solr cloud的update索引链

<updateRequestProcessorChain name="add-unknown-fields-to-the-schema" default="${update.autoCreateFields:true}"
                                 processor="uuid,remove-blank,field-name-mutating,parse-boolean,parse-long,parse-double,parse-date">
        <processor class="solr.LogUpdateProcessorFactory"/>
        <processor class="solr.DistributedUpdateProcessorFactory"/>
        <processor class="solr.RunUpdateProcessorFactory"/>
    </updateRequestProcessorChain>

public void init(PluginInfo info) {
    final String infomsg = "updateRequestProcessorChain \"" + 
      (null != info.name ? info.name : "") + "\"" + 
      (info.isDefault() ? " (default)" : "");

    log.debug("creating " + infomsg);
    // 根据solrconfig.xml中的配置创建UpdateRequestProcessorFactory的实例列表
    List<UpdateRequestProcessorFactory> list = new ArrayList<>
      (solrCore.initPlugins(info.getChildren("processor"),UpdateRequestProcessorFactory.class,null));

    // 如果solrconfig.xml中的updateRequestProcessorChain没有配置processor子属性列表，
    // 那么上一步创建的list为空，抛出异常
    if(list.isEmpty()){
      throw new SolrException(SolrException.ErrorCode.SERVER_ERROR,
                              infomsg + " require at least one processor");
    }

    int numDistrib = 0;
    int runIndex = -1;
    // 统计上一步创建的UpdateRequestProcessorFactory列表中DistributingUpdateProcessorFactory的
    // 实例的数量,和RunUpdateProcessorFactory在列表中的位置
    for (int i = list.size()-1; 0 <= i; i--) {
      UpdateRequestProcessorFactory factory = list.get(i);
      if (factory instanceof DistributingUpdateProcessorFactory) {
        numDistrib++;
      }
      if (factory instanceof RunUpdateProcessorFactory) {
        runIndex = i;
      }
    }
    // 如果上一步DistributingUpdateProcessorFactory的数量大于1，抛出异常
    if (1 < numDistrib) {
      throw new SolrException(SolrException.ErrorCode.SERVER_ERROR,
                              infomsg + " may not contain more then one " +
                              "instance of DistributingUpdateProcessorFactory");
    }
    // 如果上一步UpdateRequestProcessorFactory列表中存在RunUpdateProcessorFactory， 
    // 但是DistributingUpdateProcessorFactory不存在，向列表中RunUpdateProcessorFactory的前面
    // 插入默认的DistributedUpdateProcessorFactory实例
    if (0 <= runIndex && 0 == numDistrib) {
      // by default, add distrib processor immediately before run
      DistributedUpdateProcessorFactory distrib
        = new DistributedUpdateProcessorFactory();
      distrib.init(new NamedList());
      list.add(runIndex, distrib);

      log.debug("inserting DistributedUpdateProcessorFactory into " + infomsg);
    }
    // 根据solrconfig.xml中processor和post-processor属性生成最终的UpdateRequestProcessorFactory的列表
    chain = list;
    ProcessorInfo processorInfo = new ProcessorInfo(new MapSolrParams(info.attributes));
    if (processorInfo.isEmpty()) return;
    UpdateRequestProcessorChain newChain = constructChain(this, processorInfo, solrCore);
    chain = newChain.chain;

  }