modified: 00_Intro.asciidoc

Author: JessicaWon

060_Distributed_Search/00_Intro.asciidoc

@@ -1,3 +1,38 @@
+[[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_.
+
 [[distributed-search]]
 == 分布式检索执行