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## Elastic Index

Oak supports Elasticsearch (Elastic for short) based indexes for both property constraint and full text constraints. 
Elastic indexes support similar features as [Lucene][lucene] indexes, 
however there are differences:

* The `type` is `elasticsearch`.
* The index definition needs to be under `/oak:index`.
  Other locations are not supported.
* The `async` property needs to be set to `elastic-async`. 
  Synchronous, `nrt` or other lanes are not supported.
  Indexes are updated asynchronously.
* `refresh` is ignored.
  Changes take effect immediately after changing them.
  Existing documents in Elasticsearch are not changed.
* Indexes are NOT automatically built when needed: 
  They can be built by setting the `reindex` property to `true` or by using the `oak-run` tool.
  We recommend to build them using the `oak-run` tool.
* `evaluatePathRestrictions` is only checked at query time (to keep the compatibility with Lucene). The parent paths are
  always indexed. Changing this flag won't require a reindex then. It's strongly suggested to enable it. This control
  might be removed in the future.
* `codec` is ignored.
* `compatVersion` is ignored.
* `useIfExists` is ignored.
* `blobSize` is ignored.
* `name` is ignored.
* `indexPath` is ignored.
* `analyzers` support the Lucene configuration plus Elasticsearch specific [options][options]. Since Elasticsearch uses
  a more recent version of Lucene compared to the one in `oak-lucene` module, there might be differences in configuration options
  that could require changes when migrating from Lucene to Elasticsearch.
* `useInExcerpt` does not support regexp relative properties.
* For property definitions, `sync` and `unique` are ignored.
  Synchronous indexing, and enforcing uniqueness constraints is not currently supported in elastic indexes.
* The behavior for `dynamicBoost` is slightly different: 
  For Lucene indexes, boosting is done in indexing, while for Elastic it is done at query time.
* The behavior for `suggest` is slightly different:
  For Lucene indexes, the suggestor is updated every 10 minutes by default and the frequency
  can be changed by `suggestUpdateFrequencyMinutes` property in suggestion node under the index definition node.
  In Elastic indexes, there is no such delay and thus no need for the above config property. This is an improvement in ES over lucene.

[lucene]: https://jackrabbit.apache.org/oak/docs/query/lucene.html
[options]: https://www.elastic.co/guide/en/elasticsearch/reference/current/configure-text-analysis.html
