Solr4.8.0源码分析(21)之SolrCloud的Recovery策略(二)

• Solr4.8.0源码分析(21)之SolrCloud的Recovery策略(二)

题记：  前文<Solr4.8.0源码分析(20)之SolrCloud的Recovery策略(一)>中提到Recovery有两种策略，一是PeerSync和Replication。本节将具体介绍下PeerSync策略。

     PeeySync是Solr的优先选择策略，每当需要进行recovery了，Solr总是会先去判断是否需要进入PeerSync，只有当PeerSync被设置为跳过或者PeerSync时候发现没符合条件才会进入到Replication。这是由PeeySync的特性决定的，PeeySync是面向中断时间短，需要recovery的document个数较少时使用的策略，因此它Recovery的速度较快，对Solr的影响较小。而Replication则是对中断时间长，需要recovery数量多的情况下进行的，耗时较长。

     前文已经介绍了Recovery的总体流程，那么本文就直接来介绍PeerSync的流程了，请看下图所示：

• 首先 Solr会向所有Replica发送getversion的请求，来获取最新的nupdate个version(默认是100个)。

    // Fire off the requests before getting our own recent updates (for better concurrency)
    // This also allows us to avoid getting updates we don't need... if we got our updates and then got their updates, they would
    // have newer stuff that we also had (assuming updates are going on and are being forwarded).
    for (String replica : replicas) {
      requestVersions(replica);
    }

  private void requestVersions(String replica) {
    SyncShardRequest sreq = new SyncShardRequest();
    sreq.purpose = 1;
    sreq.shards = new String[]{replica};
    sreq.actualShards = sreq.shards;
    sreq.params = new ModifiableSolrParams();
    sreq.params.set("qt","/get");
    sreq.params.set("distrib",false);
    sreq.params.set("getVersions",nUpdates);
    shardHandler.submit(sreq, replica, sreq.params);
  }

• 获取本分片最新的nupdate个version(默认是100个)，并对这些version进行排序。

    recentUpdates = ulog.getRecentUpdates();
    try {
      ourUpdates = recentUpdates.getVersions(nUpdates);
    } finally {
      recentUpdates.close();
    }

    Collections.sort(ourUpdates, absComparator);

• 获取recovery之前的version信息startingversions。通过比较startingversions与ourUpdates可以来比较recovery期间是否有索引更新。
• 检查ourUpdates和startingversions是否有交集，由于ourUpdates和startingversions的version个数是限制为nUpdates的，也就是判断索引更新的个数是否大于nUpdate。如果需要更新的索引太多即ourUpdates和startingversions无交集，则进入Replication。

      // now make sure that the starting updates overlap our updates
      // there shouldn't be reorders, so any overlap will do.

      long smallestNewUpdate = Math.abs(ourUpdates.get(ourUpdates.size()-1));

      if (Math.abs(startingVersions.get(0)) < smallestNewUpdate) {
        log.warn(msg() + "too many updates received since start - startingUpdates no longer overlaps with our currentUpdates");
        return false;
      }

• 如果ourUpdates和startingversions有交集，则合并两个列表，即求并集。

      // let's merge the lists
      List<Long> newList = new ArrayList<>(ourUpdates);
      for (Long ver : startingVersions) {
        if (Math.abs(ver) < smallestNewUpdate) {
          newList.add(ver);
        }
      }

      ourUpdates = newList;

• 本分片的version比别的分片低，则进入Replication策略。这里进行分片version的比较，并没有按version的最大或者最小值，而是比较0.8和0.2比例处的version。

    long otherHigh = percentile(otherVersions, .2f);
    long otherLow = percentile(otherVersions, .8f);

    if (ourHighThreshold < otherLow) {
      // Small overlap between version windows and ours is older
      // This means that we might miss updates if we attempted to use this method.
      // Since there exists just one replica that is so much newer, we must
      // fail the sync.
      log.info(msg() + " Our versions are too old. ourHighThreshold="+ourHighThreshold + " otherLowThreshold="+otherLow);
      return false;
    }

• 如果本分片的version比其他分片高，则说明不需要进行recovery直接退出peersync。

    if (ourLowThreshold > otherHigh) {
      // Small overlap between windows and ours is newer.
      // Using this list to sync would result in requesting/replaying results we don't need
      // and possibly bringing deleted docs back to life.
      log.info(msg() + " Our versions are newer. ourLowThreshold="+ourLowThreshold + " otherHigh="+otherHigh);
      return true;
    }

• 对本分片的version和其他分片的version求差，获取本分片缺少的version。

    for (Long otherVersion : otherVersions) {
      // stop when the entries get old enough that reorders may lead us to see updates we don't need
      if (!completeList && Math.abs(otherVersion) < ourLowThreshold) break;

      if (ourUpdateSet.contains(otherVersion) || requestedUpdateSet.contains(otherVersion)) {
        // we either have this update, or already requested it
        // TODO: what if the shard we previously requested this from returns failure (because it goes
        // down)
        continue;
      }

      toRequest.add(otherVersion);
      requestedUpdateSet.add(otherVersion);
    }

• 最后向其他分片发送getupdate命令，根据处理后的version获取相应的document，至此完成peersync过程

  private boolean requestUpdates(ShardResponse srsp, List<Long> toRequest) {
    String replica = srsp.getShardRequest().shards[0];

    log.info(msg() + "Requesting updates from " + replica + "n=" + toRequest.size() + " versions=" + toRequest);

    // reuse our original request object
    ShardRequest sreq = srsp.getShardRequest();

    sreq.purpose = 0;
    sreq.params = new ModifiableSolrParams();
    sreq.params.set("qt", "/get");
    sreq.params.set("distrib", false);
    sreq.params.set("getUpdates", StrUtils.join(toRequest, ','));
    sreq.params.set("onlyIfActive", onlyIfActive);
    sreq.responses.clear();  // needs to be zeroed for correct correlation to occur

    shardHandler.submit(sreq, sreq.shards[0], sreq.params);

    return true;
  }

总结：

      本文具体介绍PeerSync的过程，由此可见PeerSync策略的recovery过程还是比较简单的，下一节将具体介绍Replication策略，这个较PeerSync复杂。