This change adds a new "filter_path" parameter that can be used to filter and reduce the responses returned by the REST API of elasticsearch.
For example, returning only the shards that failed to be optimized:
```
curl -XPOST 'localhost:9200/beer/_optimize?filter_path=_shards.failed'
{"_shards":{"failed":0}}%
```
It supports multiple filters (separated by a comma):
```
curl -XGET 'localhost:9200/_mapping?pretty&filter_path=*.mappings.*.properties.name,*.mappings.*.properties.title'
```
It also supports the YAML response format. Here it returns only the `_id` field of a newly indexed document:
```
curl -XPOST 'localhost:9200/library/book?filter_path=_id' -d '---hello:\n world: 1\n'
---
_id: "AU0j64-b-stVfkvus5-A"
```
It also supports wildcards. Here it returns only the host name of every nodes in the cluster:
```
curl -XGET 'http://localhost:9200/_nodes/stats?filter_path=nodes.*.host*'
{"nodes":{"lvJHed8uQQu4brS-SXKsNA":{"host":"portable"}}}
```
And "**" can be used to include sub fields without knowing the exact path. Here it returns only the Lucene version of every segment:
```
curl 'http://localhost:9200/_segments?pretty&filter_path=indices.**.version'
{
"indices" : {
"beer" : {
"shards" : {
"0" : [ {
"segments" : {
"_0" : {
"version" : "5.2.0"
},
"_1" : {
"version" : "5.2.0"
}
}
} ]
}
}
}
}
```
Note that elasticsearch sometimes returns directly the raw value of a field, like the _source field. If you want to filter _source fields, you should consider combining the already existing _source parameter (see Get API for more details) with the filter_path parameter like this:
```
curl -XGET 'localhost:9200/_search?pretty&filter_path=hits.hits._source&_source=title'
{
"hits" : {
"hits" : [ {
"_source":{"title":"Book #2"}
}, {
"_source":{"title":"Book #1"}
}, {
"_source":{"title":"Book #3"}
} ]
}
}
```
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