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。
private void doFinish() {
// TODO: if not a forward and replication req is not specified, we could
// send in a background thread
cmdDistrib.finish();
List<Error> errors = cmdDistrib.getErrors();
// TODO - we may need to tell about more than one error...
// if its a forward, any fail is a problem -
// otherwise we assume things are fine if we got it locally
// until we start allowing min replication param
if (errors.size() > 0) {
// if one node is a RetryNode, this was a forward request
if (errors.get(0).req.node instanceof RetryNode) {
rsp.setException(errors.get(0).e);
} else {
if (log.isWarnEnabled()) {
for (Error error : errors) {
log.warn("Error sending update", error.e);
}
}
}
// else
// for now we don't error - we assume if it was added locally, we
// succeeded
}
// if it is not a forward request, for each fail, try to tell them to
// recover - the doc was already added locally, so it should have been
// legit
for (final SolrCmdDistributor.Error error : errors) {
if (error.req.node instanceof RetryNode) {
// we don't try to force a leader to recover
// when we cannot forward to it
continue;
}
// TODO: we should force their state to recovering ??
// TODO: do retries??
// TODO: what if its is already recovering? Right now recoveries queue up -
// should they?
final String recoveryUrl = error.req.node.getBaseUrl();
Thread thread = new Thread() {
{
setDaemon(true);
}
@Override
public void run() {
log.info("try and ask " + recoveryUrl + " to recover");
HttpSolrServer server = new HttpSolrServer(recoveryUrl);
try {
server.setSoTimeout(60000);
server.setConnectionTimeout(15000);
RequestRecovery recoverRequestCmd = new RequestRecovery();
recoverRequestCmd.setAction(CoreAdminAction.REQUESTRECOVERY);
recoverRequestCmd.setCoreName(error.req.node.getCoreName());
try {
server.request(recoverRequestCmd);
} catch (Throwable t) {
SolrException.log(log, recoveryUrl
+ ": Could not tell a replica to recover", t);
}
} finally {
server.shutdown();
}
}
};
ExecutorService executor = req.getCore().getCoreDescriptor().getCoreContainer().getUpdateShardHandler().getUpdateExecutor();
executor.execute(thread);
}
}
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。
// if true, we are recovering after startup and shouldn't have (or be receiving) additional updates (except for local tlog recovery)
boolean recoveringAfterStartup = recoveryStrat == null; recoveryStrat = new RecoveryStrategy(cc, cd, this);
recoveryStrat.setRecoveringAfterStartup(recoveringAfterStartup);
recoveryStrat.start();
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。
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。
if (recoveringAfterStartup) {
// if we're recovering after startup (i.e. we have been down), then we need to know what the last versions were
// when we went down. We may have received updates since then.
recentVersions = startingVersions;
try {
if ((ulog.getStartingOperation() & UpdateLog.FLAG_GAP) != 0) {
// last operation at the time of startup had the GAP flag set...
// this means we were previously doing a full index replication
// that probably didn't complete and buffering updates in the
// meantime.
log.info("Looks like a previous replication recovery did not complete - skipping peer sync. core="
+ coreName);
firstTime = false; // skip peersync
}
} catch (Exception e) {
SolrException.log(log, "Error trying to get ulog starting operation. core="
+ coreName, e);
firstTime = false; // skip peersync
}
}
- 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。