Solr4.8.0源码分析(23)之SolrCloud的Recovery策略(四)
题记：本来计划的SolrCloud的Recovery策略的文章是3篇的，但是没想到Recovery的内容蛮多的，前面三章分别介绍了Recovery的原理和总体流程，PeerSync策略，Replication策略。本章主要介绍我在实际生产环境中碰到的recovery的几个问题，以及前面漏下的几个点。
一. 日志中多次出现"Stopping recovery for zkNodeName= ..."
我在公司的生产环境中总是会看到连续多次出现 " WARN : Stopping recovery for zkNodeName= ..." 或者 "INFO : Starting recovery process. core=..." 这样的日志(由于公司的东西无法拿出了，所以只能意会下日志了)。
这种现象的原因是因为：前文讲到过出现Recovery的原因之一是Leader转发update request到replica后没有接收到replica的表示成功的返回，那么这是Leader会发送RequestRecovery request给replia，命令它进行recovery。这是一次转发失败的过程。而每当Solr出现Leader转发update失败时候往往不会只出现一次，所以Leader会发送多次RequestRecovery request给replia。
Relica的Recovery过程起始于DefaultSolrCoreState类的doRecovery()函数，在进行doRecovery()时候Replica会取消之前的Recovery。所以出现上述现象的根本原因就在于cancelRecovery上。需要指出的是DefaultSolrCoreState类的doRecovery()函数不但在RequestRecovery请求后会被调用，在leader 选举失败的时候也会被掉用。
1 @Override
2 public void cancelRecovery() {
3 synchronized (recoveryLock) {
4 if (recoveryStrat != null && recoveryRunning) {
5 recoveryStrat.close();
6 while (true) {
7 try {
8 recoveryStrat.join();
9 } catch (InterruptedException e) {
10 // not interruptible - keep waiting
11 continue;
12 }
13 break;
14 }
15
16 recoveryRunning = false;
17 recoveryLock.notifyAll();
18 }
19 }
20 }
1 @Override
2 public void close() {
3 close = true;
4 try {
5 prevSendPreRecoveryHttpUriRequest.abort();
6 } catch (NullPointerException e) {
7 // okay
8 }
9 log.warn("Stopping recovery for zkNodeName=" + coreZkNodeName + "core=" + coreName);
10 }
二. Recovery过程中的rollback
之前有@从前 网友给我留言说出现了"持续向solrcloud提交数据的同时调用了optimize 方法。导致索引文件同步失败，就一直无法recovery。"的现象。造成这个现象的原因大致由以下两点：
- optimize的操作的本质是Merge策略中的forceMerge，默认情况下一旦触发了forceMerge，那么Solr会把所有的Segment合并成一个Segment。可以想象下，几十甚至几百GB的数据合成一个Segment，这样的符合会有多大？而且这还不算，一旦触发了forceMerge，如果有实时数据进来，那么它会把新进来的数据也merge进去，也就是说会一直merge进去根本不会停下来。关于forceMerge的具体情况，将在接下来介绍Merge的文章中详述。
- Replication策略介绍的时候提到，如果isFullCopyNeeded为false，那么Solr就会调用closeIndexWriter.
1 if (!isFullCopyNeeded) {
2 // rollback - and do it before we download any files
3 // so we don't remove files we thought we didn't need
4 // to download later
5 solrCore.getUpdateHandler().getSolrCoreState()
6 .closeIndexWriter(core, true);
7 }
我们很容会忽视closeIndexWriter传入的true参数，如果传入的为true，表示Solr关闭IndexWriter时候会进行回滚rollback，它的作用就是将IndexWriter退回到上次commit之后的状态，清空上次commit之后的所有add进来的数据。
1 if (indexWriter != null) {
2 if (!rollback) {
3 try {
4 log.info("Closing old IndexWriter... core=" + coreName);
5 indexWriter.close();
6 } catch (Exception e) {
7 SolrException.log(log, "Error closing old IndexWriter. core="
8 + coreName, e);
9 }
10 } else {
11 try {
12 log.info("Rollback old IndexWriter... core=" + coreName);
13 indexWriter.rollback();
14 } catch (Exception e) {
15 SolrException.log(log, "Error rolling back old IndexWriter. core="
16 + coreName, e);
17 }
18 }
19 }
那么问题就出在rollback中，Lucene的IndexWriter在进行回滚的时候会尝试去关闭正在进行的mergePolicy和mergeScheduler，如果发现还有segment正在进行那么它会一直等待，所以当optimize(forceMerge)进行时且有实时数据进来，那么Recovery就会一直停在那里直到超时。
1 /** Wait for any running merge threads to finish. This call is not interruptible as used by {@link #close()}. */
2 public void sync() {
3 boolean interrupted = false;
4 try {
5 while (true) {
6 MergeThread toSync = null;
7 synchronized (this) {
8 for (MergeThread t : mergeThreads) {
9 if (t.isAlive()) {
10 toSync = t;
11 break;
12 }
13 }
14 }
15 if (toSync != null) {
16 try {
17 toSync.join();
18 } catch (InterruptedException ie) {
19 // ignore this Exception, we will retry until all threads are dead
20 interrupted = true;
21 }
22 } else {
23 break;
24 }
25 }
26 } finally {
27 // finally, restore interrupt status:
28 if (interrupted) Thread.currentThread().interrupt();
29 }
30 }
所以解决的方法有两个：
- optimize时候保证没有实时数据进来。
- 修改forceMerge的策略，只对启动forceMerge时候的Segment进行合并，之后的Segment选择无视(我司采用的策略)。
三. Recovery触发的三个地方
触发Recovery有三个地方，也就是上文中doRecovery()被调用的三个地方：
- 之前一直在讲的RequestRecovery请求
1 protected void handleRequestRecoveryAction(SolrQueryRequest req,
