[[java-rest-high-explain]]
=== Explain API

The explain api computes a score explanation for a query and a specific document.
This can give useful feedback whether a document matches or didn’t match a specific query.

[[java-rest-high-explain-request]]
==== Explain Request

An `ExplainRequest` expects an `index` and an `id` to specify a certain document,
and a query represented by `QueryBuilder` to run against it (the way of <<java-rest-high-query-builders, building queries>>).

["source","java",subs="attributes,callouts,macros"]
--------------------------------------------------
include-tagged::{doc-tests}/SearchDocumentationIT.java[explain-request]
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===== Optional arguments

["source","java",subs="attributes,callouts,macros"]
--------------------------------------------------
include-tagged::{doc-tests}/SearchDocumentationIT.java[explain-request-routing]
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<1> Set a routing parameter

["source","java",subs="attributes,callouts,macros"]
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include-tagged::{doc-tests}/SearchDocumentationIT.java[explain-request-preference]
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<1> Use the preference parameter e.g. to execute the search to prefer local
shards. The default is to randomize across shards.

["source","java",subs="attributes,callouts,macros"]
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include-tagged::{doc-tests}/SearchDocumentationIT.java[explain-request-source]
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<1> Set to true to retrieve the _source of the document explained. You can also
retrieve part of the document by using _source_include & _source_exclude
(see <<java-rest-high-document-get-request-optional-arguments, Get API>> for more details)

["source","java",subs="attributes,callouts,macros"]
--------------------------------------------------
include-tagged::{doc-tests}/SearchDocumentationIT.java[explain-request-stored-field]
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<1> Allows to control which stored fields to return as part of the document explained
(requires the field to be stored separately in the mappings).

[[java-rest-high-explain-sync]]
==== Synchronous Execution

The `explain` method executes the request synchronously:

["source","java",subs="attributes,callouts,macros"]
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include-tagged::{doc-tests}/SearchDocumentationIT.java[explain-execute]
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[[java-rest-high-explain-async]]
==== Asynchronous Execution

The `explainAsync` method executes the request asynchronously,
calling the provided `ActionListener` when the response is ready:

["source","java",subs="attributes,callouts,macros"]
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include-tagged::{doc-tests}/SearchDocumentationIT.java[explain-execute-async]
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<1> The `ExplainRequest` to execute and the `ActionListener` to use when
the execution completes.

The asynchronous method does not block and returns immediately. Once the request
completes, the `ActionListener` is called back using the `onResponse` method
if the execution successfully completed or using the `onFailure` method if
it failed.

A typical listener for `ExplainResponse` is constructed as follows:

["source","java",subs="attributes,callouts,macros"]
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include-tagged::{doc-tests}/SearchDocumentationIT.java[explain-execute-listener]
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<1> Called when the execution is successfully completed.
<2> Called when the whole `ExplainRequest` fails.

[[java-rest-high-explain-response]]
==== ExplainResponse

The `ExplainResponse` contains the following information:

["source","java",subs="attributes,callouts,macros"]
--------------------------------------------------
include-tagged::{doc-tests}/SearchDocumentationIT.java[explain-response]
--------------------------------------------------
<1> The index name of the explained document.
<2> The id of the explained document.
<3> Indicates whether or not the explained document exists.
<4> Indicates whether or not there is a match between the explained document and
the provided query (the `match` is retrieved from the lucene `Explanation` behind the scenes
if the lucene `Explanation` models a match, it returns `true`, otherwise it returns `false`).
<5> Indicates whether or not there exists a lucene `Explanation` for this request.
<6> Get the lucene `Explanation` object if there exists.
<7> Get the `GetResult` object if the `_source` or the stored fields are retrieved.

The `GetResult` contains two maps internally to store the fetched `_source` and stored fields.
You can use the following methods to get them:

["source","java",subs="attributes,callouts,macros"]
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include-tagged::{doc-tests}/SearchDocumentationIT.java[get-result]
--------------------------------------------------
<1> Retrieve the `_source` as a map.
<2> Retrieve the specified stored fields as a map.