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Scroll

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Scroll

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While a search request returns a single “page” of results, the scroll API can be used to retrieve large numbers of results (or even all results) from a single search request, in much the same way as you would use a cursor on a traditional database.

Scrolling is not intended for real time user requests, but rather for processing large amounts of data, e.g. in order to reindex the contents of one index into a new index with a different configuration.

The results that are returned from a scroll request reflect the state of the index at the time that the initial search request was made, like a snapshot in time. Subsequent changes to documents (index, update or delete) will only affect later search requests.

In order to use scrolling, the initial search request should specify the scroll parameter in the query string, which tells Elasticsearch how long it should keep the “search context” alive (see Keeping the search context alive), eg ?scroll=1m.

curl -XGET 'localhost:9200/twitter/tweet/_search?scroll=1m' -d '
{
    "query": {
        "match" : {
            "title" : "elasticsearch"
        }
    }
}
'

The result from the above request includes a _scroll_id, which should be passed to the scroll API in order to retrieve the next batch of results.

curl -XGET 'localhost:9200/_search/scroll' -d' 
{
    "scroll" : "1m", 
    "scroll_id" : "c2Nhbjs2OzM0NDg1ODpzRlBLc0FXNlNyNm5JWUc1" 
}
'

	`GET` or `POST` can be used and the URL should not include the `index` or `type` name — this is specified in the original `search` request instead.
	The `scroll` parameter tells Elasticsearch to keep the search context open for another `1m`.
	The `scroll_id` parameter

Each call to the scroll API returns the next batch of results until there are no more results left to return, ie the hits array is empty.

For backwards compatibility, scroll_id and scroll can be passed in the query string. And the scroll_id can be passed in the request body

curl -XGET 'localhost:9200/_search/scroll?scroll=1m' -d 'c2Nhbjs2OzM0NDg1ODpzRlBLc0FXNlNyNm5JWUc1'

The initial search request and each subsequent scroll request returns a new _scroll_id — only the most recent _scroll_id should be used.

If the request specifies aggregations, only the initial search response will contain the aggregations results.

Scroll requests have optimizations that make them faster when the sort order is _doc. If you want to iterate over all documents regardless of the order, this is the most efficient option:

curl -XGET 'localhost:9200/_search?scroll=1m' -d '
{
  "sort": [
    "_doc"
  ]
}
'

Keeping the search context alive

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The scroll parameter (passed to the search request and to every scroll request) tells Elasticsearch how long it should keep the search context alive. Its value (e.g. 1m, see Time units) does not need to be long enough to process all data — it just needs to be long enough to process the previous batch of results. Each scroll request (with the scroll parameter) sets a new expiry time.

Normally, the background merge process optimizes the index by merging together smaller segments to create new bigger segments, at which time the smaller segments are deleted. This process continues during scrolling, but an open search context prevents the old segments from being deleted while they are still in use. This is how Elasticsearch is able to return the results of the initial search request, regardless of subsequent changes to documents.

Keeping older segments alive means that more file handles are needed. Ensure that you have configured your nodes to have ample free file handles. See File Descriptors.

You can check how many search contexts are open with the nodes stats API:

curl -XGET localhost:9200/_nodes/stats/indices/search?pretty

Clear scroll API

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Search context are automatically removed when the scroll timeout has been exceeded. However keeping scrolls open has a cost, as discussed in the previous section so scrolls should be explicitly cleared as soon as the scroll is not being used anymore using the clear-scroll API:

curl -XDELETE localhost:9200/_search/scroll -d '
{
    "scroll_id" : ["c2Nhbjs2OzM0NDg1ODpzRlBLc0FXNlNyNm5JWUc1"]
}'

Multiple scroll IDs can be passed as array:

curl -XDELETE localhost:9200/_search/scroll -d '
{
    "scroll_id" : ["c2Nhbjs2OzM0NDg1ODpzRlBLc0FXNlNyNm5JWUc1", "aGVuRmV0Y2g7NTsxOnkxaDZ"]
}'

All search contexts can be cleared with the _all parameter:

curl -XDELETE localhost:9200/_search/scroll/_all

The scroll_id can also be passed as a query string parameter or in the request body. Multiple scroll IDs can be passed as comma separated values:

curl -XDELETE localhost:9200/_search/scroll \
     -d 'c2Nhbjs2OzM0NDg1ODpzRlBLc0FXNlNyNm5JWUc1,aGVuRmV0Y2g7NTsxOnkxaDZ'

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