294 lines
11 KiB
Plaintext
294 lines
11 KiB
Plaintext
[[search-request-scroll]]
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=== Scroll
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While a `search` request returns a single ``page'' of results, the `scroll`
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API can be used to retrieve large numbers of results (or even all results)
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from a single search request, in much the same way as you would use a cursor
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on a traditional database.
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Scrolling is not intended for real time user requests, but rather for
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processing large amounts of data, e.g. in order to reindex the contents of one
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index into a new index with a different configuration.
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.Client support for scrolling and reindexing
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*********************************************
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Some of the officially supported clients provide helpers to assist with
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scrolled searches and reindexing of documents from one index to another:
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Perl::
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See https://metacpan.org/pod/Search::Elasticsearch::Client::5_0::Bulk[Search::Elasticsearch::Client::5_0::Bulk]
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and https://metacpan.org/pod/Search::Elasticsearch::Client::5_0::Scroll[Search::Elasticsearch::Client::5_0::Scroll]
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Python::
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See http://elasticsearch-py.readthedocs.org/en/master/helpers.html[elasticsearch.helpers.*]
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*********************************************
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NOTE: The results that are returned from a scroll request reflect the state of
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the index at the time that the initial `search` request was made, like a
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snapshot in time. Subsequent changes to documents (index, update or delete)
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will only affect later search requests.
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In order to use scrolling, the initial search request should specify the
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`scroll` parameter in the query string, which tells Elasticsearch how long it
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should keep the ``search context'' alive (see <<scroll-search-context>>), eg `?scroll=1m`.
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[source,js]
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--------------------------------------------------
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POST /twitter/_search?scroll=1m
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{
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"size": 100,
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"query": {
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"match" : {
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"title" : "elasticsearch"
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}
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}
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}
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--------------------------------------------------
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// CONSOLE
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// TEST[setup:twitter]
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The result from the above request includes a `_scroll_id`, which should
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be passed to the `scroll` API in order to retrieve the next batch of
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results.
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[source,js]
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--------------------------------------------------
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POST /_search/scroll <1>
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{
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"scroll" : "1m", <2>
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"scroll_id" : "DXF1ZXJ5QW5kRmV0Y2gBAAAAAAAAAD4WYm9laVYtZndUQlNsdDcwakFMNjU1QQ==" <3>
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}
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--------------------------------------------------
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// CONSOLE
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// TEST[continued s/DXF1ZXJ5QW5kRmV0Y2gBAAAAAAAAAD4WYm9laVYtZndUQlNsdDcwakFMNjU1QQ==/$body._scroll_id/]
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<1> `GET` or `POST` can be used and the URL should not include the `index`
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name -- this is specified in the original `search` request instead.
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<2> The `scroll` parameter tells Elasticsearch to keep the search context open
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for another `1m`.
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<3> The `scroll_id` parameter
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The `size` parameter allows you to configure the maximum number of hits to be
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returned with each batch of results. Each call to the `scroll` API returns the
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next batch of results until there are no more results left to return, ie the
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`hits` array is empty.
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IMPORTANT: The initial search request and each subsequent scroll request each
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return a `_scroll_id`. While the `_scroll_id` may change between requests, it doesn’t
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always change — in any case, only the most recently received `_scroll_id` should be used.
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NOTE: If the request specifies aggregations, only the initial search response
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will contain the aggregations results.
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NOTE: Scroll requests have optimizations that make them faster when the sort
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order is `_doc`. If you want to iterate over all documents regardless of the
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order, this is the most efficient option:
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[source,js]
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--------------------------------------------------
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GET /_search?scroll=1m
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{
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"sort": [
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"_doc"
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]
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}
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--------------------------------------------------
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// CONSOLE
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// TEST[setup:twitter]
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[[scroll-search-context]]
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==== Keeping the search context alive
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A scroll returns all the documents which matched the search at the time of the
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initial search request. It ignores any subsequent changes to these documents.
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The `scroll_id` identifies a _search context_ which keeps track of everything
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that {es} needs to return the correct documents. The search context is created
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by the initial request and kept alive by subsequent requests.
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The `scroll` parameter (passed to the `search` request and to every `scroll`
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request) tells Elasticsearch how long it should keep the search context alive.
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Its value (e.g. `1m`, see <<time-units>>) does not need to be long enough to
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process all data -- it just needs to be long enough to process the previous
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batch of results. Each `scroll` request (with the `scroll` parameter) sets a
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new expiry time. If a `scroll` request doesn't pass in the `scroll`
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parameter, then the search context will be freed as part of _that_ `scroll`
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request.
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Normally, the background merge process optimizes the index by merging together
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smaller segments to create new, bigger segments. Once the smaller segments are
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no longer needed they are deleted. This process continues during scrolling, but
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an open search context prevents the old segments from being deleted since they
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are still in use.
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TIP: Keeping older segments alive means that more disk space and file handles
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are needed. Ensure that you have configured your nodes to have ample free file
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handles. See <<file-descriptors>>.
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Additionally, if a segment contains deleted or updated documents then the
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search context must keep track of whether each document in the segment was live
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at the time of the initial search request. Ensure that your nodes have
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sufficient heap space if you have many open scrolls on an index that is subject
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to ongoing deletes or updates.
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NOTE: To prevent against issues caused by having too many scrolls open, the
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user is not allowed to open scrolls past a certain limit. By default, the
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maximum number of open scrolls is 500. This limit can be updated with the
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`search.max_open_scroll_context` cluster setting.
