[[scan-scroll]] === scan and scroll
The scan search type and the scroll API((("scroll API", "scan and scroll"))) are used together to retrieve
large numbers of documents from Elasticsearch efficiently, without paying the
penalty of deep pagination.
scroll::
A scrolled search allows us to((("scrolled search"))) do an initial search and to keep pulling batches of results from Elasticsearch until there are no more results left. It's a bit like a cursor in ((("cursors")))a traditional database.
A scrolled search takes a snapshot in time. It doesn't see any changes that are made to the index after the initial search request has been made. It does this by keeping the old data files around, so that it can preserve its ``view'' on what the index looked like at the time it started.
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scan::
The costly part of deep pagination is the global sorting of results, but if we
disable sorting, then we can return all documents quite cheaply. To do this, we
use the scan search type.((("scan search type"))) Scan instructs Elasticsearch to do no sorting, but
to just return the next batch of results from every shard that still has
results to return.
To use scan-and-scroll, we execute a search((("scan-and-scroll"))) request setting search_type to((("search_type", "scan and scroll")))
scan, and passing a scroll parameter telling Elasticsearch how long it
should keep the scroll open:
GET /old_index/_search?search_type=scan&scroll=1m <1> { "query": { "match_all": {}}, "size": 1000
<1> Keep the scroll open for 1 minute.
The response to this request doesn't include any hits, but does include a
_scroll_id, which is a long Base-64 encoded((("scroll_id"))) string. Now we can pass the
_scroll_id to the _search/scroll endpoint to retrieve the first batch of
results:
GET /_search/scroll?scroll=1m <1> c2Nhbjs1OzExODpRNV9aY1VyUVM4U0NMd2pjWlJ3YWlBOzExOTpRNV9aY1VyUVM4U0 <2> NMd2pjWlJ3YWlBOzExNjpRNV9aY1VyUVM4U0NMd2pjWlJ3YWlBOzExNzpRNV9aY1Vy UVM4U0NMd2pjWlJ3YWlBOzEyMDpRNV9aY1VyUVM4U0NMd2pjWlJ3YWlBOzE7dG90YW
<1> Keep the scroll open for another minute.
<2> The _scroll_id can be passed in the body, in the URL, or as a
query parameter.
Note that we again specify ?scroll=1m. The scroll expiry time is refreshed
every time we run a scroll request, so it needs to give us only enough time
to process the current batch of results, not all of the documents that match
the query.
The response to this scroll request includes the first batch of results.
Although we specified a size of 1,000, we get back many more
documents.((("size parameter", "in scanning"))) When scanning, the size is applied to each shard, so you will
get back a maximum of size * number_of_primary_shards documents in each
batch.
NOTE: The scroll request also returns a new _scroll_id. Every time
we make the next scroll request, we must pass the _scroll_id returned by the
previous scroll request.
When no more hits are returned, we have processed all matching documents.
TIP: Some of the http://www.elasticsearch.org/guide[official Elasticsearch clients] provide scan-and-scroll helpers that provide an easy wrapper around this functionality.((("clients", "providing scan-and-scroll helpers")))