2 SolrQueryResponse rsp) throws IOException {
3 final SolrParams params = req.getParams();
4 log.info("It has been requested that we recover");
5 Thread thread = new Thread() {
6 @Override
7 public void run() {
8 String cname = params.get(CoreAdminParams.CORE);
9 if (cname == null) {
10 cname = "";
11 }
12 try (SolrCore core = coreContainer.getCore(cname)) {
13
14 if (core != null) {
15 // try to publish as recovering right away
16 try {
17 coreContainer.getZkController().publish(core.getCoreDescriptor(), ZkStateReader.RECOVERING);
18 } catch (InterruptedException e) {
19 Thread.currentThread().interrupt();
20 SolrException.log(log, "", e);
21 } catch (Throwable e) {
22 SolrException.log(log, "", e);
23 if (e instanceof Error) {
24 throw (Error) e;
25 }
26 }
27
28 core.getUpdateHandler().getSolrCoreState().doRecovery(coreContainer, core.getCoreDescriptor());
29 } else {
30 SolrException.log(log, "Could not find core to call recovery:" + cname);
31 }
32 }
33 }
34 };
35
36 thread.start();
37 }
- 当Leader选举失败的时候，它会先进行recovery，然后再重新加入选举。
1 private void rejoinLeaderElection(String leaderSeqPath, SolrCore core)
2 throws InterruptedException, KeeperException, IOException {
3 // remove our ephemeral and re join the election
4 if (cc.isShutDown()) {
5 log.info("Not rejoining election because CoreContainer is shutdown");
6 return;
7 }
8
9 log.info("There may be a better leader candidate than us - going back into recovery");
10
11 cancelElection();
12
13 core.getUpdateHandler().getSolrCoreState().doRecovery(cc, core.getCoreDescriptor());
14
15 leaderElector.joinElection(this, true);
16 }
- Register 注册shard的时候，会去检测shard是否处于recovery状态。如果满足recovery条件就会触发recovery。
1 /**
2 * Returns whether or not a recovery was started
3 */
4 private boolean checkRecovery(String coreName, final CoreDescriptor desc,
5 boolean recoverReloadedCores, final boolean isLeader,
6 final CloudDescriptor cloudDesc, final String collection,
7 final String shardZkNodeName, String shardId, ZkNodeProps leaderProps,
8 SolrCore core, CoreContainer cc) {
9 if (SKIP_AUTO_RECOVERY) {
10 log.warn("Skipping recovery according to sys prop solrcloud.skip.autorecovery");
11 return false;
12 }
13 boolean doRecovery = true;
14 if (!isLeader) {
15
16 if (core.isReloaded() && !recoverReloadedCores) {
17 doRecovery = false;
18 }
19
20 if (doRecovery) {
21 log.info("Core needs to recover:" + core.getName());
22 core.getUpdateHandler().getSolrCoreState().doRecovery(cc, core.getCoreDescriptor());
23 return true;
24 }
25 } else {
26 log.info("I am the leader, no recovery necessary");
27 }
28
29 return false;
30 }
四. recoverFromLog
之前写到Recovery过程中在Replicate之后都进行一次applyBufferedUpdates来实现doplay以获取UpdateLog内保存的request。那么除了applyBufferedUpdates还有一种方式recoverFromLog来获取UpdateLog内保存的request。它跟applyBufferedUpdates不同之处在于，它主要用于单机的Solr模式下。当创建core的时候就会触发：
1 /**
2 * Creates a new core based on a descriptor but does not register it.
3 *
4 * @param dcore a core descriptor
5 * @return the newly created core
6 */
7 public SolrCore create(CoreDescriptor dcore) {
8
9 if (isShutDown) {
10 throw new SolrException(ErrorCode.SERVICE_UNAVAILABLE, "Solr has shutdown.");
11 }
12
13 try {
14
15 ConfigSet coreConfig = coreConfigService.getConfig(dcore);
16 log.info("Creating SolrCore '{}' using configuration from {}", dcore.getName(), coreConfig.getName());
17 SolrCore core = new SolrCore(dcore, coreConfig);
18 solrCores.addCreated(core);
19
20 // always kick off recovery if we are in non-Cloud mode
21 if (!isZooKeeperAware() && core.getUpdateHandler().getUpdateLog() != null) {
22 core.getUpdateHandler().getUpdateLog().recoverFromLog();
23 }
24
25 return core;
26
27 }
28 catch (Exception e) {
29 throw recordAndThrow(dcore.getName(), "Unable to create core: " + dcore.getName(), e);
30 }
31
32 }
总结：
本节列举了几个Recovery过程中遇到的问题，以及补充说明了之前漏下的内容。下文会介绍Recovery系列的最后一文，Replication主从模式的配置。
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