[ solr入门 ]

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　　Distributed Searching基础
　　在单机的情况下，当索引越来越大，检索就显得力不从心了。
　　solr容许我们将索引切开（多个适当大小的索引，称之为shards），并分布到多台“服务器”上。
　　solr通过一台服务器（single shard）接受检索任务，并将其分发到各个shards上，最后合并检索结果。

　　详细信息参见：http://wiki.apache.org/solr/DistributedSearch
　　
　　1.通过shards参数执行Distributed Searching
　　我们可以检索请求中加入shards参数执行Distributed Searching，其格式为：
　　host:port/base_url[,host:port/base_url]*
　　例如：



http://localhost:8983/solr3.5/core1/
select/?q=*%3A*&version=2.2&start=0&rows=10&indent=on
&shards=localhost:8983/solr3.5/core0,localhost:8983/solr3.5/core1

　　
　　
　　2.Distributed Searching支持的组件
　　只有以下组件支持Distributed Searching：

• The Query component that returns documents matching a query
  
• The Facet component, for facet.query and facet.field requests where facets are sorted by count (the default). Solr 1.4 and later also support sorting by name.
  
• The Highlighting component
  
• The Stats component
  
• The Spell Check Component
  
• The Terms Component
  
• The Term Vector Component
  
• The Debug component
  

　　
　　3.Distributed Searching的限定（不足）
　　Distributed Searching还有种种限定条件，如下：

• Each document indexed must have a unique key.
  （每个doc都要有唯一标识，因为solr要对结果进行合并）
  
• If Solr discovers duplicate document IDs, Solr selects the first document and discards subsequent ones.
  （solr如果发现重复的id，取首！）
  
• Inverse-document frequency (IDF) calculations cannot be distributed.
  （idf计算失效，idf牵涉到总文档数，distributed在各个shards进行检索时不方便计算文档总数。）
  
• Distributed searching does not support the QueryElevationComponent, which configures the top results for a given query regardless of Lucene's scoring. For more information, see http://wiki.apache.org/solr/QueryElevationComponent.
  （QueryElevationComponent不顾及scoring，有用户对结果进行编辑，那么简单的结果合并也就无从谈起。）
  
• The index for distributed searching may become out of date; for example, a document that once matched a query and was subsequently changed may no longer match the query but will still be retrieved.
  （索引会在distributed searching过程中过时。？？？）
  
• Distributed searching supports only sorted-field faceting, not date faceting
  （distributed searching仅支持sorted-field faceting）
  
• The number of shards is limited by number of characters allowed for GET method's URI; most Web servers generally support at least 4000 characters, but many servers limit URI length to reduce their vulnerability to Denial of Service (DoS) attacks.
  （shards数量受GET地址长度的限制）
  
• TF/IDF computations are per shard. This may not matter if content is well (randomly) distributed.
  （和第三点类似，tf/idf在各自shard上计算，因此合并出来的scoring排序也不是很“公正”。）