进行完整的索引快照恢复。
- Replication, 如果节点下线太久以至于不能从leader那进行同步,它就会使用solr的基于http进行索引的快照恢复。
- 最后设置分片的状态为active。并判断是否是sucessfulrrecovery,如果否则会多出尝试Recovery。
总结:
本文主要介绍了Recovery的起因以及Recovery过程,由于是简述所以内容较简单,主要提到了两种不同的Recovery策略,后续两文种将分别详细介绍。
Solr4.8.0源码分析(20)之SolrCloud的Recovery策略(一)