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   }

 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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原文地址：https://www.cnblogs.com/rcfeng/p/4152183.html