117 lines
4.7 KiB
Plaintext
117 lines
4.7 KiB
Plaintext
[role="xpack"]
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[testenv="basic"]
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[[point-in-time]]
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==== Point in time
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A search request by default executes against the most recent visible data of
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the target indices, which is called point in time. Elasticsearch pit (point in time)
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is a lightweight view into the state of the data as it existed when initiated.
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In some cases, it's preferred to perform multiple search requests using
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the same point in time. For example, if <<indices-refresh,refreshes>> happen between
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search_after requests, then the results of those requests might not be consistent as
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changes happening between searches are only visible to the more recent point in time.
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A point in time must be opened explicitly before being used in search requests. The
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keep_alive parameter tells Elasticsearch how long it should keep a point in time alive,
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e.g. `?keep_alive=5m`.
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[source,console]
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--------------------------------------------------
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POST /my-index-000001/_pit?keep_alive=1m
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--------------------------------------------------
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// TEST[setup:my_index]
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The result from the above request includes a `id`, which should
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be passed to the `id` of the `pit` parameter of a search request.
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[source,console]
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--------------------------------------------------
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POST /_search <1>
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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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"pit": {
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"id": "46ToAwMDaWR4BXV1aWQxAgZub2RlXzEAAAAAAAAAAAEBYQNpZHkFdXVpZDIrBm5vZGVfMwAAAAAAAAAAKgFjA2lkeQV1dWlkMioGbm9kZV8yAAAAAAAAAAAMAWICBXV1aWQyAAAFdXVpZDEAAQltYXRjaF9hbGw_gAAAAA==", <2>
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"keep_alive": "1m" <3>
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}
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}
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--------------------------------------------------
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// TEST[catch:missing]
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<1> A search request with the `pit` parameter must not specify `index`, `routing`,
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and {ref}/search-request-body.html#request-body-search-preference[`preference`]
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as these parameters are copied from the point in time.
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<2> The `id` parameter tells Elasticsearch to execute the request using contexts
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from this point int time.
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<3> The `keep_alive` parameter tells Elasticsearch how long it should extend
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the time to live of the point in time.
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IMPORTANT: The open point in time request and each subsequent search request can
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return different `id`; thus always use the most recently received `id` for the
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next search request.
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[[point-in-time-keep-alive]]
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===== Keeping point in time alive
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The `keep_alive` parameter, which is passed to a open point in time request and
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search request, extends the time to live of the corresponding point in time.
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The 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 for the next 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. However, open point-in-times prevent the
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old segments from being deleted since they 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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point in time must keep track of whether each document in the segment was live at
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the time of the initial search request. Ensure that your nodes have sufficient heap
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space if you have many open point-in-times on an index that is subject to ongoing
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deletes or updates.
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You can check how many point-in-times (i.e, search contexts) are open with the
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<<cluster-nodes-stats,nodes stats API>>:
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[source,console]
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---------------------------------------
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GET /_nodes/stats/indices/search
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---------------------------------------
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===== Close point in time API
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Point-in-time is automatically closed when its `keep_alive` has
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been elapsed. However keeping point-in-times has a cost, as discussed in the
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<<point-in-time-keep-alive,previous section>>. Point-in-times should be closed
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as soon as they are no longer used in search requests.
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[source,console]
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---------------------------------------
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DELETE /_pit
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{
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"id" : "46ToAwMDaWR4BXV1aWQxAgZub2RlXzEAAAAAAAAAAAEBYQNpZHkFdXVpZDIrBm5vZGVfMwAAAAAAAAAAKgFjA2lkeQV1dWlkMioGbm9kZV8yAAAAAAAAAAAMAWIBBXV1aWQyAAA="
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}
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---------------------------------------
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// TEST[catch:missing]
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The API returns the following response:
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[source,console-result]
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--------------------------------------------------
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{
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"succeeded": true, <1>
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"num_freed": 3 <2>
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}
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--------------------------------------------------
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// TESTRESPONSE[s/"succeeded": true/"succeeded": $body.succeeded/]
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// TESTRESPONSE[s/"num_freed": 3/"num_freed": $body.num_freed/]
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<1> If true, all search contexts associated with the point-in-time id are successfully closed
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<2> The number of search contexts have been successfully closed
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