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You can check how many search contexts are open with the
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<<cluster-nodes-stats,nodes stats API>>:
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[source,js]
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---------------------------------------
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GET /_nodes/stats/indices/search
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---------------------------------------
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// CONSOLE
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==== Clear scroll API
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Search context are automatically removed when the `scroll` timeout has been
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exceeded. However keeping scrolls open has a cost, as discussed in the
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<<scroll-search-context,previous section>> so scrolls should be explicitly
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cleared as soon as the scroll is not being used anymore using the
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`clear-scroll` API:
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[source,js]
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---------------------------------------
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DELETE /_search/scroll
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{
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"scroll_id" : "DXF1ZXJ5QW5kRmV0Y2gBAAAAAAAAAD4WYm9laVYtZndUQlNsdDcwakFMNjU1QQ=="
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}
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---------------------------------------
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// CONSOLE
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// TEST[catch:missing]
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Multiple scroll IDs can be passed as array:
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[source,js]
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---------------------------------------
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DELETE /_search/scroll
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{
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"scroll_id" : [
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"DXF1ZXJ5QW5kRmV0Y2gBAAAAAAAAAD4WYm9laVYtZndUQlNsdDcwakFMNjU1QQ==",
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"DnF1ZXJ5VGhlbkZldGNoBQAAAAAAAAABFmtSWWRRWUJrU2o2ZExpSGJCVmQxYUEAAAAAAAAAAxZrUllkUVlCa1NqNmRMaUhiQlZkMWFBAAAAAAAAAAIWa1JZZFFZQmtTajZkTGlIYkJWZDFhQQAAAAAAAAAFFmtSWWRRWUJrU2o2ZExpSGJCVmQxYUEAAAAAAAAABBZrUllkUVlCa1NqNmRMaUhiQlZkMWFB"
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]
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}
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---------------------------------------
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// CONSOLE
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// TEST[catch:missing]
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All search contexts can be cleared with the `_all` parameter:
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[source,js]
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---------------------------------------
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DELETE /_search/scroll/_all
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---------------------------------------
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// CONSOLE
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The `scroll_id` can also be passed as a query string parameter or in the request body.
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Multiple scroll IDs can be passed as comma separated values:
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[source,js]
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---------------------------------------
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DELETE /_search/scroll/DXF1ZXJ5QW5kRmV0Y2gBAAAAAAAAAD4WYm9laVYtZndUQlNsdDcwakFMNjU1QQ==,DnF1ZXJ5VGhlbkZldGNoBQAAAAAAAAABFmtSWWRRWUJrU2o2ZExpSGJCVmQxYUEAAAAAAAAAAxZrUllkUVlCa1NqNmRMaUhiQlZkMWFBAAAAAAAAAAIWa1JZZFFZQmtTajZkTGlIYkJWZDFhQQAAAAAAAAAFFmtSWWRRWUJrU2o2ZExpSGJCVmQxYUEAAAAAAAAABBZrUllkUVlCa1NqNmRMaUhiQlZkMWFB
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---------------------------------------
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// CONSOLE
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// TEST[catch:missing]
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[[sliced-scroll]]
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==== Sliced Scroll
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For scroll queries that return a lot of documents it is possible to split the scroll in multiple slices which
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can be consumed independently:
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[source,js]
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--------------------------------------------------
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GET /twitter/_search?scroll=1m
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{
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"slice": {
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"id": 0, <1>
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"max": 2 <2>
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},
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"query": {
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"match" : {
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"title" : "elasticsearch"
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}
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}
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}
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GET /twitter/_search?scroll=1m
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{
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"slice": {
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"id": 1,
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"max": 2
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},
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"query": {
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"match" : {
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"title" : "elasticsearch"
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}
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}
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}
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--------------------------------------------------
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// CONSOLE
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// TEST[setup:big_twitter]
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<1> The id of the slice
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<2> The maximum number of slices
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The result from the first request returned documents that belong to the first slice (id: 0) and the result from the
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second request returned documents that belong to the second slice. Since the maximum number of slices is set to 2
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the union of the results of the two requests is equivalent to the results of a scroll query without slicing.
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By default the splitting is done on the shards first and then locally on each shard using the _id field
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with the following formula:
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`slice(doc) = floorMod(hashCode(doc._id), max)`
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For instance if the number of shards is equal to 2 and the user requested 4 slices then the slices 0 and 2 are assigned
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to the first shard and the slices 1 and 3 are assigned to the second shard.
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Each scroll is independent and can be processed in parallel like any scroll request.
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NOTE: If the number of slices is bigger than the number of shards the slice filter is very slow on the first calls, it has a complexity of O(N) and a memory cost equals
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to N bits per slice where N is the total number of documents in the shard.
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After few calls the filter should be cached and subsequent calls should be faster but you should limit the number of
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sliced query you perform in parallel to avoid the memory explosion.
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To avoid this cost entirely it is possible to use the `doc_values` of another field to do the slicing
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but the user must ensure that the field has the following properties:
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* The field is numeric.
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* `doc_values` are enabled on that field
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* Every document should contain a single value. If a document has multiple values for the specified field, the first value is used.
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* The value for each document should be set once when the document is created and never updated. This ensures that each
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slice gets deterministic results.
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* The cardinality of the field should be high. This ensures that each slice gets approximately the same amount of documents.
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[source,js]
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--------------------------------------------------
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GET /twitter/_search?scroll=1m
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{
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"slice": {
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"field": "date",
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"id": 0,
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"max": 10
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},
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"query": {
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"match" : {
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"title" : "elasticsearch"
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}
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}
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}
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--------------------------------------------------
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// CONSOLE
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// TEST[setup:big_twitter]
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For append only time-based indices, the `timestamp` field can be used safely.
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NOTE: By default the maximum number of slices allowed per scroll is limited to 1024.
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You can update the `index.max_slices_per_scroll` index setting to bypass this limit.
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