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Elastic Docs ›Elasticsearch Guide [1.6] ›Mapping ›Fields

_source

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_source

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The _source field is an automatically generated field that stores the actual JSON that was used as the indexed document. It is not indexed (searchable), just stored. When executing "fetch" requests, like get or search, the _source field is returned by default.

Disabling source

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Though very handy to have around, the source field does incur storage overhead within the index. For this reason, it can be disabled as follows:

PUT tweets
{
  "mappings": {},
  "tweet": {
    "_source": {
      "enabled": false
    }
  }
}

Think before disabling the source field

Users often disable the _source field without thinking about the consequences, and then live to regret it. If the _source field isn’t available then a number of features are not supported:

  • The update API.
  • On the fly highlighting.
  • The ability to reindex from one Elasticsearch index to another, either to change mappings or analysis, or to upgrade an index to a new major version.
  • The ability to debug queries or aggregations by viewing the original document used at index time.
  • Potentially in the future, the ability to repair index corruption automatically.

The metrics use case

The metrics use case is distinct from other time-based or logging use cases in that there are many small documents which consist only of numbers, dates, or keywords. There are no updates, no highlighting requests, and the data ages quickly so there is no need to reindex. Search requests typically use simple queries to filter the dataset by date or tags, and the results are returned as aggregations.

In this case, disabling the _source field will save space and reduce I/O. It is also advisable to disable the _all field in the metrics case.

Including / Excluding fields from source

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An expert-only feature is the ability to prune the contents of the _source field after the document has been indexed, but before the _source field is stored. The includes/excludes parameters (which also accept wildcards) can be used as follows:

PUT logs
{
  "mappings": {
    "event": {
      "_source": {
        "includes": [
          "*.count",
          "meta.*"
        ],
        "excludes": [
          "meta.description",
          "meta.other.*"
        ]
      }
    }
  }
}

PUT logs/event/1
{
  "requests": {
    "count": 10,
    "foo": "bar" 
  },
  "meta": {
    "name": "Some metric",
    "description": "Some metric description", 
    "other": {
      "foo": "one", 
      "baz": "two" 
    }
  }
}

GET logs/event/_search
{
  "query": {
    "match": {
      "meta.other.foo": "one" 
    }
  }
}

These fields will be removed from the stored _source field.

We can still search on this field, even though it is not in the stored _source.

Removing fields from the _source has similar downsides to disabling _source, especially the fact that you cannot reindex documents from one Elasticsearch index to another. Consider using source filtering or a transform script instead.

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