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。

@Override
public void cancelRecovery() {
synchronized (recoveryLock) {
if (recoveryStrat != null && recoveryRunning) {
recoveryStrat.close();
while (true) {
try {
recoveryStrat.join();
} catch (InterruptedException e) {
// not interruptible - keep waiting
continue;
}
break;
}

     recoveryRunning = false;  
     recoveryLock.notifyAll();  
   }  
 }  

}

@Override
public void close() {
close = true;
try {
prevSendPreRecoveryHttpUriRequest.abort();
} catch (NullPointerException e) {
// okay
}
log.warn("Stopping recovery for zkNodeName=" + coreZkNodeName + "core=" + coreName);
}

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.

   if (!isFullCopyNeeded) {  
     // rollback - and do it before we download any files  
     // so we don't remove files we thought we didn't need  
     // to download later  
     solrCore.getUpdateHandler().getSolrCoreState()  
     .closeIndexWriter(core, true);  
   }

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。

   if (indexWriter != null) {  
     if (!rollback) {  
       try {  
         log.info("Closing old IndexWriter... core=" + coreName);  
         indexWriter.close();  
       } catch (Exception e) {  
         SolrException.log(log, "Error closing old IndexWriter. core="  
             + coreName, e);  
       }  
     } else {  
       try {  
         log.info("Rollback old IndexWriter... core=" + coreName);  
         indexWriter.rollback();  
       } catch (Exception e) {  
         SolrException.log(log, "Error rolling back old IndexWriter. core="  
             + coreName, e);  
       }  
     }  
   }

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。

/** Wait for any running merge threads to finish. This call is not interruptible as used by {@link #close()}. */
public void sync() {
boolean interrupted = false;
try {
while (true) {
MergeThread toSync = null;
synchronized (this) {
for (MergeThread t : mergeThreads) {
if (t.isAlive()) {
toSync = t;
break;
}
}
}
if (toSync != null) {
try {
toSync.join();
} catch (InterruptedException ie) {
// ignore this Exception, we will retry until all threads are dead
interrupted = true;
}
} else {
break;
}
}
} finally {
// finally, restore interrupt status:
if (interrupted) Thread.currentThread().interrupt();
}
}

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

  protected void handleRequestRecoveryAction(SolrQueryRequest req,
  SolrQueryResponse rsp) throws IOException {
  final SolrParams params = req.getParams();
  log.info("It has been requested that we recover");
  Thread thread = new Thread() {
  @Override
  public void run() {
  String cname = params.get(CoreAdminParams.CORE);
  if (cname == null) {
  cname = "";
  }
  try (SolrCore core = coreContainer.getCore(cname)) {

     if (core != null) {  
       // try to publish as recovering right away  
       try {  
         coreContainer.getZkController().publish(core.getCoreDescriptor(), ZkStateReader.RECOVERING);  
       }  catch (InterruptedException e) {  
         Thread.currentThread().interrupt();  
         SolrException.log(log, "", e);  
       } catch (Throwable e) {  
         SolrException.log(log, "", e);  
         if (e instanceof Error) {  
           throw (Error) e;  
         }  
       }
   core.getUpdateHandler().getSolrCoreState().doRecovery(coreContainer, core.getCoreDescriptor());  
     } else {  
       SolrException.log(log, "Could not find core to call recovery:" + cname);  
     }  
   }  

  }
  };

  thread.start();
  }

• 当Leader选举失败的时候，它会先进行recovery，然后再重新加入选举。

  private void rejoinLeaderElection(String leaderSeqPath, SolrCore core)
  throws InterruptedException, KeeperException, IOException {
  // remove our ephemeral and re join the election
  if (cc.isShutDown()) {
  log.info("Not rejoining election because CoreContainer is shutdown");
  return;
  }

  log.info("There may be a better leader candidate than us - going back into recovery");

  cancelElection();

  core.getUpdateHandler().getSolrCoreState().doRecovery(cc, core.getCoreDescriptor());

  leaderElector.joinElection(this, true);
  }

• 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。

  /**
  * Returns whether or not a recovery was started
  */
  private boolean checkRecovery(String coreName, final CoreDescriptor desc,
  boolean recoverReloadedCores, final boolean isLeader,
  final CloudDescriptor cloudDesc, final String collection,
  final String shardZkNodeName, String shardId, ZkNodeProps leaderProps,
  SolrCore core, CoreContainer cc) {
  if (SKIP_AUTO_RECOVERY) {
  log.warn("Skipping recovery according to sys prop solrcloud.skip.autorecovery");
  return false;
  }
  boolean doRecovery = true;
  if (!isLeader) {

  if (core.isReloaded() && !recoverReloadedCores) {
  doRecovery = false;
  }

  if (doRecovery) {
  log.info("Core needs to recover:" + core.getName());
  core.getUpdateHandler().getSolrCoreState().doRecovery(cc, core.getCoreDescriptor());
  return true;
  }
  } else {
  log.info("I am the leader, no recovery necessary");
  }

  return false;
  }

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：

/**
* Creates a new core based on a descriptor but does not register it.
*
* @param dcore a core descriptor
* @return the newly created core
*/
public SolrCore create(CoreDescriptor dcore) {

 if (isShutDown) {  
   throw new SolrException(ErrorCode.SERVICE\_UNAVAILABLE, "Solr has shutdown.");  
 }

 try {

   ConfigSet coreConfig = coreConfigService.getConfig(dcore);  
   log.info("Creating SolrCore '{}' using configuration from {}", dcore.getName(), coreConfig.getName());  
   SolrCore core = new SolrCore(dcore, coreConfig);  
   solrCores.addCreated(core);

   // always kick off recovery if we are in non-Cloud mode  
   if (!isZooKeeperAware() && core.getUpdateHandler().getUpdateLog() != null) {  
     core.getUpdateHandler().getUpdateLog().recoverFromLog();  
   }

   return core;

 }  
 catch (Exception e) {  
   throw recordAndThrow(dcore.getName(), "Unable to create core: " + dcore.getName(), e);  
 }

}

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。