#分布式搜索的執行方式 在繼續之前，我們將繞道講一下搜索是如何在分布式環境中執行的。 <!--"distributed search execution"--> 它比我們之前講的基礎的_增刪改查_(_create-read-update-delete_ ，CRUD)<!--"CRUD (create-read-update-delete) operations"-->請求要復雜一些。 > ####注意： > 本章的信息只是出于興趣閱讀，使用Elasticsearch并不需要理解和記住這里的所有細節。 > 閱讀這一章只是增加對系統如何工作的了解，并讓你知道這些信息以備以后參考，所以別淹沒在細節里。 一個CRUD操作只處理一個單獨的文檔。文檔的唯一性由`_index`, `_type`和`routing-value`（通常默認是該文檔的`_id`）的組合來確定。這意味著我們可以準確知道集群中的哪個分片持有這個文檔。 由于不知道哪個文檔會匹配查詢（文檔可能存放在集群中的任意分片上），所以搜索需要一個更復雜的模型。一個搜索不得不通過查詢每一個我們感興趣的索引的分片副本，來看是否含有任何匹配的文檔。 但是，找到所有匹配的文檔只完成了這件事的一半。在搜索（`search`）API返回一頁結果前，來自多個分片的結果必須被組合放到一個有序列表中。因此，搜索的執行過程分兩個階段，稱為_查詢然后取回_（_query then fetch_）。 <!-- [[distributed-search]] == Distributed Search Execution Before moving on, we are going to take a detour and talk about how search is executed in a distributed environment.((("distributed search execution"))) It is a bit more complicated than the basic _create-read-update-delete_ (CRUD) requests((("CRUD (create-read-update-delete) operations"))) that we discussed in <<distributed-docs>>. .Content Warning **** The information presented in this chapter is for your interest. You are not required to understand and remember all the detail in order to use Elasticsearch. Read this chapter to gain a taste for how things work, and to know where the information is in case you need to refer to it in the future, but don't be overwhelmed by the detail. **** A CRUD operation deals with a single document that has a unique combination of `_index`, `_type`, and <<routing-value,`routing` values>> (which defaults to the document's `_id`). This means that we know exactly which shard in the cluster holds that document. Search requires a more complicated execution model because we don't know which documents will match the query: they could be on any shard in the cluster. A search request has to consult a copy of every shard in the index or indices we're interested in to see if they have any matching documents. But finding all matching documents is only half the story. Results from multiple shards must be combined into a single sorted list before the `search` API can return a ``page'' of results. For this reason, search is executed in a two-phase process called _query then fetch_. -->